Before 1978, lead was a common ingredient in paint because it added strength, shine, and extended the life of the paint. In 1978, the United States banned the use of lead pigments in paints used on interior & exterior residential surfaces. 

The presence of old lead-based paint in housing represents the most significant hazard remaining for lead poisoning, particularly for young children. The most common means of exposure is young children eating peeled and flaked pieces of paint, which is a significant problem in inner-city, lower-income areas where housing may be older and poorly maintained. Ingestion of peeling or flaking paint (or dust) remains a significant problem for any child where lead-based paint may be present. 

Lead paint regulations were not issued until 1977 and any paint manufactured after that date should not contain harmful amounts of lead. However, the use of lead paint was not prohibited until 1980. Buildings constructed or renovated between 1940-1980 have a much higher probability of having lead paint than those constructed @ later dates. Look for chipped or peeling paint found anywhere on the inside or outside of a building. If the building was constructed before 1980 lead paint should be suspected. 

Source: Environmental Assessment Assoc.