Philadelphia, Burlington and Montgomery Counties and 93 other counties are part of a new, 100,000-child study of environmental influences on health and development, the federal government said this week.

Montgomery County is one of eight "vanguard" areas where the study is likely to begin next year; other counties will follow. Researchers will recruit pregnant women and others of child-bearing age, and will track the health and environmental exposures of their children over a 21-year period.

The term environment is broadly defined to include natural and artificial factors, such as chemicals, physical surroundings and geography. The counties were selected to provide as broad a representation of U.S. children as possible.