1.   A state study identified the pollutants as the leading environmental cause of premature death.

2.   Open-ended mandates in the Clean Air Act require the Environmental Protection Agency to identify hazardous pollutants and then set standards that eliminate health effects.

3.   Scientists have used chemical tracers to track and identify pollutants drifting hundreds of miles from cities and power plants to the Canyon.

4.   With most flowers or fruits, the fragrance hunters used a technique originally designed to trap and identify air pollutants.