I should add that I first started wondering about these, esp. the first, after being in Japan and camping for a night in a place where the only water available to us was volcanic in nature, so it was probably loaded with heavy metals, which we knew that the filter we had on hand wouldn't get rid of. Also, while boiling water is an easy way to kill microbes, if one is lacking a cup or going to be in a single place for a long time, other options might be worth consideration.