Mil Skibinski
Mil Skibinski voted up Danae Hitch's answer

Sulfur dioxide (SO2) kills by combining with water (H2O) to form sulfuric acid (H2SO4), which, incidentally, is a major component of acid rain (along with nitric acid).

Remember that humans are just great bags of water walking around.  When the SO2 touches us, it can't help but find water.  You inhale it, there's … Read more