The different types of radiation have differing abilities to penetrate surfaces. Alpha particles don’t travel far and can’t even penetrate paper; they do not penetrate skin but can be harmful if inhaled or ingested in food or water. The much smaller beta particles can penetrate the upper layers of the skin but are easily stopped by a thin sheet of aluminum or very heavy clothing. High-energy gamma rays can penetrate the deepest but can be stopped by thick or very dense material such as 20 centimeters (8 inches) of concrete or 2.5 centimeters (1 inch) of lead. Exposure to radiation, especially gamma rays, can damage a wide variety of body organs, leading to radiation sickness; it can also cause mutations that lead to cancer or cause birth defects.