How can I learn about my local air?

The Environmental Protection Agency (EPA) and local governments monitor thousands of sites nationwide. The results are given an air-quality-index (AQI) rating between 0 and 500. A reading of more than 100 means the air is unhealthy for children, older people, and anyone with heart or lung problems. A reading of more than 150 means the air is unhealthy for everyone. You can get daily readings and forecasts from local newspapers and news broadcasts or at www.airnow.gov, which has an e-mail and cell-phone alert service for some areas.

What creates outdoor air pollution?

Ozone and particulate matter (PM), commonly called soot, are the main culprits — the air-quality index is based on their concentrations. Ground-level ozone (as opposed to the protective stuff in the stratosphere) is the brown haze that blurs the horizon. It forms when automobile exhaust and fumes from chemical solvents and gasoline react with heat, sunlight, and stagnant air. More than 140 million Americans — 47 percent of the population — live in areas with unhealthy ozone levels, which peak in the late afternoon and early evening during the summer months. (See U.S. City Air Quality Ratings.)

Ozone can irritate your airways, leading to symptoms such as coughing, wheezing, shortness of breath, tightness in the chest, and pain when taking deep breaths, says Susan Stone, an environmental health scientist with the EPA. In the long run, exposure to ozone can lead to more severe asthma, decreased lung function, and permanent lung damage. Ozone can even be fatal: EPA-funded research found that when ozone levels go up, so does the number of deaths from heart and lung disease and other natural causes in the days that follow. Many experts say the EPA’s ozone standards (which were under review at press time) are too lax. A major study of the air in 95 cities, published in the Journal of the American Medical Association in 2004, showed that ozone levels considered acceptable by the EPA are linked to thousands of premature deaths a year.

Particulate matter is a cocktail of tiny pieces of dust, chemicals, and gases suspended in droplets of liquid. More than 64 million people — over 20 percent of the U.S. population — in areas where PM sometimes exceeds EPA standards. PM is even more dangerous than ozone: Over time, exposure to high levels of the pollutant can lead to lung disease and cancer.