Air quality in Utah is often some of the worst in the United States. Poor air quality in Utah is due to the mountainous topography which can cause pollutants to build up near the surface (especially during inversions) combined with the prevalence of emissions from gasoline and diesel powered vehicles, especially older models. Burning wood fuel for home heating can also contribute significantly to poor air quality. Homes heated with wood contribute about 3000 times the amount of pollution as homes heated with natural gas. About 50% of air pollution in Salt Lake County is from vehicles.