Fred, the south-western arm of the lake (it's shaped like an inverted Y) is a dead end, and furthest from the Adda river which feeds it, so it's the most polluted (or least fresh). In 2007 it was reported the water in that part of the lake was 68 times above acceptable levels. I haven't found any more recent measurements (but I'll look) although the Italian government, to meet EU standards, has been installing water purification stations. Still, just to be safe, swim in the northern parts of the lake.... or at least check with locals before taking the plunge.
Phil at he safety hub about 6 years ago