The word smog was originally derived from a combination of the words smoke and fog, and today it describes air pollution that, once in the atmosphere, reacts with sunlight to form additional pollutants. The main sources of this pollution are commonly motor vehicles, factories, power plants, and burning of fuels. The main chemical components of smog are ozone, nitric oxide, nitric dioxide, and volatile organic compounds. Smog is present in pretty much every large city around the world. It can be hazardous to human health and has been blamed for health problems and even deaths.