Although lead poisoning does not often exhibit obvious symptoms, blood levels as low as 10 micrograms per deciliter (mg/dl) have been associated with learning disabilities, growth impairment, permanent hearing and visual impairment, and other damage to the brain and nervous system. Lead exposure can also alter fetal development and cause miscarriages. It is especially damaging to children under six and pregnant women. In 1991, the secretary of Health and Human Services characterized lead poisoning as the number-one environmental threat to the health of children in the United States. A national health survey indicated that over the past 20 years, the average child's blood lead level has decreased from 12.8 mg/dl to 2.8 mg/dl. In 1991, 1.7 million U.S. children under the age of six had blood-lead levels exceeding 10 mg/dl. Efforts to reduce lead exposure from gasoline and food cans have been successful and have contributed to the decline in blood lead levels. However, lead-based paint still poses a threat. About 83% of the privately owned housing units built in the United States before 1980 contain some lead-based paint (approximately 64 million homes). The lead from paint can flake off and dust caused during normal wear can create a hard-to-see film over surfaces. Even regular cleaning and dusting can disperse fine dust particles containing lead into the air. If managed improperly, the exposed lead may be inhaled or ingested.