The recently released report by the National Academies of Sciences stating that any exposure to low-level radiation can lead to an increased risk of cancer once again raises the thorny question of whether the benefits derived from medically related radiation exposures outweigh the risks associated.
Before addressing that question directly, some facts regarding radiation exposure need to be understood. For starters, all of us receive a certain amount of exposure to low-level radiation, whether we have any medical procedures performed or not. This is called background radiation, and it essentially comes from two sources.
The first source is the ground at our feet, which contains trace amounts of radioactive elements. Among the various elements found in ground soil is uranium, and the natural decay of this uranium gives rise to a radioactive by-product gas called radon.
In areas such as central Pennsylvania where the ground soil has relatively high concentrations of uranium, radon can collect in basements of homes and can pose a significant health risk. Indeed, according to the Environmental Protection Agency, as many as 22,000 lung cancer deaths annually throughout the United States can be traced to radon gas exposure.
The second source of background radiation is cosmic rays, which rain down on us from outer space. Because the atmosphere acts as a filter to cosmic rays, when we go to higher altitudes, whether in an airplane or on a mountain trail, we receive a higher dose from cosmic ray background.
Natural background radiation exposure is the predominant source of exposure to individuals, accounting for about 82 percent of our annual total. Obviously, it is a source that we can do little to reduce or eliminate.
The remaining portion of our average annual radiation exposure is from man-made sources. This includes medical devices, such as X-ray machines, CT scanners and linear accelerators used in cancer treatment, as well as radioactive agents used in nuclear medicine. Other sources of man-made exposure include trace amounts of radioactive materials used in consumer products, residual fallout from nuclear weapons tests
conducted in the past and radioactive waste from both the nuclear power industry and the weapons program.
To put things in better perspective, let's attach some numbers to these various sources of radiation exposures, including some common medical procedures. The total average annual individual exposure from background sources (ground soil and cosmic rays) is roughly 3.0 mSv. (This unit, the millisievert, is the internationally accepted unit of effective radiation dose.) An international flight from Chicago to London results in an additional dose of around 0.04 mSv, a chest X-ray adds about 0.1 mSv, a mammogram about 0.7 mSv and a CT scan of the chest gives around 8 mSv.
This average exposure figure can vary greatly, however, depending on local conditions such as soil makeup and altitude. Persons who live at high altitudes, such as the residents of Denver, Colorado, receive a significantly higher background radiation exposure from cosmic rays for the reasons mentioned above than those who live at sea level, for example.
These facts and factors all need to be taken into consideration when trying to determine whether radiation exposure we receive from various diagnostic and therapeutic medical represent a significant risk.
Charles Wild is a physicist in the Radiation Oncology Department at Mount Nittany Medical Center.