Cosmic radiation, often referred to as cosmic microwave background radiation, is a form of electromagnetic radiation that exists throughout the entire universe. Cosmic radiation is one of the major sources of background radiation on earth. Background radiation is radiation that is constantly present in the environment; it can originate from natural sources such as radon gas and from man-made sources such as nuclear power plants. Cosmic background radiation is specifically radiation present in the environment that originated from the sun and from other sources in space. Such radiation is composed of many different kinds of charged and uncharged particles; many of the particles that reach earth are formed when radiation from space interacts with particles in the atmosphere.
Most of the cosmic radiation that reaches the Earth is shielded by the atmosphere. Many high-energy particles that could be harmful to organisms on Earth impact particles in the atmosphere; the collision of these particles greatly decreases their energy. As such, they are considerably less dangerous and can not significantly harm organisms on the Earth's surface. Some human activities damage the atmosphere, however; some chemicals humans use, for example, can damage the ozone layer which plays a vital role in blocking out harmful ultraviolet cosmic radiation.
Ultraviolet cosmic radiation tends to be the most dangerous form of cosmic radiation; it is high-energy radiation that comes from the sun. Limited exposure to ultraviolet light is important to health as it causes the human body to develop vitamin D, which is important to many body processes, such as immunity and blood pressure regulation. Excessive exposure to ultraviolet radiation, however, can lead to sunburn and can severely harm the skin's genetic information by damaging the DNA contained within skin cells. This can lead to several different forms of skin cancer, which is extremely dangerous and can be fatal. Excessive exposure to ultraviolet radiation can be caused simply by spending too much time in the sun.
Cosmic radiation levels tend to increase with increasing altitudes, so those who fly with great frequency can sometimes receive unhealthy doses of radiation. Most casual travelers do not need to worry about this, but full-time flight crews and pilots occasionally suffer from radiation exposure. The precise amount of damage that altitude-related exposure can cause is not yet fully known. The World Health Organization, however, advises that air crews and frequent fliers keep themselves aware of health effects of cosmic radiation. They also recommend that pregnant women record their radiation doses and ensure they do not reach any harmful levels.