Answer to Question #1097 Submitted to "Ask the Experts"

• 0.27 mSv Cosmic radiation (muons)
• 0.01 mSv Cosmogenic radionuclides (³H, ¹⁴C))
• 0.28 mSv Terrestrial radionuclides (uranium series, thorium series, ⁴⁰K))
• 2.0 mSv Inhaled radionuclides (radon decay products))
• 0.4 mSv Radionuclides in the body (⁴⁰K, ²¹⁰Pb, and decay products)

The rounded total is 3.0 mSv per year. Adding in an estimated 0.5 mSv as an average medical exposure and an estimated 0.1 mSv exposure from radioactive consumer products gives an average annual exposure from all sources of 3.6 mSv. Note that the biggest single source of exposure is the inhalation of radon decay products. Radon is a naturally occurring radioactive gas that under some circumstances can accumulate to relatively high concentrations inside the home. Individuals whose homes have higher than average levels of radon (the average is approximately 1.5 picocuries per liter) receive correspondingly higher exposures. An individual in a home with radon levels at the suggested Environmental Protection Agency (EPA) guideline of 4 picocuries per liter would receive an annual exposure of 7 to 8 mSv. In Europe, the annual dose due to natural background radiation is estimated to range from 2 mSv in the United Kingdom to 7 mSv in Finland. Miscellaneous Exposures: The crew on passenger aircraft receive annual exposures of several mSv due to their increased exposures to cosmic rays. Moving from a coastal location to an elevation of 5,000 feet (for example, Denver) would result in an extra 0.3 to 0.4 mSv per year due to the increased exposure to cosmic rays. In Leadville, Colorado,(ca. 10,000 feet) the annual cosmic ray exposure is 1.25 mSv.

• A round-trip transcontinental flight across the United States results in an exposure of approximately 0.05 mSv.
   
• An average chest x ray results in a exposure of 0.1 mSv.

In the state of Kerala on the southwest coast of India, the average exposure due to the unusually high levels of terrestrial radionuclides is 5 to 6 mSv per year. Similar exposures occur to occupants of several regions near Rio de Janiero in Brazil. Inhabitants of another similar area in Guangdong Province in China receive annual exposures from terrestrial radionuclides on the order of 3 mSv. The Issue of Risk: As mentioned previously, the quantity "dose equivalent" has been developed as a means to describe the risk to people who are exposed to ionizing radiation. For the most part, the risk being referred to is the risk of dying of cancer as a result of the exposure. There is no solid evidence that radiation exposures below 100 mSv (or so) carry an increased risk of cancer. However, the risks observed at high doses can be used to estimate what the risks might be at lower doses. Using this approach, we can derive an estimated risk of 5 in 100,000 per mSv. In other words, an individual exposed to one mSv would have a 5 in 100,000 chance of dying of cancer due to the exposure. This would be in addition to the one in five risk of dying of cancer that the person would have in the absence of the exposure. It is worth mentioning once again that an increased risk of cancer has not been seen at such low exposures (1 mSv). It should also be mentioned that such risk estimates are intended for large populations rather than individuals. In industry, the risk of accidental death is approximately 1 in 10,000 per year. Industries with this risk, or less, are referred to as "safe" industries. The following activities would have the same risk assumed for a 1 mSv radiation exposure (5 in 100,000 of dying in any year) according to the data in William Allman's "Staying Alive in the 20th Century" which appeared in the October 1985 issue of Science:

• Smoking 70 cigarettes (cancer, heart disease)
• Drinking 25 liters of wine (cirrhosis of the liver)
• Spending 50 hours in a coal mine (black lung disease)
• Traveling 300 minutes by canoe (accident)
• Traveling 500 miles by bicycle (accident)
• Traveling 7,500 miles by car (accident)
• Eating 2,000 tablespoons of peanut butter (liver cancer caused by aflatoxin B)

References:

• An excellent source of information is What You Need to Know About Radiation by Lauriston Taylor.
   
• A somewhat simpler but still excellent source of information is Radiation and Life by Eric Hall.
   
• Unless noted otherwise, Environmental Radioactivity by Eisenbud and Gesell (Academic Press 1997) was the source of most of the above information.

Paul Frame, CHP, PhD