Sources of radiation

People are constantly exposed to natural radiation (background radiation). Background radiation includes cosmic radiation, most of which is absorbed by the atmosphere. Thus, the background affects people living in high mountains or flying in an airplane more. Radioactive elements, especially radon gas, are found in many rocks or minerals. These elements end up in various substances, including food and building materials. Radon exposure usually accounts for 2/3 of the total dose of natural radiation.

People are also exposed to radiation from artificial sources, including nuclear weapons (for example, during testing) and various medical tests and treatments. The average person receives approximately 3-4 mSv/year from natural and artificial sources.

Average annual dose of ionizing radiation (USA)

Source	Dose (mSv)
Natural sources
Radon gas	2.00
Other terrestrial sources	0.28
Cosmic radiation	0.27
Natural internal radioactive elements	0.39
Total	2.94
Artificial sources
Diagnostic X-ray (for the average person)	0.39
Nuclear medicine	0.14
Consumer goods	0.10
Fallout from nuclear weapons tests	<0.01
Nuclear industry	<0.01
Total	0.63
Total annual radiation	3.6
Other sources of radiation
Flight	0.005 per flight hour
Dental X-ray	0.09
Chest X-ray	0.10
X-ray with barium enema	8.75

There have been known radiation leaks from nuclear power plants, such as Three Mile Island in Pennsylvania in 1979 and Chernobyl in Ukraine in 1986. The release at Three Mile Island was minimal; people living within 1 mile (1.6 km) of the plant received only about 0.08 mSv. However, people living near the Chernobyl nuclear power plant received a dose of approximately 430 mSv. More than 30 people died, many more were infected, and radiation reached other parts of Europe, Asia, and the United States. In total, besides Chernobyl, radiation from reactors in the first 40 years of nuclear power use resulted in 35 serious releases with 10 deaths, none of which were from commercial power plants. Other significant events include the detonations of atomic bombs in Japan in August 1945, which resulted in the deaths of more than 100,000 people directly from the blast and hundreds of thousands more from radiation sickness and other related injuries.

The possibility of terrorists using radiation exposure is of great concern to the public worldwide. Possible terrorist scenarios range from limited dispersal of radioactive materials without explosion to dispersal by means of conventional explosives ("dirty bombs") and attempts to seize and detonate nuclear reactors or nuclear weapons.

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