Originally Posted by link

Interesting observation. Evidently no one knows what the top source of lead poisoning may be. Avgas is widely attributed as causing about half of the lead pollution as of about 2012 Does the Continued Use of Lead in Aviation Fuel Endanger Public Health and the Environment?: Scientific American

According to another source about 200,000 helicopters and airplanes use leaded fuel.

One group sued recently to get the EPA to remove lead from avgas.
Aviation Fuel's Toxic Lead Emissions Draws Lawsuit Against EPA and this is what motivated the EPA to begin to study the issue of lead levels near airports where avgas is sold.

Accordingly a 2011 Duke University study found that kids living within 500 meters of an airport where leaded avgas is used have higher blood lead levels than other children, with elevated lead levels in blood found in kids as far as one kilometer away.

More than 3 million children attend school in close proximity to the some 20,000 airports where avgas may be used. I didn't find a statement of how many adults live in proximity to these 20,000 airports.