This chapter demonstrates that air pollutants can have both long-term (chronic) and short-term (acute) effects on plants, which are economically significant. They also affect visual range by absorbing and scattering light, and they damage materials by chemical action. The original Clean Air Act made it a goal to remedy any existing impairment of visual range due to air pollution, but recognised that understanding of cause and effect was too limited at that time to bring in legislation. Children are particularly susceptible to poor air quality, as they show enhanced response to atmospheric contaminants, both in terms of the severity and nature of the impact. The change in scattering coefficient is expressed as the growth factor, which is the ratio of the scattering coefficient in the humid air to a reference scattering coefficient in dry air. Fundamental ozone exposure is a direct function of the time sequence of airborne concentration inhaled by the individual, both outdoors and indoors.