Nearly half of the world’s population relies on fuels such as wood or dung for cooking and heating. In the 1980s, Kirk R. Smith, a member of the National Academy of Sciences and a professor of global health at the University of California at Berkeley, sounded the alarm that these fuels, when burned in open fires or traditional cook stoves, produce high levels of indoor air pollution that prematurely kill about 2 million people each year—more than either malaria or tuberculosis, according to Smith. Cleaner alternatives to traditional cook stoves exist, but convincing funding agencies and decision makers to invest in these technologies requires substantive evidence of their health benefits, he says. Today, Smith—a 2012 recipient of the Tyler Prize for Environmental Achievement—develops inexpensive, portable electronic monitors to measure exposures to indoor air pollution in developing countries. Here, he explains how his research can aid the design and dissemination of solutions to tackle this ancient but still widespread problem.