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I heard that the Earth's atmosphere is opaque at most wavelengths and only allows visible light and radio waves through, so why have plants evolved to only absorb visible light?
Even if the atmosphere is mostly transparent in the radio domain, most of sunlight is visible light.
Also, to make photosynthesis work, you need photons of high energy (say: wavelength shorter than 700nm) in order to excite electrons in the reaction centres. Radio waves cannot do that.
The most widespread photosynthetic organisms make use of visible light and water, which are plentiful on Earth.
How are Radio Waves Blocked? (with pictures)
Radio waves are a type of electromagnetic radiation and the primary mode of global communication. In addition to radio broadcasts, other applications such as television sets, cell phones and radio-controlled cars all use forms of the technology. The challenge with this form of communication is the fact that different naturally-occurring phenomena like mountains, along with certain materials such as copper and aluminum, can block the waves.
As a simple form of electromagnetic radiation, radio waves are not harmful to humans and move from location to location with limited impact. The benefits and drawbacks of blocking radio waves come in the form of either intentional or accidental events. A military may choose to block radio waves of an enemy for instance. Other situations result in the natural interruption of a broadcast.
To understand what blocks radio waves, the fundamental theory behind the technology must be illustrated. A radio signal is sent from one location via a device known as a transmission antenna, basically creating an electromagnetic field projected from the unit to the broader world. Each wave moves out in every direction until they hit a receiving antenna, a device designed to pick up the wave.
Transmittance is the theory that makes radio waves travel through materials without being stopped. Either a material is a good or bad transmitter of the radiation. An example of good transmission material is the lower atmosphere of the Earth, which allows radiation to travel for long distances. The same cannot be said for the upper layer of the atmosphere, however, which is known as the ionosphere. This contains ionized radiation from the sun, which reflects the radio waves back towards the lower atmosphere.
The attenuation coefficient is the level by which a material will block or interfere with radio waves. This coefficient depends heavily on the thickness and composition of the material. Cardboard, paper, many plastics, water, and glass are all substances with very low attenuation coefficients. Wood, brick, and cement have a limited effect on radio waves. Metallic compounds, steel-reinforced concrete, and the Earth reflect signals, however, preventing radio waves from passing through.
One major consideration that determines if radio waves are blocked involves the concept of diffraction. This depends on the wavelength of the radiation and the size of the obstacle that it is attempting to penetrate. Low frequencies have a easier time passing over large objects such as hills, while higher frequencies work better with small obstacles such as rooftops. This can be very useful for blocking radio waves using the knife-edge diffraction method. If a wave does not have a line of sight over an object, a sharp edge can be created that causes the wave to be blocked and redirected to where the broadcast should go.
Is the universe a graveyard? This theory suggests humanity may be alone.
Ever since we've had the technology, we've looked to the stars in search of alien life. It's assumed that we're looking because we want to find other life in the universe, but what if we're looking to make sure there isn't any?
Here's an equation, and a rather distressing one at that: N = R* × fP × ne × f1 × fi × fc × L. It's the Drake equation, and it describes the number of alien civilizations in our galaxy with whom we might be able to communicate. Its terms correspond to values such as the fraction of stars with planets, the fraction of planets on which life could emerge, the fraction of planets that can support intelligent life, and so on. Using conservative estimates, the minimum result of this equation is 20. There ought to be 20 intelligent alien civilizations in the Milky Way that we can contact and who can contact us. But there aren't any.
The Drake equation is an example of a broader issue in the scientific community—considering the sheer size of the universe and our knowledge that intelligence life has evolved at least once, there should be evidence for alien life. This is generally referred to as the Fermi paradox, after the physicist Enrico Fermi who first examined the contradiction between high probability of alien civilizations and their apparent absence. Fermi summed this up rather succinctly when he asked, “Where is everybody"?
But maybe this was the wrong question. A better question, albeit a more troubling one, might be “What happened to everybody?" Unlike asking where life exists in the universe, there's a clearer potential answer to this question: the Great Filter.
A radio transmission is electromagnetic radiation that is made up of electrical and magnetic fields perpendicular to one another. They both move as a wave, cycling at a specific frequency. Energy in the wave moves back and forth between the magnetic and electrical fields. A radio signal propagates from its point of transmission in a spherical shape, as with higher-frequency radio waves as a more focused, narrower beam. The radio frequency range begins with the Extremely Low Frequency band at 3 hertz and extends to the Extremely High Frequency band at 300 gigahertz.
Cellular phone networks utilize multiple bands of EM spectrum, one of which is called UHF, or ultra-high frequency, sometimes known as microwave The frequency range for microwave radiation is between 300 megahertz and 300 gigahertz. UHF waves are also utilized in radar, microwave ovens and wireless local area networks. Microwaves on the electromagnetic spectrum can be further divided into different bands, depending on the frequency.
The interaction of photons with matter is complicated. The electromagnetic spectrum covers many orders of magnitude in frequency and photon energy, and there are qualitatively different processes that occur in different regimes. The results depend on the electrical properties of the material, such as conductivity and permittivity. We have materials like glass that are transparent to visible light, and low-energy x-rays that are strongly absorbed.
But speaking very broadly, it is possible to understand the main trends over the whole spectrum. We have a region (1) in the visible spectrum, where the frequency of the light is similar to the frequency of condensed-matter resonances, which in many cases you can think of as resonances of the electrons, as if the electrons are little objects attached to atoms by springs and region (2) in low-energy x-rays, where the wavelength of the photon is comparable to the wavelength of the electrons in an atom. This splits up the spectrum into three parts.
At low frequencies $f$ , below region 1, we have a skin depth, which depends on $f^<-1/2>$ . As $f$ gets smaller, the skin depth grows without bound. Hence radio waves tend to be penetrating.
Around region 1, you get strong classical resonant behavior. You can see this if you look at a plot of the index of refraction of glass as a function of frequency. It has a series of spectacular peaks. Each of these peaks has a classic Lorentzian shape, in which the response on the right-hand side of the peak approaches zero. So if you ignore the peaks themselves, which are narrow, then you get a series of stair steps. At frequencies above region 1, you've gone down all the stair steps, and the response approaches zero. This is why, classically, we expect high-frequency electromagnetic radiation to interact with matter very weakly.
But in region 2 you get the photoelectric effect. In first-order perturbation theory, this depends on the extent to which the electric field overlaps with the wavefunction of the electron. When the two wavelengths are similar, you get a strong cross-section. This is why matter strongly absorbs soft x-rays, but not gammas and hard x-rays.
8 Answers 8
In optical engineering, the choice between lenses and mirrors often comes down to aperture diameter: less than a few inches and lenses can be made cheaply and with high accuracy. Larger and costs increase exponentially, so even 6" diameter systems usually work better reflective.
At RF frequencies, a 6 inch lens is on the order of a wavelength, and so not useful for focusing. It isn't until you get towards the edge of the microwave spectrum that the wavelength gets short enough for lenses to start to become practical.
Of course if you don't care about cost, and you don't mind it being extremely heavy, you could build a lens to use with a WiFi antenna. It just doesn't make much practical sense.
Why Isn't Desalination the Answer to All California's Water Problems?
The massive new Carlsbad desalination plant is the biggest in the country, capable of supplying water to around 7 percent of the population of San Diego County. (Adam Keigwin/Poseidon Water)
Desalination just took a huge leap forward in California. The biggest plant in North America, able to purify tens of millions of gallons each day, is now pumping water near San Diego.
The $1 billion Carlsbad facility is a &ldquotest case&rdquo to backers like Cal Desal executive director Ron Davis, who quipped last year, &ldquoOnly the entire future of desal is riding on this project. No pressure.&rdquo
Now the plant&rsquos completion is a feather in the cap for the builder, Poseidon Water, which hopes to follow suit with a similar desalination project in Huntington Beach.
First though, Poseidon engineers must resolve the question of how the Huntington Beach plant would draw in water. State regulators prefer an intake below the seafloor, to make sure it doesn&rsquot suck in fish and their tiny eggs &ndash but a feasibility study this summer said building that type of intake would cost too much.
Further north, a smaller plant is expected to provide water for several towns around the Monterey Peninsula. But it won&rsquot come online for four years, long after a deadline for the local water company, California American, to stop sucking water from the Carmel River. Cal Am and local officials recently asked the state water board to delay that cutoff order &ndash currently set for the end of 2016 &ndash until the plant can be finished around 2020.
Meantime, a test well for the plant&rsquos subsurface intake, on a beach near the town of Marina, is pulling up a couple thousand gallons of saltwater per minute. Carmel Mayor Jason Burnett says that bolsters hopes that, pending the proper approvals, drilling of more slant wells could get underway in 2017.
Nowhere near enough water has fallen on California in years, and there&rsquos nothing you can do to make it rain.
So where else can we get water? One idea gaining traction is desalination: converting seawater into drinking water. While desal has long been confined by steep costs and environmental concerns, even some critics now say it merits a place in the state&rsquos water portfolio.
South of Los Angeles, in the city of Carlsbad, what will be the nation&rsquos largest desalination facility is nearly ready. For roughly a billion dollars, the plant will produce 7 percent of San Diego County&rsquos water. In Santa Barbara, a plant built amid the drought of the early 1990&rsquos, and idled by the return of rain, could come back online soon and provide 30 percent of the community&rsquos water.
Farther north, another desalination plant is expected to serve several towns in Monterey County. Jason Burnett, the mayor of Carmel, sometimes acts as a kind of spokesman for the planned project -- but he&rsquos hardly an evangelist.
&ldquoI&rsquoll say at the outset, I am not a fan of desal generally,&rdquo says Burnett.
Listen to the Story:
Why Isn't Desalination the Answer to All California's Water Problems?
Apart from concerns about the expense, Burnett has a personal stake in desalination&rsquos environmental challenges. He&rsquos the son of two marine biologists, and his grandfather David Packard&rsquos Silicon Valley fortune was integral to founding the Monterey Bay Aquarium. Burnett himself worked on climate rules for the U.S. Environmental Protection Agency before becoming Carmel's mayor.
Carmel Mayor Jason Burnett gestures toward the Carmel River, near its mouth at the Pacific. Burnett says he's not a fan of desalination, but the Monterey Peninsula is out of alternatives. (Daniel Potter/KQED)
&ldquoI&rsquove dedicated my professional life to working on climate change," Burnett says. "My family is very dedicated to the health of our oceans. So here I am advocating a project that has a large carbon footprint, and, if not done correctly, can hurt the oceans.&rdquo
Burnett met me on a beach where the Carmel River flows out to the Pacific Ocean. Nearby, ladies in straw hats were hauling easels and paints out to the sand to capture the picturesque landscape. Wearing designer sunglasses and a crisp blue shirt, Burnett told me desalination was the community&rsquos last resort.
&ldquoWe&rsquove explored a wide range of options," he says. "Everything was on the table -- harnessing icebergs and bringing them down, filling up huge balloons of water from up north and bringing them down."
It came to desal because the area&rsquos for-profit water supplier, California American Water Company, was told it had to find a new source. For decades Cal Am had relied on the Carmel River, but then came a cease-and-desist order intended to protect the river&rsquos threatened steelhead trout. There were years of wrangling and competing designs. A deadline was set for the end of next year &ndash- a deadline Cal Am&rsquos proposed desal plant will not hit. All the same, a plan is moving forward.
&ldquoThis is, at its core,&rdquo says Burnett, &ldquoan environmental project.&rdquo
Intakes and Outfalls
There are three main environmental considerations when building a desalination plant: how seawater is brought in, how the drinkable water is separated out, and what happens to the salt afterward.
The simplest intake is essentially a straw in the ocean -&ndash a design that risks trapping and killing sea life. One solution is to affix a grate to the end of such a pipe, but even then, tiny larvae and fish eggs can still be sucked in. Instead, regulators tend to prefer what&rsquos known as a "subsurface intake."
At a cement company&rsquos beachside site on Monterey Bay, California American is currently working on a proof-of-concept for this approach. They&rsquore using directional drilling, similar to the technology oil companies use to extract fossil fuels. The idea is to run a slant well hundreds of feet out, passing beneath the dunes to a spot under the waves. From below 200 feet of sand, and well insulated from any vulnerable sea life, Cal Am hopes to suck up a couple thousand gallons of water per minute.
California American is using directional drilling extend a pipe some 735 feet under the beach, in hopes of sucking in a couple thousand gallons of seawater per minute from below the ocean floor. (Luke Gianni/California American Water Co.)
It will take a huge amount of power to pump that much water, that far.
&ldquoOur energy bill is going up, no question,&rdquo an engineer on the project told me.
This is the second concern with desalination: once the seawater gets to the plant, it has to be pushed through membranes fine enough that salt can&rsquot pass through them. That requires immense pressure &ndash on the order of a pressure-washer.
An official at a smaller desal facility told me it took $25,000 of electricity per month to produce enough water for 1,200 homes. In Cal Am&rsquos case, they&rsquore hoping to reach a deal to power the plant using methane from a nearby landfill.
One other still-tentative design element addresses the third challenge of the desalination process: all that salt has to go somewhere.
Only about half of the saltwater piped into a desal plant is made drinkable. All the salt that&rsquos separated out ends up concentrated into the other half, in a kind of brine that&rsquos much denser than seawater. As a result, it doesn&rsquot easily mix back in.
If it's just dumped carelessly back into the ocean, it sinks, and can kill any marine life having the misfortune of dwelling on the seafloor below.
Blending the briny byproduct back into the ocean may involve sprayers, or in Cal Am&rsquos case, an existing outfall that the nearby Monterey Regional Water Pollution Control Agency uses to dispose of wastewater. It's a pipe that runs thousands of feet out to sea, with small holes spaced ten feet apart, so not too much brine would pour out in any one place.
The desal facility isn&rsquot expected to start delivering water to customers for several years, and in the meantime, it has to navigate a regulatory thicket of needed approvals.
Optional or Inevitable?
In recent years, desalination projects were considered in places like Marin County and Santa Cruz, only to end up sidelined amid skepticism. Between the environmental headaches and the cost of engineering work-arounds, critics argued the technology is often more trouble than it&rsquos worth.
To the extent that conservation's an option, it&rsquos much simpler and cheaper to do. Mayor Burnett says the towns along the Monterey Peninsula have just about wrung out that sponge for all it&rsquos worth: people there get by on 60 gallons per day -- less than half what many Californians use.
Susan Jordan with the California Coastal Protection Network is a longtime critic of desal. She says, indeed, communities should first exhaust their other options.
&ldquoIf you&rsquore going to do something like desal," Jordan says, "you want to make sure you&rsquore doing everything you can in terms of conservation, water recycling, water re-use, and you don&rsquot want unsustainable development that just perpetuates your problem, or the state&rsquos problem.&rdquo
That question of what constitutes sustainable development underpins the debate around desal. The counter-argument I heard from Scott Maloni, vice president at Poseidon Water, is: what if there are no alternatives?
&ldquoThe larger concern is climate change, and what happens ten years from now and twenty years from now," says Maloni, whose company is building the big plant outside San Diego and hopes to add another like it in Huntington Beach. "Can you really count on the Colorado River or Northern California to continue to supply the vast majority of the state&rsquos population with water?&rdquo
I asked several people what percentage of California&rsquos overall water portfolio desalination might someday make up, and only Maloni was willing to venture a guess. He says such plants are most efficient when they&rsquore built big, thereby reaping economies of scale. Between that and the stringent permitting process, he says, you could probably count the number of viable sites on two hands.
&ldquoAnd so I think you could be looking at somewhere between 10 to 20 percent of the state&rsquos municipal and industrial demand,&rdquo Maloni says.
It&rsquos worth noting that would seem to leave out agriculture Maloni envisions desal serving the state's coastal urban populations.
Maloni and several others I spoke with also made the point that, while the technical challenges of designing and constructing an environmentally sound desalination plant are serious, the permitting process is lengthy and could well last longer than the drought itself.
Plant Responses to High Frequency Electromagnetic Fields
High frequency nonionizing electromagnetic fields (HF-EMF) that are increasingly present in the environment constitute a genuine environmental stimulus able to evoke specific responses in plants that share many similarities with those observed after a stressful treatment. Plants constitute an outstanding model to study such interactions since their architecture (high surface area to volume ratio) optimizes their interaction with the environment. In the present review, after identifying the main exposure devices (transverse and gigahertz electromagnetic cells, wave guide, and mode stirred reverberating chamber) and general physics laws that govern EMF interactions with plants, we illustrate some of the observed responses after exposure to HF-EMF at the cellular, molecular, and whole plant scale. Indeed, numerous metabolic activities (reactive oxygen species metabolism, α- and β-amylase, Krebs cycle, pentose phosphate pathway, chlorophyll content, terpene emission, etc.) are modified, gene expression altered (calmodulin, calcium-dependent protein kinase, and proteinase inhibitor), and growth reduced (stem elongation and dry weight) after low power (i.e., nonthermal) HF-EMF exposure. These changes occur not only in the tissues directly exposed but also systemically in distant tissues. While the long-term impact of these metabolic changes remains largely unknown, we propose to consider nonionizing HF-EMF radiation as a noninjurious, genuine environmental factor that readily evokes changes in plant metabolism.
High frequency electromagnetic fields (HF-EMF, i.e., frequencies from 300 MHz to 3 GHz, wavelengths from 1 m to 10 cm) are mainly human-produced, nonionizing electromagnetic radiations that do not naturally occur in the environment, excluding the low amplitude VHF (very high frequency) cosmic radiation. HF-EMF are increasingly present in the environment  because of the active development of wireless technology, including cell phones, Wi-Fi, and various kinds of connected devices. Since living material is not a perfect dielectric, it readily interferes with HF-EMF in a way that depends upon several parameters, including (but not restricted to) its general shape, the conductivity and density of the tissue, and the frequency and amplitude of the EMF. The interaction between the living material and the electromagnetic radiation may (or not) induce an elevation of the tissue temperature, thus defining the thermal (versus nonthermal) associated metabolic responses. In the case of a thermal response, the resulting heat dissipation is normalized with the specific absorption rate (SAR) index. This has led to considerable research efforts to study the possible biological effects due to exposure to HF-EMF. While the vast majority of these studies have focused on animals and humans because of health concerns, with contradictory or nonconclusive results , numerous experiments have also been performed on plants. Plants are outstanding models compared to animals to conduct such investigations: they are immobile and therefore keep a constant orientation in the EMF and their specific scheme of development (high surface area to volume ratio) makes them ideally suited to efficiently intercept EMF . It is also quite easy in plants to achieve genetically stable plant lines through the selection of species that favor asexual reproduction  or self-pollination . Furthermore, metabolic mutants are easily available for several species and constitute invaluable tools to understand the way the EMF signal is transduced . Indeed, several reports have pointed out that plants actually perceive HF-EMF of even small amplitudes and transduce them into molecular responses and/or alterations of their developmental scheme [3–9]. The way that HF-EMF interact with plants remains essentially unanswered. However, since EMF evoke a multitude of responses in plants, they might be considered as a genuine environmental stimulus. Indeed, EMF exposure alters the activity of several enzymes, including those of reactive oxygen species (ROS) metabolism , a well-known marker of plant responses to various kinds of environmental factors. EMF exposure also evokes the expression of specific genes previously implicated in plant responses to wounding [5, 8] and modifies the development of plants . Furthermore, these responses are systemic insofar as exposure of only a small region of a plant results in almost immediate molecular responses throughout the plant . These responses were abolished in the presence of calcium chelators  or inhibitors of oxidative phosphorylation  which implies the involvement of ATP pools. In the present review, we describe exposure devices, SAR determination methods, and biological responses (at both the cellular/molecular and whole plant levels) observed after plant exposure to EMF. We focused this review on radiated (i.e., EMF that are emitted through an antenna) HF-EMF (mainly within the range of 300 MHz–3 GHz) and consequently will not address the biological effects of static magnetic fields (SMF), extremely low frequency electromagnetic fields (ELF), or HF current injection, since their inherent physical properties are dramatically different from those of high frequencies. Therefore, the HF-EMF we consider should be viewed through the prism of classical electromagnetism: macroscopic electrodynamics phenomena described in terms of vector and scalar fields.
2. Exposure Systems and Dosimetry
HF-EMF are a combination of an electric field and a magnetic field governed by Maxwell’s equations. At high frequency, these vector quantities are coupled and obey wave equations whether for propagating waves or for standing waves. In vacuum, the former travel at the speed of light (≈3
10 8 m s −1 ) and have the structure of a plane wave (Figure 1(a)). In other media, the speed decreases and the spatial distribution for the electric and the magnetic fields are generally arbitrary (thus not being a plane wave). The latter, which do not propagate but vibrate up and down in place, appear in some particular conditions (e.g., bounded medium like metallic cavity) and play important roles in many physical applications (resonator, waveguide, etc.).
) is the distance between two crests. DOP: direction of propagation. (b) A TEM cell (transverse electromagnetic cell). (c) A GTEM cell (gigahertz transverse electromagnetic cell). (d) MSRC (mode stirred reverberation chamber). Note the double-sided metallic walls, the emitting antenna, the rotating stirrer, and the specialized culture chamber that stands in the “working volume” where the electromagnetic field characteristics have been extensively characterized.
In both cases, HF-EMF are characterized by an amplitude of the electric ( ) or magnetic (
) components (measured in volts or amperes per meter), a frequency
(number of cycles per second of the wave quantity, measured in hertz), and a wavelength (distance between wave crests, measured in meters). These properties are related through the following equation:
where is the speed of the wave in the considered medium and is the period of the wave (time between successive wave crests, measured in seconds). The wavelength is then the distance traveled by the wave during a period .
The electromagnetic power density associated with an electromagnetic wave (measured in watts per square meter) is obtained by a vector product between the electric and magnetic field vectors (namely, the Poynting vector) for every point in space. The total HF-EMF power crossing any given surface is derived from Poynting’s theorem . For an incident plane wave in vacuum, the time-averaged electromagnetic power
(measured in watts) illuminating a surface of 1 m 2 orthogonal to the direction of propagation is given by the following equation:
where is the characteristic impedance of free vacuum space (377 Ω).
The absorbed electromagnetic power (
), converted to heat by Joule effect in a volume (
) and averaged over a time period, is given by (3) for an electrically and magnetically linear material that obeys Ohm’s law (conductivity
2.1. Diversity of Exposure Devices
Due to the wide variety of electromagnetic waves, physicians developed a lot of electromagnetic exposure facilities, mainly for electromagnetic compatibility (EMC) test purposes. Some of these devices are used for plant exposure to HF-EMF.
HF-EMF exposure set-up is usually made up with the following two basic elements: (i) HF source (radio frequency generator, Gunn oscillator) associated with a radiating element (antenna, strip-line) and (ii) a structure that allows the propagation of EM waves and the exposure of the sample. The simplest exposure set-up relies on the use of standard cell phones as a source of HF-EMF [12, 13] radiating in an open-area test site. While this apparatus has the advantage of being simple and economical, it poses many limitations that may compromise the quality of the exposure. Indeed, these communication devices are operated with different protocols that may modify or even interrupt the emitted power. Also, the biological samples are placed in the immediate neighborhood of the antenna, which is a region where the electromagnetic field is not completely established (near-field conditions) and therefore is difficult to measure this situation may constitute an issue for bioelectromagnetics studies. These apparatuses are nowadays used only in a small proportion of studies. Moreover, the use of open-area test sites exposes the biological samples to the uncontrolled electromagnetic ambient environment. The use of shielded rooms is a good solution to overcome this issue. Indeed, anechoic chambers provide shielded enclosures, which are designed to completely absorb reflected electromagnetic waves. However, these facilities are often large structures requiring specific equipment and costly absorbers to generate an incident plane wave (far-field illumination) and are consequently seldom used for plant exposure [14, 15].
In contrast, numerous studies are based upon dedicated apparatus of relatively small volume (Figure 1(b)), namely, the transverse electromagnetic (TEM) cell . TEM cells are usually quite small (about 50 cm long × 20 cm wide) and therefore only allow the use of seeds or seedlings as plant models. Many TEM cells are based upon the classic “Crawford cell” . They consist of a section of rectangular coaxial transmission line tapered at each end to adapt to standard coaxial connectors. A uniform plane wave of fixed polarization and direction is generated in the sample space for experiments between the inner conductor (septum) and the upper metallic wall. Because this cost-efficient device is enclosed, high amplitude EMF can be developed with relatively little injected power. Under some conditions, two parallel walls of the TEM cell can be removed (therefore constituting the so-called open TEM cell) without dramatically compromising the performances. This configuration is adequate to allow plant lighting. Special attention must still be paid to the relative position of the samples in the system since the disposition of the different organs within the EMF could severely affect the efficiency of the plant samples’ coupling with the electromagnetic field. The main TEM cell limitation is that the upper useful frequency is bound by its physical dimensions limiting the practical size of samples at high frequency.
The gigahertz transverse electromagnetic (GTEM) cell has emerged as a more recent EMF emission test facility (Figure 1(c)) . It is a hybrid between an anechoic chamber and a TEM cell and could therefore be considered as a high frequency version of the TEM cell. The GTEM cell comprises only a tapered section, with one port and a broadband termination. This termination consists of a 50 Ω resistor board for low frequencies and pyramidal absorbers for high frequencies. This exposure device removes the inherent upper frequency limit of TEM cell while retaining some of its advantages (mainly the fact that no antenna set-up is required and the fact that high field strength could be achieved with low injected power).
Waveguides are another kind of screened enclosures that are seldom used in plant exposure [19, 20]. These classical and easy to use exposure devices generate traveling waves along the transmission coordinate and standing waves along the transverse coordinates. In contrast to the TEM cell, waveguides do not generate uniform plane waves but rather allow the propagation of more complex EMF, namely, propagation modes. Each mode is characterized by a cutoff frequency below which the mode cannot propagate. When the ends of the waveguide are short-circuited, a so-called resonant cavity is constituted, from which a recent large facility, originally designed for EMC studies, namely, the mode stirred reverberation chamber (MSRC, Figure 1(d)), is based. While this equipment is expensive and technically difficult to set up, it is the state of the art in terms of electromagnetic field characteristics, allowing the establishment of an isotropic and homogeneous field in a volume large enough to hold a dedicated plant culture chamber (either transparent or shielded toward EMF ). This latter characteristic permits experiments on large plants that are kept in an adequate controlled environment . Our group pioneered the use of this facility, based on judicious combinations of standing waves patterns in a complex screened enclosure, in plant bioelectromagnetics studies  and extensively described the MSRC functionality . Finally, each exposure set-up may differ in concept, polarization, frequency, or incident power but these setups always need to be optimally designed and based on well-understood physical concepts in order to assess well-controlled HF-EMF exposure conditions (homogeneity, repeatability, reproducibility, etc.).
2.2. Different Types of Exposure Signals
From each of the previous exposure devices, two very different types of EMF can be used to expose plants. The most commonly encountered mode is the continuous wave (CW) mode, in which the biological samples are continuously exposed for a specific duration to an EMF of given frequency and amplitude (rarely more than a few dozen V m −1 ). The second mode is the pulsed electromagnetic field (PEMF) mode, in which the biological samples are subjected to several series of discontinuous pulses of ultrashort duration EMF (within the range of μs to ns) and usually of very high amplitude (up to several hundred kV m −1 ). This last kind of exposure [22, 23] is seldom used because of the scarcity and great complexity of the equipment needed to generate the EMF and the difficulty to design the dedicated antennae able to deliver such ultrashort power surges .
The HF-EMF could also be modulated (i.e., varied in time at a given, usually much lower frequency). Only a few studies explicitly addressed modulation effect on biological responses. Răcuciu et al.  exposed maize caryopses to low levels (7 dBm), 900 MHz RF field, for 24 h in either continuous wave (CW), amplitude modulated (AM), or frequency modulated (FM) modes. They found that 12-day-old plant lengths were reduced by about 25% in modulated EMF (AM or FM type) compared to control (unexposed samples), while CW exposure had an opposite (growth stimulation) effect, suggesting that EMF modulation actually modifies biological responses.
In order to compare the biological effects observed in different exposure conditions, the National Council on Radiation Protection and Measurements officially introduced in 1981 an EMF exposure metric, the specific absorption rate (SAR). The formal definition of this basic dosimetry (the amount of dose absorbed) is “the time derivative of the incremental energy absorbed (
) by (dissipated in) an incremental mass contained in a volume ( ) of a given density
.” From this definition and (3), the SAR (measured in W kg −1 ) is given by the following equation:
SAR is the power absorbed by living tissue during exposure to CW-EMF (this quantity does not apply to PEMF mode because of the very short duration of the pulses that do not cause temperature increase in the samples). SAR can be calculated from the dielectric characteristics of plant tissues at the working frequencies, using (4). While could be easily determined, the value of is dependent upon the frequency and is difficult to assess in the range of GHz. It is usually evaluated from the literature , since the experimental set-up to measure this parameter at a given frequency (waveguide, open waveguide, and coaxial line technique, e.g., D-Line) is rarely used because of its complex set-up. From the biological heat-transfer equation, the SAR can also be determined using the temperature increase evoked in plant tissue after exposure to EMF, using the following equation:
where is the heat capacity (J K −1 kg −1 , which is available for some tissues in the literature) and (measured in Kelvin) is the sample temperature increase corresponding to the elapsed time (measured in second) since the beginning of HF-EMF exposure. Either for animals or plants, the SAR measurement is subject to uncertainty . Since the specific heat is frequency independent and the temperature distribution is usually more uniform than the internal electric field, (5) provides, for detectable temperature increases, a better way for SAR estimation.
In animal and human tissue, SAR is determined using dedicated phantoms  filled with a special liquid that mimics the dielectric properties of biological fluids. While this approach is adequate in animals, in which the developmental scheme produced volumes, it could not be adapted to most plant organs (e.g., leaves) that have a high surface area to volume ratio  but could be used in fruits and tuberous structures. In contrast, surface temperature can be easily assessed with dedicated instruments (e.g., Luxtron® fiber optic temperature probe) and used to feed (5) . The SAR can also be determined using the differential power method based on the measurement of power absorption (reviewed in ) that takes place in the absence or presence of biological samples . The SAR is then calculated by dividing the absorbed power by the mass of the living material.
3. Biological Responses
Biological responses should be considered as reporters of, and evidence for, the plant’s ability to perceive and interact with EMF. These responses can take place at the subcellular level, implying molecular events or modification of enzymatic activities, or at the level of the whole plant, taking the form of growth modification. Tables 1–3 summarize some work reporting HF-EMF effects observed at the scale of the whole plant, biochemical processes, or gene regulation, respectively.
3.1. Cellular and Molecular Level
Numerous reports [4, 7, 33] indicate an increase in the production of malondialdehyde (MDA, a well-known marker of membrane alteration) along with ROS metabolism activation after exposing plants to HF-EMF (Table 1). Membrane alteration and ROS metabolism activation are likely to establish transduction cascades that enable specific responses. Indeed, the critical role of calcium, a crucial second messenger in plants, has long been pointed out [6, 10]: the responses (e.g., changes in calm-n6, lecdpk-1, and pin2 gene expression) to EMF exposure are severely reduced when plants are cultivated with excess of calcium or in the presence of calcium counteracting agents (Figure 2) such as chelators (EGTA and BAPTA) or a channel blocker (LaCl3). The importance of calcium in the establishment of the plant response is also highlighted by the fact that early gene expression associated with EMF exposure involves at least 2 calcium-related products (calmodulin and calcium-dependent protein kinase) [5, 10]. This response is also energy-dependent: an important drop (30%, Figure 3) in ATP content and adenylate energy charge (AEC) occurs after HF-EMF exposure . It is not clear for now if the AEC drop is the consequence of altered membranes allowing passive ATP exit or if higher consumption of ATP occurred because of increased metabolic activity. Indeed, it is well known that a drop in AEC stimulates the catabolic enzymatic pathways through allosteric modulations. Nevertheless, inhibiting ATP biosynthesis with the decoupling agent carbonyl cyanide m-chlorophenyl hydrazone (CCCP) abolished plant responses to EMF exposure . Nitric oxide (NO) is another signaling molecule that is tightly related to environmental factors’ impact on plants . NO rapidly increases after various kinds of stimuli including drought stress or wounding. Chen et al.  recently demonstrated the increased activity of nitric oxide synthase and accumulation of NO after exposing caryopses of wheat for 10 s to high power 2.45 GHz EMF. Similarly, Qiu et al.  showed in wheat that the tolerance to cadmium evoked by microwave pretreatment was abolished by the addition of 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (carboxy-PTIO), an NO scavenger, suggesting that microwave-induced NO production was involved in this mechanism. Taken together, these results advocate for the EMF induction of NO synthase. However, these studies used high power EMF (modified microwave oven) as stimulating tool and the fact that a temperature increase of the sample was the cause of NO increase is not excluded. To our knowledge, the involvement of NO has not yet been demonstrated after low power (i.e., nonthermal) EMF exposure. Furthermore, well-known actors of plant responses to environmental stimuli are also involved: the tomato mutants sitiens and JL-5 for abscisic (ABA) or jasmonic (JA) acids biosynthesis, respectively, display normal responses (accumulation of stress-related transcripts) when whole plants are exposed to EMF . In contrast, very rapid distant responses to local exposure that occur in the wild plants (Figure 4(a)) are impaired in sitiens ABA mutant (Figure 4(b)) and JL-5 mutants, highlighting the existence of a transmitted signal (whose genesis and/or transmission is dependent on ABA and JA) in the whole plant after local exposure . The nature of this signal is still unknown, but very recent work has demonstrated that membrane potential is affected after exposure to EMF . It could therefore be hypothesized that electrical signals (action potential and/or variation potential) could be the transmitted signal, strongly implying that HF-EMF is a genuine environmental factor.
test. Reproduced from , with permission.
3.1.1. Alterations of Enzymatic Activities
Table 1 summarizes some of the enzymatic activities that are modified after exposing plants to HF-EMF. As previously noted, ROS metabolism is very often activated after plant exposure to EMF. Enzymatic activities such as peroxidase, catalase, superoxide dismutase, and ascorbate peroxidase have twofold to fourfold increase [4, 7, 18, 27, 33]. The question remains open to determine if this could be the consequence of a direct action of EMF on living tissue. Indeed, the very low energy that is associated with the EMF at these frequencies makes them nonionizing radiations. Side effects of elevated ROS metabolism are also noted: H2O2 production [4, 7], MDA increases [4, 7, 33], and protein damage . An increase in polyphenol oxidase  and phenylalanine ammonia-lyase  may indicate stress responses linked to an increased lignification, a common response of plants to environmental stress.
Protein content is reduced in Vigna and Phaseolus [27, 32] as well as in Triticum . It is not yet known if the decrease in protein content results from an increase in protein degradation and/or a decrease in protein synthesis, but this may constitute a stimulating field of investigation, since evidence shows that mRNA selection from translation occurs after plant exposure to HF-EMF . Hydrolytic enzymatic activities (α- and β-amylases and invertases) responsible for the production of soluble sugar increase in germinating seeds after exposure to HF-EMF [12, 28, 32], while the starch phosphorylase activity, phosphorolytic and potentially reversible, is diminished. In contrast, HF-EMF exposure causes a drop of soluble sugar that may be related to the inhibition of Krebs cycle and pentose phosphate pathway in Plectranthus (Lamiaceae) leaves after exposure to 900 MHz EMF , suggesting that seeds and adult leaves respond in a different way to HF-EMF exposure. The accumulation of proline, reported by several authors [7, 33], and an increase in terpenoid emission and content in aromatic plants  are also classical responses of plants to environmental stresses.
3.1.2. Modification of Gene Expression
While numerous reports focused on enzymatic activities alterations after exposure to EMF, only a few studies concentrate on gene expression modifications (Table 2). Tafforeau et al.  demonstrated using Gunn generator (105 GHz) several reproducible variations in 2D gel electrophoresis profiles, showing that gene expression is likely to be altered by the exposure treatment. Jangid et al.  provided indirect proof (RAPD profiles) suggesting that high power microwave irradiation (2450 MHz, 800 W cm −2 ) modifies gene expression in Vigna aconitifolia, while these results do not exclude a possible thermal effect of microwave treatment. Arabidopsis thaliana suspension-cultured cells exposed to HF-EMF (1.9 GHz, 8 mW cm −2 ) showed differential expression of several genes (
values < 0.05) compared to the control (unexposed) condition in microarray analysis . Most of them are downregulated (while At4g39675, At5g10040, and AtCg00120 displayed a slight increase see Table 2). However, the RT-PCR value lowers the significance of these variations and these authors consequently concluded the absence of HF-EMF effect on plant gene expression. In contrast, short duration, high frequency, low amplitude EMF exposure (10 min, 900 MHz, 5 V m −1 ) performed on whole 3-week-old tomatoes in MSRC [5, 6, 8, 10] demonstrated altered expressions of at least 5 stress-related genes (Table 2), suggesting that whole plants are more sensitive to HF-EMF than cultured cells. These experiments have been independently replicated by Rammal et al. , using a longer exposure period and a far less sophisticated exposure set-up (cell phone). Stress responses of plants quite often display a biphasic pattern : a very rapid increase in transcript accumulation that lasts 15–30 min, followed by a brief return to basal level, and then a second increase (after 60 min). This pattern was observed after tomato exposure to EMF so we questioned the meaning of the early and late population of transcripts in terms of physiological significance by measuring their association to polysomes (which reflects their putative translation to proteins). We found that the early (0–15 min) mRNA population was only faintly associated with polysomes, yet being poorly translated, while the late mRNA population (60 min) is highly associated with polysomes . This result strongly suggests that only the late mRNA population may have a physiological importance since it is the only one to be efficiently translated into proteins.
3.2. Whole Plant Level
The biochemical and molecular modifications observed after plant exposure to EMF and described in the previous paragraphs might induce morphogenetic alterations of plant development. Indeed, an increasing number of studies report modifications of plant growth after exposure to HF-EMF (Table 3). These treatments are effective at different stages of plant development (seeds, seedlings, or whole plants) and may affect different organs or developmental processes including seeds germination and stem and root growth, indicating that biological samples of even small sizes (a few mm) are able to perceive HF-EMF. Seed exposure to EMF generally results in a reduced germination rate [27, 37, 39], while in other cases germination is unaffected  or even stimulated . The seedlings issued from EMF-exposed seeds displayed reduced growth of roots and/or stem [13, 28, 32, 37–39, 41] but rarely a stimulatory effect . This point strongly differs from exposure to static magnetic fields or extremely low frequency EMF, in which the stimulatory effects on growth are largely predominant . Ultrashort pulsed high power EMF (PEMF, 4 μs, 9.3 GHz, 320 kV m −1 ) also tends to stimulate germination of seeds of radish, carrot, and tomato and increase plant height and photosynthetic surface area in radish and tomato  and roots of tobacco seedlings . These different effects of PEMF compared to HF-EMF on plants may be related to their fundamental difference in terms of physical properties. Exposure to HF-EMF of seedlings or plants (rather than seeds) also generally resulted in growth inhibition [9, 18, 27, 28, 39]. Singh et al.  showed that rhizogenesis (root number and length) is severely affected in mung bean after exposure to cell phone radiation, possibly through the activation of several stress-related enzymes (peroxidases and polyphenol oxidases). Akbal et al.  showed that root growth was reduced by almost 60% in Lens culinaris seeds exposed in the dormant state to 1800 MHz EMF radiation. Concomitantly, these authors reported an increase in ROS-related enzymes, lipid peroxidation, and proline accumulation, with all of these responses being characteristic of plant responses to stressful conditions. Afzal and Mansoor  investigated the effect of a 72 h cell phone exposure (900 MHz) on both monocotyledonous (wheat) and dicotyledonous (mung bean) plants seeds: germination was not affected, while the seedlings of both species displayed growth inhibition, protein content reduction, and strong increase in the enzymatic activities of ROS metabolism. It is however worth noting that growth of mung bean and water convolvulus seedlings exposed at a lower frequency (425 MHz, 2 h, 1 mW) is stimulated because of higher elongation of primary root , while duckweed (Lemna minor, Araceae) growth was significantly slowed down not only by exposure at a similar frequency (400 MHz, 4 h, 23 V m −1 ) but also after exposure at 900 and 1900 MHz for different field amplitudes (23, 41, and 390 V m −1 ) at least in the first days following the exposure . Surducan et al.  also found stimulation of seedling growth in bean and maize after exposure to EMF (2.452 GHz, 0.005 mW cm −2 ). Senavirathna et al.  studied real-time impact of EMF radiation (2 GHz, 1.42 W m −2 ) on instantaneous growth in the aquatic plant, parrot’s feather (Myriophyllum aquaticum, Haloragaceae), using nanometer scale elongation rate fluctuations. These authors demonstrated that EMF-exposed plants displayed reduced fluctuation rates that lasted for several hours after the exposure, strongly suggesting that plants’ metabolism experienced a stressful situation. It is worth noting that the exposure did not cause any plant heating (as measured using sensitive thermal imaging). Some other kind of morphological changes also occurred after plant exposure to HF-EMF: induction of epidermal meristems in flax , flower bud abscission , nitrogen-fixation nodule number increase in leguminous , or delayed reduced growth of secondary axis in Rosa .
These growth reductions may be related to a lower photosynthetic potential since Răcuciu et al.  showed that exposing 12-day-old maize seedlings to 0.47 W kg −1 1 GHz EMF induces a drop in photosynthetic pigment content: the diminution was especially important in chlorophyll a, which was reduced by 80% after 7 h of exposure. Ursache et al.  showed that exposure of maize seedlings to microwave (1 mW cm −2 , 10.75 GHz) also caused a drop in chlorophyll a and b content. Similarly, Hamada  found a decrease in chlorophyll content in 14-day-old seedlings after exposing the caryopses for 75 min at 10.5 GHz. Kumar et al. showed a 13% decrease in total chlorophyll after 4 h exposure of maize seedlings to 1800 MHz (332 mW m −2 ). These modifications may be related to abnormal photosynthetic activity, which relies on many parameters, including chlorophyll and carotenoid content. Senavirathna et al.  showed that exposing duckweeds to 2–8 GHz, 45–50 V m −1 EMF induced changes in the nonphotosynthetic quenching, indicating a potential stressful condition. Three aromatic species belonging to Apiaceae family (Petroselinum crispum, Apium graveolens, and Anethum graveolens) strongly respond to global system for mobile communications radiation (GSM, 0.9 GHz, 100 mW cm −2 ) or wireless local area network (WLAN, 2.45 GHz, 70 mW cm −2 ) exposure by decreasing the net assimilation rate (over 50%) and the stomatal conductance (20–30%) .
4. Conclusion and Future Prospects
An increasing number of reports highlight biological responses of plants after exposure to HF-EMF at the molecular and the whole plant level. The exposure conditions are, however, far from being standardized and illustrate the diversity of exposure conditions employed. However, future work should avoid exposure in near-field conditions (i.e., in immediate vicinity of the emission antenna) where the field is instable and difficult to characterize. Similarly, the use of communication devices (i.e., cell phones) should be avoided as emission sources since it may be difficult to readily control the exposure conditions because of built-in automation that may overcome the experimental set-up. The use of specialized devices (TEM cells, GTEM cell, waveguides, MSRC, etc.) in which a precise control of exposure condition can be achieved is highly preferable.
Shckorbatov  recently reviewed the possible interactions mechanisms of EMF with living organisms. While the classical targets (interaction with membranes, free radicals, and intracellular regulatory systems) have all been observed in plants, a convincing interpretation of the precise mechanism of HF-EMF interaction with living material is still needed. Alternative explanation (i.e., electromagnetic resonance achieved after extremely high frequency stimulation which matches some kind of organ architecture) has also been proposed for very high frequency EMF (several dozen GHz) . However, the reality of this phenomenon in vivo (studied for now only through numerical simulations) and its formal contribution to the regulation of plant development have not yet been experimentally established. Amat et al.  proposed that light effects on plants arose not only through chromophores, but also through alternating electric fields which are induced in the medium and able to interact with polar structures through dipole transitions. The possible associated targets (ATP/ADP ratio, ATP synthesis, and Ca 2+ regulation) are also those affected by exposure to HF-EMF . It could therefore be speculated that HF-EMF may use similar mechanisms. The targeted pathways, especially Ca 2+ metabolism, are well known to modulate numerous responses of plants to environmental stress. While deeper understanding of plant responses to HF-EMF is still needed, these treatments may initiate a set of molecular responses that may affect plant resistance to environmental stresses, as already demonstrated in wheat for CaCl2  or UV  tolerances, and constitute a valuable strategy to increase plant resistance to environmental stressful conditions.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to gratefully acknowledge the LRC BIOEM (“Laboratoire de Recherche Conventionné”—LRC no. 002-2011-CEA DAM/Institut Pascal) for its financial support.
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Can radio waves hurt you?
It’s impossible not to overlook the benefits of wireless technology that heavily relies on radio waves. Technology has greatly improved the quality of life and the way we can stay in touch with our loved ones. While discussing the threats posed by radio waves and trying to answer “Can radio waves hurt you?” we should understand that there are two main categories of electromagnetic waves:
Ionizing electromagnetic waves like x-rays and gamma-rays have the potential to cause severe damage. However, it is generally only when excessive exposure occurs that damage to our DNA and cells will occur. Ionizing radio waves have the potential to cause sickness, burns, or cancer.
Non-ionizing electromagnetic waves don’t carry enough energy in their photons to ionize or break the atoms and cells.
Radio waves which are emitted by mobile phones have lower energy radiation which can’t cause damage to our cells. Infrared, visible light and microwaves also produce lower energy radiation and are entirely safe.
For more practical and visual representation of which electromagnetic waves are considered ionizing and non-ionizing, please refer to the chart below.
5 Answers 5
As I stated in my answer to the linked question, the evolutionary prerequisites for radio communication is in a species that is able to precipitate a variety of metals in a variety of forms and an environment high in metals.
Initially, a precursor species would evolve to use metal to enhance its neural transmission rates as electrical transmission is vastly faster than human nerves' sodium-gate depolarization system, in itself a highly advantageous strategy in evolutionary terms.
It is likely that creatures using metal as a nerve conduction rate booster would have found that unshielded nerves would cause radiation detectable not only within a creature's own body, but in other creatures too. As faster nerve conduction is too great an advantage to give up, shielding would have evolved, quite possibly by running nerves through the centres of metal bones, or perhaps by sheathing the individual neurons in metal.
However, the possibilities of transmission and detection of EM radiation means that not all metal nerves would have evolved to be completely shielded, some could be partially shielded and be used to detect EM radiation.
So, we have evolved creatures that emit RF energy as a by-product of their neural activity. From there, once shielding has evolved to reduce cross-talk between nerve fibres, detection of RF leakage requires more sensitive receiver organs. Along with this, any deliberately unshielded neurons would emit RF energy detectable at greater range.
As there is almost always an advantage in being able to communicate at longer distances, the evolution of a stacked pile of depolarising cells (as occurs in electric eels) allows higher transmitter voltages, and hence higher power and range.
We then get to the point of bandwidth. EM radiation emission will most likely begin at lower radio frequencies, but it is entirely possible that mechanisms could evolve to increase the frequency of emitted radiation. Since a system of this type could have practically each neuron driving an EM transmitter of a different frequency, high bandwidth can be achieved by rapid changes in signal amplitude and frequency that is allowed by using high-frequency EM radiation, and also by multiplexing - using many frequencies simultaneously. This could ultimately allow an evolved bandwidth many times greater than our own Wi-Fi communication, which could also be somewhat directional. Another argument for higher radio frequencies and microwaves is that smaller antennas are required.
Since all this bandwidth is relatively easily achieved in evolutionary terms - simply by duplicating the relevant organs - there is no reason why the beings would not evolve to make use of this bandwidth. Since the highest intelligences of species on earth are found in those creatures with an active social life (and this ability makes for a great social life), the evolution of intelligence is pretty much a given.
Considering that a sentient, tool-using species that can communicate via RF at what are probably high bandwidths, it is unlikely that humans could easily develop an interpreter for this alien language, especially given that it would most likely be multiplexed, and both frequency- and amplitude-modulated, as well as rapid and idiosyncratic rather than following any simple grammar as in human-manufactured RF communication. It is far more likely that these creatures - should it occur to them that audio is being used to communicate ideas, a not-unlikely proposition given their inherent ability to share processing - would learn to understand and communicate with humans using human language, given its likely lower bandwidth and complexity.
Of course, since we're talking about evolution, an evolutionary feature such as metal-enhanced neurology would have to occur at a very early point in the species' evolutionary history. This means that - thanks to evolutionary divergence - there would most likely be a great number of species on this world which emit RF energy to a greater or lesser extent.
We can anticipate that in the groups of creatures with unshielded neurology, the 'noisiness' of their neurons would be a beacon to the RF senses of predators, particularly those who have shielded neurons themselves, and thus have a lower background noise over which they can 'hear' their prey. From this, we can anticipate that these may be easy prey to such predators, and would hence be prone to adopting an r-strategy.
Other species would have evolved to communicate via RF to a greater or lesser extent we can anticipate a wide variety of such creatures occupying multiple niches, though as the communication range of RF is such that it can be anticipated that many would be at least a bit smarter than a terrestrial-equivalent species mainly due to the greater opportunities for social interaction.
As to the environment, there is practically a necessity for more metals to be accessible. This does not preclude an oxygen atmosphere, but there may be levels of atmospheric dust containing heavy metals that would have toxic effects on humans not protected by respiratory filters or who eat the local life forms. This would make face mask filters highly advisable rather than essential, and we could have a human living for quite some time without one before they might start to experience symptoms of heavy metal poisoning.
A potential reason for humans to be interested in such a world is that with the biological precipitation of metals, mining metals would be an almost trivial exercise of picking up the carcasses of dead creatures, whether recently dead or fossilised. Some very interesting alloys are likely to have evolved, as is foam-metal which is both light and strong due to its internal voids.