Never heard of an explosion. Deleterious effects on health would be VERY hard to prove in the real world, but that doesn't mean they don't exist.
One thing for sure: flooded batteries under charge DO emit fumes.
WaterMiser caps can go a long way toward reducing the loss of electrolyte and reduce the need for watering. I've used them on my
boat for many years, with batteries inside the
cabin, and have had no ill effects.
A good source for them is Northern Arizona
Wind and Sun....about $4.80 each and well worth it IMHO.
Bill