More than a billion people don’t have reliable access to clean drinking water. Boiling kills most germs in water, but requires fuel and doesn’t remove dirt. In recent years, sand and ceramic filters have become more common, but these tend to be more expensive and usually don’t catch all the microbes.

So many of the poor worldwide simply drink dirty water. As a result, about 1.5 million children die of diarrhea each year.

A new generation of cheap and effective water purifiers including Pureit (made by Unilever) and Swach (made by an Indian company, Tata, with a novel rice-husk ash filter) can remove nearly all water-borne pathogens without electricity.