It's the gubmint dumping Fluorine in the water again! :)
Series....halogens are fluorine, chlorine, bromine, iodine. They chlorinate the water some to kill the bad bugs but chlorine is toxic and you don't want too much of the stuff. From my environmental days, there's always some form of chloroform in the water: that's a carbon with 3 hydrogens and a chlorine stuck on it.
But then I've never worked at a water treatment plant. I expect they have ranges for these compounds that have been exceeded.
At least it ain't cryptosporidium. :)
I think I'm glad I didn't study chemistry...
That's what the guy told us who was trying to sell us the filter (seriesly, if we could afford it, we'd get it). But he said the chlorine concentration in our drinking water is actually higher than in a standard swimming pool. You just can't smell it because of the volume.