Clinical and Health Affairs
Radiation Exposure and Air Travel: Should We Worry?
By Ronnell Hansen, M.D., and Elisa Hansen, D.O.
■ With the federal government introducing new advanced imaging scanners at airports, the traveling public has become concerned about the radiation exposure they may receive when passing through scanners as well as during flight. This article offers a primer on radiation and the extent to which exposure from various sources can affect health. It also provides advice for physicians whose patients may have concerns about radiation exposure during air travel.
The catastrophic events at Japan’s tsunami-damaged Fukushima nuclear power plant and media reports about inappropriately high radiation doses delivered during medical imaging have heightened public concern about the potential long-term consequences of radiation exposure. Although we presume a positive risk-benefit ratio from diagnostic scans and medical treatments that involve radiation, there is growing concern about the health effects of the cumulative amount of radiation individuals are exposed to over the course of their lifetime. With the recent addition of more security scanners at airports around the country, the public has expressed concern about the amount of radiation they may be exposed to when passing through security and flying.
This article provides background on radiation and discusses what is known about its potential effect on biologic systems as well as the statistical risk of radiation exposure in airports and on airplanes.
Visible light, X-rays, gamma rays (nuclear medicine), microwaves, and radio waves (MRI) are all forms of radiation; radiation can be defined simply as moving energy and occurs in ionizing and nonionizing forms. Ionizing radiation consists of particles (photons) that have enough energy to remove electrons from atoms or molecules. Alpha, beta, neutron, gamma, and X-ray waves are forms of ionizing radiation. Ionization (electron removal) results in free radicals, atoms or molecules containing unpaired electrons that tend to be chemically reactive and potentially can damage DNA. Nonionizing radiation consists of lower-energy particles/photons, which typically only change the rotational, vibrational, or electron valence of molecules and atoms, affecting their ability to bond with other atoms. Nonionizing radiation can produce nonmutagenic effects such as inciting thermal energy in biological tissue that can lead to burns. Radio waves and the magnetic waves used in diagnostic MRI are examples of nonionizing radiation.1
The two main sources of ionizing radiation are natural radiation (cosmic radiation and radiation from radon gas and radioactive materials in the body) and radiation from medical tests and procedures (imaging and cancer therapy). Quantifying their potential for biologic damage is not easy. The measure “total ionizing dose,” which is the amount of energy deposited per unit mass of medium, is not necessarily a good indicator of likely biological effect because equal doses of different types of radiation cause different amounts of damage to living tissue. A better estimate of effect is “equivalent dose,” which takes into account factors reflecting different relative biological consequences. Equivalent dose is calculated by multiplying the absorbed dose by a weighting factor that is different for each type of radiation. It is frequently reported in milliSieverts (mSv).
Health Effects of Radiation Exposure
The public’s No. 1 concern regarding radiation is that it may cause cancer. The probability of exposure to ionizing radiation causing cancer depends on both the dose rate and the sensitivity of the organism. Ionizing radiation causes harm in two ways: It forms free radicals that may indirectly damage DNA, and it may directly break down DNA molecules.2-5
The average amount of radiation a person in the United States receives from all sources is estimated to be 6.20 mSv per year.1 That may vary depending on the environment in which one lives, the work a person does, and the medical procedures he or she undergoes. Currently, international standards allow exposures up to 50 mSv per year for those working with and around radioactive material. Federal law mandates lower doses for women who are pregnant—5 mSv during the entire gestational period and 0.5 mSv during any month of pregnancy.6,7
Determining biological significance and potential damage to tissues and systems remains a challenge, however, as not all tissues (and perhaps not all individuals) react the same way to the same level of exposure. In many cases, it may be difficult if not impossible to determine the precise dose to any given tissue, and often doses are adjusted to standardized “whole-body exposures,” even though only a small portion of the body may be exposed. Adding to the confusion is the fact that medical equipment manufacturers use variable methods to quantify the dose delivered by their products. Calculating dose is further complicated by a person’s size and percentage of body fat. Patients who have very little body fat to attenuate radiation can receive higher effective doses. As a result of these factors, some CT-dose algorithms for pediatric patients may have underestimated the delivered effective dose in neonates by as much as 300%. Recently, much-improved volumetric dose methods using phantoms (simulated targets) have been developed, substantially improving such estimates, which are now thought to be within 20% of the actual delivered dose.8 As medical tests and treatments deliver so much radiation, recording patient exposures and the cumulative amount delivered in the electronic health record will likely be required in the future.
Unlike ionizing radiation, nonionizing radiation is currently considered noncarcinogenic. The effect of nonionizing forms of radiation on living tissue has only recently been studied, and different biological effects of exposure to different types of nonionizing radiation have been observed.9,10 For example, a majority of animal studies show no adverse effect of chronic exposure to microwaves, although some do suggest they might contribute to the potential for increased rates of tumor growth (which also occurs in chronically stressed animals not exposed to radiation).11 Boian S. Alexandrov and colleagues from the Center for Nonlinear Studies at Los Alamos National Laboratory constructed mathematical models to assess the effect of terahertz waves that were a frequency 10 to 100 times higher than microwaves on double-stranded DNA. Their analysis suggested that terahertz waves might “unzip” DNA, creating bubbles within the double strand that could significantly interfere with processes such as gene expression and DNA replication.12 Experimental verification of this model has not yet been performed. Recent analysis suggests this would not actually occur under reasonable physical conditions.13
Radiation Related to Air Travel
Air travelers are exposed to two main sources of radiation: cosmic radiation during flight and radiation from scanners while undergoing security clearance.
Two types of modern advanced imaging technology are used for security scanning: backscatter X-ray (BSX) scanners and millimeter wave scanners (MWS). BSX scanners, like medical imaging machines, use X-ray photons but at considerably lower doses. Unlike traditional X-ray or CT imaging, which directs higher-energy radiation through a target, BSX scanners use lower-energy radiation that reflects from a target. This technology is able to assess only one side of a target such as the front or back side of a human being, rendering a 2-D image that resembles a chalk etching. The Health Physics Society estimates that the dose of radiation from a BSX scan is approximately 0.05 μSv;14,15 American Science and Engineering Inc., a manufacturer of backscatter scanners, estimates the dose at 0.09 μSv. The Food and Drug Administration proposes an allowable scan dose of 0.1 μSv, assuming that the dose increases the individual lifetime risk of death from cancer 5×10−10.16-19 Thus, we could anticipate one additional cancer death per 200 million scans.
Active MWS systems transmit a millimeter wave through dual antennas as they rotate around a target, constructing a 3-D image using reflected energy similar to BSX. Passive MWS systems detect energy naturally emitted from the body or from objects concealed on the body and produce an image that resembles a fuzzy photo negative. Passive MWS systems direct no additional energy at the subject and are considered as safe as a digital camera for both the screener and the person being scanned.
High-altitude cosmic radiation likely poses more risk to travelers than radiation from either type of security scanner. The dose received during air travel is estimated at approximately 0.005 mSv/hr; thus, the dose received during a six-hour flight would be approximately 20 μSv, which is 200 to 400 times greater than a dose received during a BSX scan. For every 1 million people who travel by air, an estimated 600 additional cancers would occur as a result of exposure to the higher levels of radiation during flight.20
The statistical associations between exposure to radiation and cancer typically have been based on populations exposed to high levels of ionizing radiation such as the survivors of the atomic blasts at Hiroshima and Nagasaki and recipients of selected diagnostic or therapeutic medical procedures. Accurately determining potential causal relationships between exposure to smaller radiation doses and cancer is complicated by the relatively high incidence of lifetime risk of cancer for the general population independent of additional extrinsic radiation exposure. The lifetime risk of developing cancer is 44% for men and 38% for women; the lifetime risk of dying from cancer is 23% for men and 20% for women.21 Thus, extrapolations of cancer risk from minuscule exposures to radiation across large populations cannot easily be statistically supported. In an analysis of the issue, the National Council on Radiation Protection stated the following: “Summation of trivial average risks over very large populations or time periods into a single value produces a distorted image of risk, completely out of perspective with risks accepted every day, both voluntarily and involuntarily, and statistical extrapolation predicting one death in 200 million scanned is an unrealistic over-estimation.”22
To gain some perspective on the amount of risk we are confronting when we fly, we need to think in terms of relative risk. Andrew J. Einstein, director of cardiac CT research at Columbia University Medical Center in New York has estimated that a passenger would need to be scanned with BSX from front and back about 200,000 times to receive the amount of radiation equal to one typical CT scan.23 Comparably, having a BSX scan every day of your life would deliver one-tenth of the dose delivered during a typical CT scan. Most recent analyses by University of California San Francisco/University of California Berkeley,20 the Center for Radiological Research at Columbia University Medical Center New York,24 and the National Council on Radiation Protection and Measurements25 have largely alleviated concerns about radiation exposure from BSX scanners, assuming they are used correctly. Individual risk is considered sufficiently low for the security function performed, and exposure to radiation from BSX scanners is estimated to be safe for most persons flying only a few times a year. The risk is higher for frequent flyers, and the long-term consequences of such exposures are unknown.26
It is important for people to understand that they are exposed to background radiation from multiple sources in daily life. A number of additional factors contribute to the amount of radiation to which they are exposed. Comparative examples include residing in a state bordering the Gulf or the Atlantic coast, which adds a dose of 0.16 mSv; living on the Colorado Plateau (0.63 mSV); residing in stone/adobe/brick/concrete building (0.07 mSv); living within 50 miles of a nuclear power plant (0.0001 mSv); living within 50 miles of a coal-fired power plant (0.0003 mSv); consuming food (0.4 mSv); and breathing the radon gas found in air (2.28 mSv). More localized forms of radiation exposure may come from uranium in porcelain dental crowns or false teeth (0.0007 mSv per day) or smoking half a pack of cigarettes daily (0.18 mSv per day).27
Statistically, the risk posed by exposure to radiation from passing through BSX and MWS scanners in airports and during commercial flight is exceedingly low. Relatively speaking, the radiation people receive from medical imaging and procedures poses a much greater risk. For that reason, when physicians are counseling patients who are concerned about the amount of radiation they will receive going through airport security or during flight they might do better to focus the discussion on issues that are more likely to ensure safety such as appropriate use of medical radiation and developing habits including use of seat belts.
The American College of Radiology has developed educational materials for patients, providers, and radiologists through their Image Wisely and Image Gently campaigns in an effort to reduce the amount of medical radiation patients receive for routine procedures. In addition, manufacturers of imaging equipment are developing new technologies that will significantly reduce the amount of radiation delivered during imaging—in some cases by more than tenfold. Although the amount of radiation delivered during a CT scan may never be as low as that delivered by an airport security scan, we are likely to see substantial reductions in medical imaging radiation during the next three to five years. MM
Ronnell Hansen is a staff radiologist at the Minneapolis VA Medical Center and an assistant professor of radiology at the University of Minnesota. Elisa Hansen is a PGY4 in anesthesia at the university.
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