Does the lead in fuel harm passengers?
Your question is based on an incomplete picture of how aircraft work, I'm afraid.

As a passenger on any major airline, the only lead you are likely to encounter is in the shielding on the security x-ray machines.

Fact: leaded fuel is relatively uncommon

First, tetraethyl lead is only present in what we call avgas - it is most often sold these days as 100LL (100-octane low-lead) aviation gasoline. Avgas is only used in piston (reciprocating piston) engines. Very few commercial aircraft in this day and age are powered by piston engines; most airliners are turbine-powered (jet or turboprop engines) and are fueled by Jet A, a variation on kerosene.

Fact: exhaust shouldn't be in the cabin anyway

Second, even if you happen to be flying in a light aircraft with a piston engine (or old, think Douglas DC-3 in Alaska), you shouldn't actually encounter any lead. The cabin air intakes are positioned intentionally to avoid any exhaust entering the cabin.

If exhaust carrying lead particles was reaching the cabin, you've probably got bigger and more immediate problems with carbon monoxide poisoning anyway.