Ironically, ozone, which serves an essential protective function in the stratosphere, is the major culprit in tropospheric smog. In fact, surface ozone levels are used as a measure of smog. Ozone’s phytotoxicity raises particular concern with respect to trees and crops. Ozone is the smog constituent responsible for most of the respiratory system distress and eye irritation characteristic of human exposure to smog. Breathing is impaired at ozone levels approaching only about 0.1 ppm. Ozone is the “criterion” air pollutant that has been most resistant to control measures. Because of its strongly oxidizing nature, ozone attacks unsaturated bonds in fatty acid constituents of cell membranes. Other oxidants, such as PAN (Section 13.4), also contribute to the toxicity of smog, as do aldehydes produced as reactive intermediates in smog formation.