if they were that bad we'd know by now.

edit: if you are downvoting you either don't know or have forgotten how obviously bad leaded gasoline was, even with 50 year old science.

https://en.wikipedia.org/wiki/Tetraethyllead#Toxicity

The hazards of TEL content are heightened due to the compound's volatility and high lipophilicity, enabling it to easily cross the blood–brain barrier.

Chronic exposure to TEL can cause long-term negative effects such as memory loss, delayed reflexes, neurological problems, insomnia, tremors, psychosis, loss of attention, and an overall decrease in IQ and cognitive function.[101]

Concerns over the toxicity of lead[103] eventually led to the ban on TEL in automobile gasoline in many countries. Some neurologists have speculated that the lead phaseout may have caused average IQ levels to rise by several points in the US (by reducing cumulative brain damage throughout the population, especially in the young). For the entire US population, during and after the TEL phaseout, the mean blood lead level dropped from 16 μg/dL in 1976 to only 3 μg/dL in 1991.[104] The U.S. Centers of Disease control previously labelled children with 10 μg/dL or more as having a "blood lead level of concern". In 2021, the level was lowered in accordance with the average lead level in the U.S. decreasing to 3.5 μg/dL or more as having a "blood lead level of concern".[105][106]