How does UV rays kill germs but grow plants?
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UV is not a specific “color” of light, it is a range of colors and has wavelength of about 100 to 400nm.
UV-C (~100-280nm) will not grow plants and will kill germs (and many other cells) by breaking the bonds holding together DNA. The good news is that this UV is very easy to block. The atmosphere easily absorbs almost all of the Sun’s UV-C. Fluorescent bulbs are probably the most common UV-C sources you’ll encounter. The glass of the tube would block most of the UV-C even if the phosphors didn’t capture a lot of it and convert it to visible light. Germicidal UV-C source must be designed with special glass (fused quartz) which is fairly transparent to the higher range of UV-C (the extremely shortwave UV-C is blocked by just a little air).
UV-B and UV-A are useful AND dangerous for plants and animals. The UV can be used to aid in the synthesis of necessary compounds, but it can also generate others that can damage DNA. You body can clean up some of this mess, but if too much occurs then the body may not be able to keep up. Humans produced melanin (the skin pigment that produces tans and makes brown and black people darker than white people). Melanin helps regulate the skins exposure to UV.
In short, the UV that is used to kill germs is different from the UV that is beneficial to life, and too much of something is frequently bad for you.
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