On an average day in Ulaanbaatar, Mongolia, the temperature doesn't rise above freezing, and in the winter, it often dips to 40 degrees below zero on the Fahrenheit scale. Many Mongolian families, including most of the population of Ulaanbaatar, live in traditional huts called yurts or gers. To stay warm, ger dwellers typically burn raw coal in individual coal stoves, contributing to increasingly dangerous levels of air pollution throughout the city.