Also, because UV is not penetrating radiation, it only harms your skin and your eyes, it cannot get into your solid organs or bone marrow. Once you get down to regular ultraviolet (~390-100nm, 3.2-12 eV) you are no longer capable of creating ions, but you can still create chemical lesions like pyrimidine dimers. But I'm not sure what you'd die of first, the radiation poisoning (which is delayed by a few hours from the radiation dose) or suffocation as the Earth's atmosphere is ionized and blown into space. So if the Sun decided to emit 100% X-rays, you'd absorb a fatal dose of 315J in just 0.35 seconds. Multiply this by 1.73 m 2 for an average human's body surface area, divide by 2 for directionality = ~900 W/human. The same amount of energy applied as heat would increase the temperature of your body by 0.001 degrees C.ĭirect sunlight at sea level has an irradiance of ~1,050 W/m 2. The lethal dose of instantaneous whole-body irradiation is around 4.5 Gy in humans, which comes out to 315 Joules of energy for a 70kg person. It doesn't take much ionizing radiation to cause a major biological effect. (These are not ionization lesions, they're photon-induced chemical reactions) At higher energies you get more double-strand breaks, which cause far more cellular lethality. At lower energies you'll get a preponderance of DNA base damage, like pyrimidine dimers. Ionizing radiation damages DNA in a variety of ways. There is no extreme-UV in sunlight because the Earth's atmosphere strongly absorbs it. This includes the extreme-ultraviolet (100-10nm, 12-124 eV) and X-ray/gamma spectrum (124eV). Ionizing radiation includes any photon above the ~12.6eV required to ionize water. Let's start at the very highest energies. It's all based on physiology and biology. (Advanced Note: If there is an intermediate level between the 10 eV and 5 eV states, it actually is possible, but dramatically less likely, to have a two-photon event where it basically hops up in two goes.) It takes generally ~10 eV (a unit of energy) to start stripping electrons, or reconfiguring molecular bonds like in our DNA and, because of the way quantum mechanics works, you can shoot 5 eV photons at such a system until you're blue in the face and you won't get that to happen. This is why we called UV and higher "ionizing radiation" and lower energy per photon light "non-ionizing". Want to strip an electron from an atom entirely or fundamentally reconfigure the chemical bonds of a molecule in a permanent and destructive manner? You need a UV or higher photon. Continuing, want to send the electron in an atom from one energy state to another in a non-destructive manner (they'll just relax back)? Need a visible-light photon or higher. However, crucially, you cannot "make do" with, say, two radio photons, even if the sum of their energies is the same as the IR photon. Want to vibrate the bonds of an atom or molecule? You need an IR photon or higher. Want to rotate an atom or molecule? You need a radio or microwave photon or higher. The electromagnetic spectrum basically encapsulates "energy per photon" with radio having the least, then microwave, then IR, then the visible, then UV, then X-ray, then gamma. What a system can do with the energy of light is based on energy per photon. But, the broadest and simplest way to think of it is that high frequency radiation is like a stream of little damaging bullets, and low frequency radiation is like a gentle ocean wave. Of course, there is everything in the middle too, and the interactions between matter and electromagnetic radiation are more complex than I'm implying here. Rather than smashing into an atom, it's more likely to gently swash electrons back and forth (which is how an antenna works). It has a small amount of energy spread out over a larger area. On the other hand, a low-energy/long-wavelength/low-frequency photon like a radio signal is more like a gentle wave. This can be quite harmful, because it can change the chemistry of your body, and your DNA in particular. It smashes into atoms and knocks them apart. It has a lot of punch packed into a small space. To oversimplify a bit, a high-energy/low-wavelength/high-frequency photon like a gamma ray is a bit like a bullet. Photons of different energies/wavelengths/frequencies will interact with matter differently. Electromagnetic radiation is a spectrum that ranges from long-wavelength, low-frequency, low-energy photons up to short-wavelength, high-frequency, high-energy photons.
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