Question: what`s the most harmful thing that is likely to be found in tap water?

Ciarán O'Brien answered on 10 Nov 2014:

Probably bacteria. Tap water is supposed to be very strictly monitored by government agencies, and most of the time it’s fine. Every now and again though, some bacteria will make it into the water and past the filters and sterilisation, and if enough of them get swallowed by one person it can make them seriously ill, or even kill them.

Back around 2007 there was an outbreak of cryptosporidium in the Galway water supply. It’s a tiny animal that shouldn’t live in your gut, but if it gets in it makes you feel absolutely dreadful. Some times some Escherichia coli will get in the water, from maybe cow poop getting in the rivers or maybe one of the people testing the water will forget to wash their hands. E. coli is a bacteria that normally lives in your gut, but there are different strains of it that can be really harmful and kill a person in a few days if they’re not seen by a doctor quickly.

You sometimes hear about a boil notice for water in one part of the country or another. It’s usually because some kind of bacteria was found in the water, and boiling the water will kill it off and make it safe to drink.