To help urban, suburban, and rural tree owners know about air pollution's effects on trees and their tolerance and intolerance to pollutants, the USDA Forest Service has prepared this booklet. It answers the following questions about atmospheric pollution: Where does it come from? What can it do to trees? and What can we do about it? In addition, it provides a tentative list of known responses by both hardwoods and softwoods to various airborne pollutants. Tolerant, intermediate, or sensitive levels are indicated for each species (when known) with regard to sulfur dioxide, hydrogen fluoride, ozone, oxides of nitrogen, peroxyacetyl nitrate (pan), ethylene, 2-4-D, chlorine, hydrogen choloride, and mercury vapor. An explanation is made that variations should be anticipated because of genetic and environmental factors, the type and concentration of pollutant, the tree's nearness to the pollution source and length of exposure to the pollutant, and the tree's health, stage of growth, and genetic makeup. (BL)