A report this week that air pollution, particularly ground-level ozone, is a more serious problem in Canada than previously thought got me to thinking about this stuff that we breathe. What is air? It’s a question we don’t ask very often, because we generally take air for granted.
Air is the mixture of gases comprising the atmosphere. Exactly what that mixture is, however, is harder to pin down, because while the concentration of some of the gases is nearly constant everywhere, others change from place to place and from time to time.
Most people, when they think of air, think of oxygen, which from our point of view is certainly the most vital component. It sometimes surprises people to find out that oxygen only makes up 21 percent of our air. There’s actually a lot more oxygen in the ground and water (46.6 percent of the crust is oxygen, and a whopping 85.7 percent of seawater).
Almost all the free oxygen in the atmosphere comes from plants, which breathe in carbon dioxide and breathe out oxygen like we breathe in oxygen and breathe out carbon dioxide.
Mostly, air is nitrogen: 78 percent. Though we seldom think about it, nitrogen is just as vital to life as oxygen–it’s an essential component of nucleic acids and proteins. While we can’t make use of the nitrogen in the atmosphere, certain microbes can, transforming it into a form useable by plants (and thus by us), a process called nitrogen fixing.
The next-most-plentiful element of air is argon, at just under one percent. Argon is an inert gas, colourless, odourless and tasteless (like all the other gases normally present in our air). It’s mostly produced by the slow decay of the naturally radioactive isotope potassium 40. (This slow decay, which happens at a known, constant rate, gives us a powerful tool for dating rocks.)
The rest of the gases in air occur in very small quantities. Neon, at .0018 percent, is best known for emitting a bright-orange light when you run an electrical current through it. Helium, at .000524 percent, is also generated by radioactive decay. There’d be a lot more of it if it weren’t so light that most of it escapes into space.
Methane, at .0002 percent, is produced by the bacterial decay of vegetable matter under water. It’s the major component of natural gas. (And yes, it’s colorless and odorless too, though we don’t think of it that way because the decay that produces it also produces plenty of unrelated odors.)
Krypton, at .000114 percent, has nothing to do with Superman. It’s another inert gas, three times heavier than air, which is used in fluorescent lamps and flash lamps.
Hydrogen, the lightest gas of all, is next in concentration at .00005 percent. Although hydrogen is only the ninth most abundant element on Earth (11 percent of the mass of all water is hydrogen), it makes up 75 percent of the universe as a whole, and is present in all living tissues. Like helium, it, too constantly leaks off into space.
Finally, there’s nitrous oxide, “laughing gas,” matching hydrogen at .00005 percent, and xenon, a very heavy, extremely rare gas used in strobe lights and to activate lasers, at a mere .0000087 percent.
Air’s basic uniformity is insured by the constant mixing of the wind, but there are other things in the air that vary from place to place. Water vapour can range from zero to seven percent of the total, carbon dioxide from 0.01 to 0.1 percent, ozone from zero to 0.01 percent, sulfur dioxide from zero to .0001 percent, and nitrogen dioxide from zero to 0.000002 percent.
Water and carbon dioxide are important parts of the atmosphere because of their role in maintaining life: not only are they used directly by living things, they are also primarily responsible for the “greenhouse effect” that keeps the Earth’s surface from freezing over. However, as we’re beginning to discover, it is possible to have too much of a good thing.
Ozone is present primarily 10 to 50 kilometres above the Earth, where it blocks ultraviolet radiation, at least until it is eaten by man-made chlorofluorocarbons. However, it is also found at ground level, primarily due to human activities, and, as this week’s report highlighted, can have unpleasant effects on lungs (among other things).
Sulfur dioxide (produced by burning coal) is a major ingredient in acid rain, and nitrogen dioxide (produced by automobiles) is simply poisonous.
In other words, maybe it’s time we stopped taking our air for granted.
