Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 36 to 42.

    Lead poisoning in children is a major health concern. Both low and high doses of paint can have serious effects. Children exposed to high doses of lead often suffer permanent nerve damage, mental retardation, blindness, and even death. Low doses of lead can lead to mild mental retardation, short attention spans, distractibility, poor academic performance, and behavioral problems.

   This is not a new concern. As early as 1904, lead poisoning in children was linked to lead-based paint. Microscopic lead particles from paint are absorbed into the bloodstream when children ingest flakes of chipped paint,plaster,or paint dust from sanding. Lead can also enter the body through household dust, nailbiting, thumb sucking, or chewing on toys and other objects painted with lead-based paint. Although American paint companies today must comply with strict regulations regarding the amount of lead used in their paint, this source of lead poisoning is still the most common and most dangerous. Children living in older, dilapidated houses are particularly at risk.

What is the main topic of the passage?