Dr. Pryor and colleagues at Louisiana State University developed methods for measuring ozone reaction products in in vitro models of lung lining fluids exposed to ozone and in lung fluids from rats exposed to ozone. During the study, Dr. Mark Frampton of the University of Rochester provided Pryor with lung fluids from humans exposed to air or ozone under controlled conditions. Frampton and colleagues exposed exercising smokers and nonsmokers to filtered air or to 0.22 parts per million (ppm) ozone for four hours. They obtained lung fluid samples from the subjects either immediately after or 18 hours after exposure ended. Pryor and collaborators analyzed the samples for two aldehydes, nonanal and hexanal. These aldehydes are formed by ozone reacting with unsaturated fatty acids found in the lung lining fluid and cell membranes.