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Even though lead was removed from petrol, there is a lot in soil and dust in cities, and that is still agitated out into the air.

This article shows how there is lead in Londons air despite it being removed in the 90s from fuel:

Remember that avgas still uses leaded fuel (though that will hopefully improve soon).

Leaded avgas is only used in small, propeller-driven aircraft like Cessnas. Jet fuel does not contain lead.

Exactly. And to think it used to be in all passenger car gasoline!

Same thing with asbestos, there is a non-zero background level in the air because it was used in so many products for a long time.