I was wondering how toxic chlorine is, because my well water was just chlorinated yesterday and today the smell is still strong. I have a 4-year-old daughter and I’m concerned.
— Rose Smith, via e-mail

According to the U.S. Environmental Protection Agency (EPA), chlorine levels of four parts per million or below in drinking water—whether from a private well or municipal reservoir—are acceptable from a human health standpoint. Inexpensive home drinking water test kits (from $5 on up) that can detect levels of chlorine and other elements in water are widely available from online vendors. Administering the tests is easy and can provide parents with a way to involve kids in science for a practical purpose right at home.

Chlorine was first used in drinking water to reduce waterborne infectious diseases in Jersey City, New Jersey more than a century ago. It was so effective at destroying potentially harmful …