Scientists use sophisticated instruments to measure concentrations of harmful substances in the air, but it’s tough to say exactly what percentage of air pollution comes from cars. This makes sense, because many other human activities contribute to air pollution as well. In fact, the production of electricity by coal-fired power plants and other sources can cause more pollution than most cars. If that wasn't enough, we pollute the air when we heat our homes and public buildings with fuels other than electricity -- just as we do when we drive our cars. Even people who don’t drive add to pollution when they buy goods and services that involve fuel when they're made or delivered.