Our atmosphere is crawling with things called “chemical scavengers.” Inside our body, they can be deadly. Out in the sky, they do very good work. Here’s why you want do chemical scavengers in the atmosphere and you don’t want them anywhere near you.

Chemical scavengers aren’t sentient, but that doesn’t keep them from being voracious. Once unleashed, these chemicals grab up every available “food source” in a particular area. We’ve pressed a few of them into service. We put oxygen scavengers (often nothing more than iron filings that rust on contact with oxygen) into boilers or other systems to suck up all available oxygen and prevent corrosion, and insert packages of oxygen scavengers in food containers to prevent food inside from spoiling or going stale.

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Not all chemical scavengers limit themselves to “eating” one specific atom. There’s one scavenger floating around in the upper atmosphere right now that feeds on a lot of things. A hydroxyl radical comprises an oxygen atom and a hydrogen atom. It’s a simple molecule, but we don’t see it much. Oh sure, it’s made quite a bit. When a single oxygen, in an excited state, hits water, it becomes H2O2.When UV light hits H2O2, it splits into hydroxyl radicals. The excitation of oxygen, the single molecules of water, and the UV light are all abundant in the upper atmosphere, so a lot of hydroxyl radicals get made there.

We don’t see hydroxyl radicals in our atmosphere much because they’re not around for long. What do they eat? Ozone, methane, and many other greenhouse gases. Hydroxyl radicals will rip hydrogen atoms right off pollutants, converting the radical to water, at which point it can team up with excited oxygen and start the process over again. These chemical scavengers are surprisingly effective scrubbers of the atmosphere, and can actually be thought of as pollutant sinks. They’re the reason we can breathe easy.

So why don’t we want more of them around? Well, we don’t lack for them. White blood cells will produce hydroxyl radicals when exposed to certain kinds of bacteria. But the eagerness with which a hydroxyl radical tears apart pollutants isn’t great when it’s unleashed on our bodies. It rip into anything it encounters. In our body, it alters lipids, carbohydrates, and our DNA. Over-production of hydroxyl radicals by blood cells will often kill all the cells in an area.

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So while we may be grateful to these scavengers for saving our planet, let’s keep them in the upper atmosphere.

Image: USDA.