I don’t know if there is any known level of safe lead exposure. In terms of direct impact, this is going to be hard to measure but the risk is nonetheless real. In fact the difficulty of measuring the consequences of mass lead exposure was one of the best arguments out forth my oil refining companies back in the day to support leader gasoline. Not sure why all the burden of proof is on anyone who wants to avoid exposure to a dangerous chemical.

"From 1976-1980 to 2015-2016, the geometric mean blood lead level (BLL) of the US population aged 1 to 74 years dropped from 12.8 to 0.82 μg/dL, a decline of 93.6%. Yet, an estimated 500 000 children aged 1 to 5 years have BLLs at or above the blood lead reference value of 5 μg/dL established by the Centers for Disease Control and Prevention. Low levels of exposure can lead to adverse health effects. There is no safe level of lead exposure, and child BLLs less than 10 μg/dL are known to adversely affect IQ and behavior. When the exposure source is known, approximately 95% of BLLs of 25 μg/dL or higher are work-related among US adults. Despite much progress in reducing exposure to lead in the United States, there are challenges to eliminating exposure." (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6522252/)

You're right that no level of lead exposure is considered safe, but that doesn't mean that any lead exposure is going to have any kind of measurable effect on an organism.

Shouldn't the impact of leaded avgas be easier to measure, since the concentrations should be higher around airports which send up a lot of leaded avgas planes? It's a different situation than where every car was burning leaded gas and everyone was pretty much exposed to it. The effects should be much more localized and therefore easier to study.

The risk may be real, but it may be dwarfed by other sources of lead, making it a silly thing to worry yourself about. For example, you might get more lead into your body from 8oz of fruit juice than you would living a year near an airport.