Sometimes, air pollution is visible. A person can see dark smoke pour from the exhaust pipes of large trucks or factories. Polluted air can be dangerous. It can make people's eyes burn and make them have difficulty breathing. It can also increase the risk of lung cancer. Natural disasters can also cause air pollution to increase quickly. When volcanoes erupt, they eject volcanic ash and gases into the atmosphere. Volcanic ash can discolor the sky for months. Volcanic gases, such as sulfur dioxide, can kill nearby residents and make the soil infertile for years. Most air pollution is not natural, however. It comes from burning fossil fuels-coal, oil, and natural gas. When gasoline is burned to power cars and trucks, it produces carbon monoxide, a colorless, odorless gas. The gas is harmful in high concentrations, or amounts. City traffic produces highly concentrated carbon monoxide.