In the United States we have water treatment facilities. We're better off than many parts of the world, and we still had the problem in Flint. This can happen anywhere.
The top five potential toxins in tap water are as follows:
- Heavy metals (lead and mercury). Children are especially susceptible to the bad effects of heavy metals. Lead is odorless and has no taste.
- Pharmaceuticals. Don't flush them down the toilet because that moves them into the water supply.
- Herbicides & pesticides. Anything you put on your lawn and garden gets rinsed into the water supply.
- Chlorine & chloramines. People don't like the taste of chlorine in drinking water but chloramines (chlorine and ammonia) don't have the same flavor. The problem with chloramines is they're more corrosive and harder to remove from water. It can damage the transit system from the plant to your house. Most filtration doesn't remove chloramines at a very effective rate.
- Industrial chemicals. Mercury is a biproduct of industrial processes.
The only way to know exactly what's in your tap is to test your water directly with a home testing kit. Any water filter is better than nothing. Invest in a water filter that's been certified by a third party.