Ozone, a major pollutant in urban smog, can worsen chronic lung problems and lower resistance to respiratory diseases. Children, older people, and those with heart or lung problems are most sensitive to smog. But even healthy adults engaged in moderately strenuous outdoor activities can experience throat irritation and breathing discomfort. The Environmental Protection Agency and state and local governments have launched a free e-mail and pager service to warn you about bad air days. Sign up or check local conditions at www.airnow.gov.