The US exports about 40,000 barrels a day of MTBE, a probably carcinogenic chemical used to replace lead in gasoline. Almost all the exports go to Mexico and Venezuela. MTBE was banned in the US because it’s soluble in groundwater. That means that when an underground storage tank springs a leak, it isn’t just a local problem, like gasoline. It spreads quickly and pollutes drinking-water aquifers.
(Blerg inspired by this, about the ongoing exports of lead from the UK.)