Mongolia might be one of the most sparsely populated countries in the world, but one in every three Mongolians lives in the capital, Ulan Bator. The urban boom has had serious consequences for the environment. Since so many families depend on coal-fired stoves for heat in the winter, the city is blanketed by a layer of smog that is 20 times thicker than the permissible amount. Now, officials are trying to combat the problem through better city planning and more energy-efficient building renovation. And local construction companies are learning how to build green-friendly structures as they go.