The air becomes polluted in different ways. How the air becomes polluted determines the types of problems it causes. Different sources of emissions contain different chemicals. These may interact with other airborne chemicals in unknown ways. As the chemicals mix with moisture in the air they can become rain. The rain can move the chemicals through the ecosystem, including crops and livestock. Mercury, lead, and aluminum all move in this way, with adverse ecological effects. There may be other chemicals with adverse ecological effects that do not last as long as metals do and may therefore be hard to detect while present. Air pollution can expose populations to more than just airborne pollution.