Why the Sun can harm you and WiFi can’t (and how microwave ovens cook your food)
As we roll into Summer, we prepare for long days outdoors: at the beach, on the golf course, or just having a cold drink and reading a book on the backyard deck. On those Sun-drenched days, we also remember to be careful about extended exposure to the Sun, lest we suffer the immediate consequences of a nasty sunburn and increase the risk of long-term effects such as premature skin aging and even skin cancer.
It should be routine to apply sunblock or sunscreen before heading outside, and to reapply it every few hours during continued exposure. This advice is quite commonplace, although there remains some debate over whether particular ingredients in sunscreen and sunblock may actually cause more harm than good (or, to put it more alarmingly, whether they are “destroying your health“). It is certainly important to note that sunscreens are not created equal, and some can have nasty side effects – as one example, the zinc oxide in some sunscreens can cause a chemical reaction, catalysed by the very UV light that it blocks from the skin, that might damage cells near the surface of the skin. Some people remain skeptical that sun protection is even necessary. Although I will not discuss how sunscreen and sunblock actually works in this article, I find this discussion fascinating, especially when considering other discussions in past years about the supposed dangers of WiFi routers because of its use of electromagnetic radiation. It is sometimes recognized that sunlight and WiFi radiation are both forms of electromagnetic radiation, but there is a connection between the two that is rarely ever made. In fact, by understanding why exposure to the ultraviolet rays of the Sun can harm our skin, we can also understand why WiFi radiation is not harmful.
Understanding electromagnetic radiation
The light emitted from the Sun is a form of electromagnetic radiation – particles that oscillate at a certain speed (just under 300,000 km/s, also known as the “speed of light”), causing both an electric field and a magnetic field. Other forms of electromagnetic radiation includes light from lightbulbs or candles, X-rays, microwaves (which includes WiFi) and radio waves.
When electromagnetic radiation acts as a particle that carries energy, it is known as a photon. The quantity of energy in a photon is directly proportional to the frequency of the radiation (the number of oscillations per second), and can be calculated with the equation E = hv, the product of the frequency v by the Planck constant h (an extremely tiny value). The frequency v and wavelength lambda of a photon are related through the speed of light c by the equation v = c/(lambda). This means that the quantity of energy in a photon is also inversely proportional to its wavelength (as the wavelength gets longer, the energy decreases); its energy can be calculated through the equation E = hc/(lambda). It is convenient to express energy with either equation, as sometimes we describe radiation by its frequency, such as with radio stations (680 kHz or 97.3 MHz, for example), and sometimes we describe it by its wavelength, such as with categories of light (ultraviolet light in the 100-400 nm range, for example).
The energy of photons is usually expressed either in units of electron volts (eV) or kilojoules per mole of photons (kJ/mol), with 1 eV equivalent to 96.485 kJ/mol. (One mole of photons is 6.02 x 1023 photons, just like one dozen photons is 12 photons.) To obtain the energy in eV, the value of the Planck constant h to be used in the equations above is 4.136 x 10-15 eV/Hz.
When a photon strikes matter with sufficient energy, there can be excitation and promotion of electrons within the compound, putting the compound in an excited state. Every electron in an atom orbits around the nucleus at its own specific energy level; when just the right amount of energy is added to it, the highest-level electron uses absorbs this energy to move to a higher level, hence its “promotion”. The excited state is temporary; when that electron returns to its normal level, the compound returns to the ground state, and emits that surplus energy in the form of s electromagnetic radiation. A practical example of this effect is with sodium-vapour lamps, used in many cities for street lighting. The sodium vapour is excited by an electrical source, bringing the sodium into an excited state. It returns to the ground state by two different paths, emitting photons of slightly different energies, equivalent to wavelengths of 589.0 nm and 589.6 nm – both visible to us as yellow light.
How sunlight can lead to a sunburn
The light emitted by the Sun consists of radiation that encompasses the ultraviolet (UV, 250-400 nm), visible (400-700 nm) and infrared (IR, 700-2500 nm) regions. Some of this radiation never reaches the Earth’s surface. We are familiar with ozone (O3) in the atmosphere absorbing some of the UV light, dissociating the molecule into oxygen gas (O2) and an oxygen atom (O) that can react with another O2 molecule to reform ozone. The water and oxygen in the atmosphere also absorbs some portions of the infrared light. The IR and visible light that strikes our skin heats it a bit. The UV light, however, can have a serious effect on our skin, due to the energy carried by these photons.
UV light is classed by ISO 21348 into three categories: UVA, UVB and UVC. UVC has the shortest wavelength (100-280 nm) and is therefore the most energetic photons (4.43-12.4 eV); however, this radiation is absorbed by the ozone layer and other particles in the air. UVB is the middle classification (280-315 nm, 3.94-4.43 eV) and UVA encompasses the longest wavelengths (315-400 nm, 3.10-3.94 eV). While UVA is the weakest of the three groups in terms of photon energy, it accounts for up to 95% of the total UV sunlight that penetrates through the atmosphere and onto your skin. Photons from visible and IR light, with wavelengths longer than 400 nm, will carry less than 3.10 eV (300 kJ/mol) of energy.
Going back to a previous point – when a photon with sufficient energy strikes an atom with just the right energy, an electron can be promoted and bring the compound into an excited state. The figure on the right shows how a photon (green) is absorbed by an electron in ground state (yellow), then emitted when the excited electron (red) returns to the ground state. At a certain energy threshold, the electron is promoted to the point where it is completely removed from the atom, causing ionization. Considering that a chemical bond is two atoms sharing a pair of electrons, sufficient energy given to a bond can lead to cleavage of the bond to form two radicals, each holding on to one electron from the now-defunct bond. The energy needed to cause this cleavage is known as the bond-dissocation energy; this table gives approximate values for common bonds, although the actual values for each bond in a particular compound will vary by a few percent due to the atoms surrounding it. Most single bonds of carbon require 3.6-5.0 eV (350-480 kJ/mol) of energy to dissociate – just the amount offered by UV light. Visible light does not give enough energy to dissociate most bonds, and any radiation of infrared or longer wavelength has far too little energy to cause bond dissociation.
Until recently, it was not known what a sunburn really was, only that it wasn’t a thermal burn, like placing your hand on a hot stovetop. A 2012 article by Richard Gallo in Nature Medicine finally described the mechanism by which sunburn occurs. As described in this press release from UC-San Diego:
Using both human skin cells and a mouse model, Gallo, first author Jamie J. Bernard, a post-doctoral researcher, and colleagues found that UVB radiation fractures and tangles elements of non-coding micro-RNA – a special type of RNA inside the cell that does not directly make proteins.
The photon energy in UVB radiation is quite sufficient to break molecular bonds within RNA – and, as the quote continues, nearby cells spring into action to deal with the problem:
Irradiated cells release this altered RNA, provoking healthy, neighboring cells to start a process that results in an inflammatory response intended to remove sun-damaged cells.
Before burning, skin that is exposed to a bit of UV light will get darker – what is known as a suntan. Exposure to the UVA radiation causes production of melanin, a natural pigment which is then capable of dissipating up to 99.9% of UV radiation, as described in a 2004 paper by Meredith and Riesz. Darker-skinned people already have high melanin content in their skin, so it goes to work immediately in protecting the skin from damage caused by UV exposure – this is why darker skin is much less susceptible to sunburns. Lighter-skinned people, while building up some protection during initial Sun exposure, cannot produce melanin quickly enough to dissipate all of the UV light, and their skin does eventually burn.
While exposure to UV radiation can lead to skin damage, it is also the mechanism by which our skin produces vitamin D. Absorption of light in the 290-320 nm region (particularly at 295-300 nm) provides just the right energy to launch a two-step reaction where 7-dehydrocholesterol becomes cholecalciferol, better known as vitamin D3. (The vitamin D produced in our skin is a slightly different compound from the vitamin D consumed in supplements – that compound is ergocalciferol, or vitamin D2.)
Why other forms of electromagnetic radiation can or can’t affect us
As we understand what ionizing radiation does to atoms and molecules, we can appreciate why certain forms of radiation are harmful to us. X-rays, for example, are in the wavelength region of 0.01 to 10 nm, and carry photon energies in the order of 100 to 100,000 eV. X-rays are energetic enough to penetrate into most objects, which is great for scanning bags at airports and taking images of the inside of our bodies. However, as described above, the interaction of photons of such energy with our cells can cause significant damage, and prolonged exposure causes an increased risk of cancer. Consider the precautions in place when taking an X-ray: the patient wears a lead coat and the technician is in a different room to push the button.
Meanwhile, radio waves are an example of non-ionizing radiation, as the energy in that radiation is too small to cause an atom to lose an electron. Most radio stations broadcast on FM radio (88-108 MHz, or 360-450 neV) or AM radio (530-1710 kHz, or 2.2-7.0 neV), with photon energies that are on the order of a millionth to a billionth of the energy in UV light. (Note that 1,000,000,000 neV = 1 eV.) There has been no evidence over the past century of adverse health effects from radiation exposure when listening to too much radio or living too close to a radio transmission tower.
If a photon does not enough energy to cause bond cleavage or promotion of electrons, it can still have other effects on molecules. Infrared light (700 nm to 1 mm, or 1,000,000 nm; 0.0013-1.77 eV) can cause increased rate of vibration in certain chemical bonds; this increased vibration generates some heat. Saunas and cooking are two areas where infrared heating is used.
Microwave ovens cook your food, WiFi routers won’t cook anything
Microwave ovens operate on a similar principle, even though microwave radiation is even less energetic than infrared radiation. The radiation emitted by the oven source has an approximate frequency of 2.45 GHz, or a wavelength of 122 mm (0.00001 eV). When that radiation reaches the food, the oscillating magnetic field causes polar molecules such as water to sway up and down. The image on the right illustrates why water is polar – the hydrogen atoms are somewhat positively charged, and the oxygen atom is somewhat negatively charged, due to the oxygen atom pulling more strongly on the pair of electrons in each hydrogen-oxygen bond. The oscillation causes water molecules to collide into neighbouring molecules, increasing kinetic energy and, more importantly, generating heat – which eventually cooks the food. This mechanism makes microwave ovens great for defrosting meat, as the layer of ice can be heated and melt away while starting to cook the meat. This also explains why some foods cooked in the microwave oven taste dry and have odd textures – the water and the natural juices are evaporated out of the food. The radiation does not modify the molecular structure of the irradiated compounds, and will not harm the DNA of the proteins. The heat generated may be enough to denature the proteins, but that can also happen when cooking in a standard convection oven.
Oh, but wait a minute – most WiFi routers also emit radiation at approximately 2.4 GHz. Wouldn’t my WiFi router be able to cook food as well – and perhaps, “cook” some things in my body? In theory, it could – if the router emitted thousands of times more power than it does, and didn’t have an antenna broadcasting in all directions. In practice, a glass of water set next to a WiFi antenna will barely even warm up, and the heat it acquires will come from the electric circuitry, just like your computer becomes hot when in prolonged use. The power of the radiation emitted by the microwave oven is on the order of 500-2000 Watts, and that radiation is focussed within a 1-2 cubic foot space inside the oven. WiFi routers emit less than 1 Watt of energy (and regulations in Canada prohibit any router to emit more than 4 Watts), and that energy is dissipated in all directions.
WiFi is safe to use under current regulations. We know it is safe because we understand the mechanisms by which electromagnetic radiation causes harm to our bodies. Sunburns and the consequent skin damage results from exposure to UV radiation that is capable of breaking chemical bonds, causing DNA damage. Radiation from WiFi routers is much less powerful, and each photon carries approximately 2-millionths of the energy in photons of UV radiation, so it cannot break chemical bonds – the best it can do is slightly increase the rate of vibrations within molecules. It will not kill plant life, as an experiment that received worldwide media attention in May would lead you to believe. Yes, microwave ovens use the same radiation as WiFi routers to cook foods, but its radiated power is a thousand times larger and confined in a narrow box. In other words, a few thousand WiFi routers pointed directly at a piece of chicken might eventually cook it, although in that case, the heat generated by the overloaded electrical outlets and the electric circuitry within the boxes will probably be even more useful in preparing your dinner.