Environmental radiation exists in our environment from naturally occurring and man-made radiation sources. Often, environmental radiation is termed “background radiation.” On average, we receive an annual radiation dose of 300 millirem from natural radiation sources and 60 millirem from manmade radiation sources. Any one person’s annual environmental radiation dose will vary depending upon their activities and location.
Natural Background Sources
Natural background radiation comes from the following three sources:
The sun and stars send a constant stream of cosmic radiation to Earth, much like a steady drizzle of rain. Differences in elevation, atmospheric conditions, and the Earth’s magnetic field can change the amount (or dose) of cosmic radiation that we receive.
The Earth itself is a source of terrestrial radiation. Radioactive materials (including uranium, thorium, and radium) exist naturally in soil and rock. Essentially all air contains radon, which is responsible for most of the dose that Americans receive each year from natural background sources. In addition, water contains small amounts of dissolved uranium and thorium, and all organic matter (both plant and animal) contains radioactive carbon and potassium. Some of these materials are ingested with food and water, while others (such as radon) are inhaled. The dose from terrestrial sources varies in different parts of the world, but locations with higher soil concentrations of uranium and thorium generally have higher doses.
Radon gas contributes nearly two-thirds of our natural background radiation exposure.
All people have internal radiation, mainly from radioactive potassium-40 and carbon-14 inside their bodies from birth and, therefore, are sources of exposure to others. The variation in dose from one person to another is not as great as that associated with cosmic and terrestrial sources.
In general, the following man-made sources expose the public to radiation (the significant radioactive isotopes are indicated in parentheses):
- Medical sources (by far, the most significant man-made source)
- Diagnostic x-rays
- Nuclear medicine procedures (iodine-131, cesium-137, and others)
- Consumer products
- Building and road construction materials
- Combustible fuels, including gas and coal
- X-ray security systems
- Fluorescent lamp starters
- Smoke detectors (americium)
- Luminous watches (tritium)
- Lantern mantles (thorium)
- Tobacco (polonium-210)
- Ophthalmic glass used in eyeglasses
- Some ceramics