1. An extended period of a high concentration of pollutants in the atmosphere. 2. A public alert or notification of unhealthful air quality. In the U. S. , the degree of alert is often based on the pollutant standards index (PSI):

• Stage 1 episode or alert, for 200 ≤ PSI < 275. Public recommendation: avoid strenuous activities. Children and elderly people advised to stay indoors. * Stage 2 episode or alert, for 275 ≤ PSI < 400. Public recommendation: cease all physical activity, and stay indoors. Industry can be ordered to reduce emissions, and employees forced to carpool or use mass transportation for their necessary travel. * Stage 3 episode or alert, for PSI ≥ 400. Everyone ordered to remain at home with windows closed. Minimize physical exertion and avoid driving vehicles. Automobile travel could be restricted and industries ordered to reduce emissions.