Radiation Exposure

Exposure to ionizing radiation can be divided into two categories: natural and anthropogenic (i.e., associated with human activity). Background radiation is mostly due to solar radiation in the form of cosmic rays, and also radioactivity from rocks. Exposure to background radiation is continuous, although its intensity varies. The sun is also the main source of ultraviolet radiation. Each person in the United States receives an average radiation dose per year of about one millisievert (one-thousandth sievert; this is the same as 0.1 rem). About one-half of this exposure is due to radon, a natural radioactive gas released from rocks.

Radon is a breakdown product of uranium. Radon itself breaks down rapidly; its half-life is less than four days (this is the time for one-half of an initial quantity to decay through radioactivity). Unfortunately, radon decays into polonium-218, polonium-214, and polonium-220, which emit alpha particles. Alpha particles are heavy, charged particles that have trouble penetrating matter but can be dangerous if taken into the body, where they are in close contact with tissues and biochemicals (such as DNA) that are sensitive to suffering damage by ionization. Radon may be responsible for one-tenth of all deaths by lung cancer.

The actual and potential sources of anthropogenic radiation include: x rays and other types of radiation used in medicine, radioactive waste generated by nuclear power stations and scientific research centers, and radioactive fallout from nuclear weapons testing. Fallout is radioactive contamination of air, water, and land following the explosion of nuclear weapons or accidents at nuclear power stations.

Electromagnetic radiation from television sets and microwave ovens has been lowered to insignificant levels in recent years, thanks to federal regulations and improved designs. Some people consider high-voltage transmission lines a radiation threat, but scientific studies have not demonstrated a significant threat from this source.