Final answer:
Blood-lead levels among children have decreased by more than 90% since the introduction of federal legislation in the 1970s, likely indicating a reduction of at least 75%. This is attributed to the banning of lead in products and improved public health efforts, though lead exposure still remains a concern requiring ongoing prevention.
Step-by-step explanation:
Since the advent of federal legislation in the 1970s, blood-lead levels among children have fallen significantly. The question asks by what percentage these levels have fallen. While the question does not provide the exact figure, historically, lead levels in children's blood have decreased by more than 90% since the removal of lead from gasoline and since the United States banned the use of lead-based paint in housing. For the purposes of this question, although not directly provided, it can be inferred that there has been at least a 75% reduction. This is due to the extensive measures taken over the past several decades to reduce lead exposure from various sources such as paint, pipes, and contaminated soil, as well as improved public health policies and screening practices.
It's essential to note that even with the considerable decline in blood-lead levels, lead poisoning remains a serious issue, particularly in areas with older housing stock or ongoing exposure through other means. Additionally, the Centers for Disease Control and Prevention (CDC) has set a blood lead reference value to identify children with higher levels of lead exposure, highlighting the continuing need for vigilance and prevention measures.