The toxic air pollutants of greatest concern are those that cause serious health problems or affect many people. Some health problems occur very soon after a person inhales a toxic air pollutant. These immediate effects may be minor, such as watery eyes or coughing. Or they may be serious, such as life-threatening. Other health problems may not appear until many months or years after a person's first exposure to the toxic air pollutant, or only after prolonged or repeated exposure.
Cancer is one example of a delayed health problem.