Acid rain is one type of atmospheric deposition. Atmospheric deposition includes any precipitation, airborne particles or gases deposited from the atmosphere to the Earth's surface. Other forms of atmospheric deposition may also be by wet or dry methods. Much of the material in atmospheric deposition may be a nuisance, but does not harm the environment. Some air pollutants, such as those in acid rain, can cause environmental problems. Over many decades, the combined input of contaminants to sensitive environments can lead to widespread environmental problems. Most smaller particles with a diameter of 10 microns (.004 inches) or less are too light to be deposited, and so remain in the atmosphere where they can cause health problems. They pose a different problem, and are regulated as particulates, or PM.