Sources may be naturally occurring or artificial (see table Average Annual Radiation Exposure in the US).

People are constantly exposed to low levels of naturally occurring radiation called background radiation. Background radiation comes from cosmic radiation and from radioactive elements in the air, water, and ground. Cosmic radiation is concentrated at the poles by the earth’s magnetic field and is attenuated by the atmosphere. Thus, exposure is greater for people living at high latitudes, at high altitudes, or both and during airplane flights. Terrestrial sources of external radiation exposure are primarily due to the presence of radioactive elements with half-lives comparable to the age of the earth (~ 4.5 billion years). In particular, uranium-238 and thorium-232 along with several dozen of their radioactive progeny and a radioactive isotope of potassium (K-40) are present in many rocks and minerals. Small quantities of these radionuclides are in the food, water, and air and thus contribute to internal exposure as these radionuclides are invariably incorporated into the body. The majority of the dose from internally incorporated radionuclides is from radioisotopes of carbon (C-14) and potassium (K-40), and because these and other elements (stable and radioactive forms) are constantly replenished in the body by ingestion and inhalation, there are approximately 7000 atoms undergoing radioactive decay each second.

Internal exposure from the inhalation of radioactive isotopes of the noble gas radon (Rn-222 and Rn-220 ), which are also formed from the uranium-238 decay series, accounts for the largest portion (73%) of the US population's average per capita naturally occurring radiation dose. Cosmic radiation accounts for 11%, radioactive elements in the body for 9%, and external terrestrial radiation for 7%. In the US, people receive an average effective dose of about 3 millisieverts (mSv)/year from natural sources (range ~ 0.5 to 20 mSv/year). However, in some parts of the world, people receive > 50 mSv/year. The doses from natural background radiation are far too low to cause radiation injuries; they may result in a small increase in the risk of cancer, although some experts think there may be no increased risk.

In the US, people receive on the average about 3 mSv/year from man-made sources, the vast majority of which involve medical imaging. On a per capita basis, the contribution of exposure from medical imaging is highest for CT and nuclear cardiology procedures. However, medical diagnostic procedures rarely impart doses sufficient to cause radiation injury, although there is a small theoretical increase in the risk of cancer. Exceptions may include certain prolonged fluoroscopically guided interventional procedures (eg, endovascular reconstruction, vascular embolization, cardiac and tumor radiofrequency ablation); these procedures have caused injuries to skin and underlying tissues. Radiation therapy can also cause injury to normal tissues near the target tissue.

A very small portion of average public exposure results from radiation accidents and fallout from nuclear weapons testing. Accidents may involve industrial irradiators, industrial radiography sources, and nuclear reactors. These accidents commonly result from failure to follow safety procedures (eg, interlocks being bypassed). Radiation injuries have also been caused by lost or stolen medical or industrial sources containing large quantities of the radionuclide. People seeking medical care for these injuries may be unaware that they were exposed to radiation.

Unintended releases of radioactive material have occurred, including from the Three Mile Island plant in Pennsylvania in 1979, the Chernobyl reactor in Ukraine in 1986, and the Fukushima Daiichi nuclear power facility in Japan in 2011. Exposure from Three Mile Island was minimal because there was no breach of the containment vessel as occurred at Chernobyl and no hydrogen explosion as occurred at Fukushima. People living within 1.6 km of Three Mile Island received at most only about 0.08 mSv (a fraction of what is received from natural sources in a month). However, the 115,000 people who were eventually evacuated from the area around the Chernobyl plant received an average effective dose of about 30 mSv and an average thyroid dose of about 490 mGy. People working at the Chernobyl plant at the time of the accident received significantly higher doses. More than 30 workers and emergency responders died within a few months of the accident, and many more experienced acute radiation sickness. Low-level contamination from that accident was detected as far away as Europe, Asia, and even (to a lesser extent) North America. The average cumulative exposure for the general population in various affected regions of Belarus, Russia, and Ukraine over a 20-year period after the accident was estimated to be about 9 mSv.

The earthquake and tsunami in Japan in 2011 led to releases of radioactive material into the environment from several reactors at the Fukushima Daiichi nuclear power plant. There were no serious radiation-induced injuries to on-site workers. Among nearly 400,000 residents in Fukushima prefecture, the estimated effective dose (based on interviews and dose reconstruction modeling) was < 2 mSv for 95% of the people and < 5 mSv for 99.8%. WHO estimates were somewhat higher because of intentionally more conservative assumptions regarding exposure. The effective dose in prefectures not immediately adjacent to Fukushima was estimated to be between 0.1 to 1 mSv, and the dose to populations outside of Japan was negligible (< 0.01 mSv).

Another significant radiation event was the detonation of 2 atomic bombs over Japan in August 1945, which caused about 110,000 deaths from the immediate trauma of the blast and heat. A much smaller number (< 1000) of excess deaths due to radiation-induced cancer have occurred over the ensuing 70 years. Ongoing health surveillance of the survivors remains among the most important sources of estimates of radiation-induced cancer risk.

While several criminal cases of intentional contamination of individuals have been reported, radiation exposure to a population as a result of terrorist activities has not occurred but remains a concern. A possible scenario involves the use of a device to contaminate an area by dispersing radioactive material (eg, from a discarded radiotherapy or industrial source of cesium-137 or cobalt-60). A radiation dispersal device (RDD) that uses conventional explosives is referred to as a dirty bomb. Other terrorist scenarios include using a hidden radiation source to expose unsuspecting people to large doses of radiation, attacking a nuclear reactor or radioactive material storage facility, and detonating a nuclear weapon (eg, an improvised nuclear device [IND], a stolen weapon).