When we think about the health impacts of air pollution, it’s important to know which pollutants are in the air we breathe. Fortunately, the U.S. has a large network of air monitoring instruments designed to measure this “nose-level” pollution, from ozone to particulates. In fact, the U.S. has made excellent progress toward clean air in the past four decades thanks to these instruments. Using historic data only goes so far, since many land areas do not have monitors and there aren’t any over lakes, oceans or beyond the troposphere, the lowest layer of the atmosphere.