http://en.wikipedia.org/wiki/Malathion

I'll quote: "However, malathion breaks down into malaoxon, which is 60
times more toxic than malathion."

"Malathion present in untreated water is converted to malaoxon during
the chlorination phase of water treatment"

Also:

"Malathion was used in the 1980s in California to combat the
Mediterranean Fruit Fly."

"Malathion was sprayed in many cities to combat West Nile virus."