Radiation has always been a part of the natural environment.
Natural Radiation - Naturally-occurring radiation accounts for approximately 80 per cent of our radiation exposure. Background radiation comes from natural sources always present in the environment, including solar and cosmic radiation, radiation emitted from the earth's crust, radon, and internal radiation from the foods we eat and drink.
Man-Made Radiation - Man-made radiation sources include medical (x-rays and diagnostic or therapeutic materials), industry, consumer products (such as smoke detectors and television sets), and atmospheric testing of nuclear weapons.
The radionuclides present in our environment can give both internal and external doses. Internal dose is received as a result of the intake of radionuclides. The major routes of intake of radionuclides for members of the public are ingestion and inhalation. Ingestion includes the intake of the radionuclides from drinking milk and water, and consumption of food products. Inhalation includes the intake of radionuclides through breathing dust particles containing radioactive materials.
The background radiation level for an average American is about 360 mrem per year.