not so welcome at the ground level, where people can breathe it in. Ground-level ozone is commonly called smog and is the primary component of the orange-hued haze that one sometimes sees when flying into a big city on a hot summer day. The original ozone target in the first Clean Air Act was a concentration of 120 parts per billion (ppb) in the air. In other words, if cities controlled air pollution well enough to reduce ozone in the ambient air to 120 ppb or less, then the EPA would define the air as clean – at least with respect to smog.
Around the mid-1990s, it became obvious that just about every metropolitan area, except those located in Southern California, was going to meet the 120 ppb standard. (Southern California is a special and virtually unsolvable case because of the region’s geography and weather.) What to do? The Sierra Club and the American Lung Association, along with other environmental groups, pushed for a new lower ozone standard, 80 ppb, and the EPA under the Clinton administration agreed. (That “averaging period” was also changed during the Clinton administration. Ozone concentrations used to be averaged over a one-hour period, but following this change, they have been averaged over an eight-hour period.) More emissions reductions were made, and it became clear that this standard would be met as well. So in 2008 the EPA under the Bush administration lowered the standard again, this time to 75 ppb.
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