This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.
The short answer is no. However that answer is not global, as some countries
have replaced the lead compound octane-improvers with aromatic or olefin
octane-improvers without introducing exhaust catalysts. The aromatics
contents may increase to around 40%, with high octane unleaded fuels reaching
50% in countries where oxygenates are not being used, and the producers have
not reconfigured refineries to produce high octane paraffins. In general,
aromatics are significantly more toxic than paraffins. Exhaust catalysts
have a limited operational life, and will be immediately poisoned if
misfuelled with leaded fuel. Catalyst failure can result in higher levels of
toxic emissions if catalysts or engine management systems are not replaced or
repaired when defective. Maximum benefit of the switch to unleaded are
obtained when the introduction of unleaded is accompanied by the introduction
of exhaust catalysts and sophisticated engine management systems.
Unfortunately, the manufacturers of alkyl lead compounds have embarked on a
worldwide misinformation campaign in countries considering emulating the
lead-free US. The use of lead precludes the use of exhaust catalysts, thus
the emissions of aromatics are only slightly diminished, as leaded fuels
typically contain around 30-40% aromatics. Other toxins and pollutants that
are usually reduced by exhaust catalysts will be emitted at significantly
higher levels if leaded fuels are used .
The use of unleaded on modern vehicles with engine management systems and
catalysts can reduce aromatic emissions to 10% of the level of vehicles
without catalysts . Alkyl lead additives can only substitute for some of
the aromatics in gasoline, consequently they do not eliminate aromatics,
which will produce benzene emissions . Alkyl lead additives also require
toxic organohalogen scavengers, which also react in the engine to form and
emit other organohalogens, including highly toxic dioxin . Leaded fuels
emit lead, organohalogens, and much higher levels of regulated toxins
because they preclude the use of exhaust catalysts. In the USA the gasoline
composition is being changed to reduce fuel toxins ( olefins, aromatics )
as well as emissions of specific toxins.