Publisher Summary The natural sources of many common air contaminants make a contribution to the overall atmospheric pollutant loading. Volatile organic compounds are contributed to the atmosphere by many forms of plant life. Deserts contribute significant masses of dust and particulate matter to the atmosphere. Sulfur dioxide and hydrogen sulfide from volcanic activity also contribute pollutants to the atmosphere. Biological contaminants in the atmosphere may not represent a large mass but are an important component of atmospheric pollution because of their potent effect. Air pollutants can be classified into three main categories based on their physical characteristics: coarse particulate matter (PM), aerosol class, and gases. A wide variety of wet chemical, instrumental, and biological procedures exist for the analysis of gaseous air pollutants. Instrumental analysis of air samples can be frequently conducted directly on the air or gas sample; ultraviolet and infrared spectrophotometric methods are commonly used for that purpose. High concentrations of air pollutants are known to kill many annual plants and trees, while less severe exposures can cause premature senescence of sensitive species of trees and shrubs. These two conditions can be used as a coarse biological indicator. One logical approach that may be used to organize the consideration of air pollution problems and solutions is to start from the individual level of exposure with treatment of indoor air pollution and then move to the consideration of larger scale macroscopic effects such as smog, acid rain, arctic haze, climate effects, and damage to the stratospheric ozone layer.