Observation of vegetation injury induced by exposure to air pollutants provides a simple and low cost means of monitoring pollutant emissions from a particular source as well as for determining the distribution of pollutants over a wide area. This use of plants has long been accepted and has been extensively reported (2, 7, 8, 14). However, many other environmental factors may also cause plant injury and their effects are often difficult to distinguish from pollution-induced damage (13). In addition, factors such as temperature, humidity, wind velocity, light, nutrition, water supply, and the presence of pesticides and other chemicals have all been shown to markedly influence plant sensitivity to air pollutants.