Ionizing radiation comprises sub-atomic particles or photons that are able to cause ionization in the medium they traverse. Examples are alpha-particles, electrons, x-rays and gamma-rays. Humans are exposed to ionizing radiation from natural and artificial sources. Ionizing radiation from natural sources comprises mainly, ranked from the largest dose: indoor radon; radionuclides in rocks, soils, etc.; cosmic radiation; and radionuclides ingested with foods and drink. The most important artificial source of radiation is medical diagnostic procedures, but there are much smaller collective exposures from historical nuclear weapons testing fallout, and from industrial and occupational settings.1
Until the late-nineteenth century, exposure to ionizing radiation was from natural sources, but over the past 120 years this has increasingly changed and, in some countries, it is estimated that radiation received from artificial sources currently exceeds that received from natural sources. Indeed, a report published by the US National Council on Radiation Protection & Measurements (NCRP) estimated that half of the ionizing radiation dose received by an average American comes from artificial sources, predominantly medical procedures.1 Ten years have elapsed since the publication of that report, so it can be anticipated that the amount of anthropogenic radiation received–including from routine screening procedures—has increased since then. Exposure to radiation from natural sources is, in general, approximately constant for humans living in a given area, implying that, on average, overall exposure to ionizing radiation received by the global population is increasing. We aim to highlight how human exposure to ionizing radiation is changing and is increasing globally due to the more frequent use of medical imaging, which is obviously needed on many occasions.
IONIZING RADIATION AND CANCER RISK.
All types of ionizing radiation have been classified as carcinogenic to humans (Group 1) by the International Agency for Research on Cancer (IARC). The relationship of ionizing radiation with the onset of different cancer types has been extensively reviewed by, among others, IARC in an updated monograph published in 2012.2 The magnitude of this association varies for different cancers and the basic radiobiologic mechanism is understood to be radiation damage to DNA, although the exact pathway from DNA damage to cancer is not completely known and is beyond the scope of this article. The BEIR (Biologic Effects of Ionizing Radiations) VII Report (from the US National Academy of Sciences), which was focused on exposure to low levels of ionizing radiation, concluded that “there is a linear dose–response relationship between exposure to ionizing radiation and the development of radiation-induced solid cancers in humans”, and that a potential threshold dose was not apparent in this association.3
The slope of the dose–response varies between different cancers and different types of radiation, and is modified by factors such as age at exposure and sex. Evidence is derived from studies that have assessed the effect of radiation on cancer in different settings or exposures: survivors of the Japanese atomic bombings4,5; lung cancer following exposure to radon in underground hard-rock (e.g., uranium) mines6 and more recently due to residential radon exposure7,8; patients exposed during medical procedures (mainly radiotherapy, but also radiography)9,10; children exposed to high levels of radioactive iodine after the Chernobyl accident11; and various other exposed groups.12,13
It is important to note that there are different metrics to quantify exposure to ionizing radiation. The same absorbed dose in tissue can cause different damage relevant to health effects depending on the ionization density of the radiation and the tissues irradiated. For the purposes of radiological protection, the International Commission on Radiological Protection (ICRP) defines radiation weighting factors for particular radiations. ICRP also accounts for tissue sensitivity through the use of tissue weighting factors that are applied to equivalent doses. The effective dose is the sum of the equivalent doses to each tissue/organ, each multiplied by the appropriate tissue weighting factor, is measured in Sievert (Sv), and approximates to the whole-body dose of gamma-rays that would produce the same excess risk.9 However, the effective dose is a radiologic protection quantity averaged over sexes, ages at exposure and populations, and outside this context it must be used with caution.
EXPOSURE TO NATURAL SOURCES OF IONIZING RADIATION
The main radioisotopes of radon found in the environment are 222Rn (in the 238U decay chain) and 220Rn (“thoron”, in the 232Th decay chain); exposure to 222Rn comprises >90% of radon exposure in the United States. The decay products of inhaled radon deliver a dose predominantly to the lung, increasing the risk of lung cancer, and there is strong evidence for a linear dose–response. Residential radon exposure is by far the most important source of natural background radiation. For each increase of 100 Bq/m3 of radon activity concentration, lung cancer risk increases by around 16%.7 This prompted the World Health Organization (WHO) to propose a reference level for indoor radon that should not exceed 300 Bq/m3, with a target level below 100 Bq/m3.14 Different countries have progressively lowered their action levels for indoor radon15 while the United States has maintained an action level of 148 Bq/m3 since 1987.16 Radon exposure strongly depends on the geologic nature of the underlying rock and therefore is highly variable; the dose received can vary by orders of magnitude depending on the place of residence. In the United States, the highest concentrations occur in the Rocky Mountains and the Colorado Plateau, and also in the Appalachians. In Europe, there are radon-prone areas in Northwest Spain, the French Massif Central, and Southwest England. Indoor radon can reach high concentrations in dwellings but also in certain workplaces.
The source of cosmic radiation is the sun and beyond. On earth, we are shielded by the atmosphere; so, the less air above us the more we are exposed to cosmic radiation, e.g., someone living at Mexico City receives three times more cosmic radiation than someone living at sea level.17 This also means that aircrews receive higher doses, and they and frequent flyers can receive higher doses than from all other sources. The highest doses are received by astronauts, and this is a major consideration in any trip to Mars.18
There are some parts of the world, such as Kerala in India, where high levels of terrestrial gamma radiation are to be found because of local geology, and some of these areas have been studied epidemiologically, but unlike radon, large variations are unusual. Some foodstuffs and water contain higher levels of radionuclides than others, but generally the variation is not great.
EXPOSURE TO ARTIFICIAL SOURCES OF IONIZING RADIATION: MEDICAL PROCEDURES
As explained above, human exposure to anthropogenic ionizing radiation comes from different sources, but the most relevant without any doubt is that due to medical procedures. Other artificial sources, apart from those rare exposures due to accidents or nuclear bombings, are generally small and related to exposure to consumer devices (e.g., smoke detectors), occupationally (e.g., nuclear workers), certain industrial uses, and radioactive contamination.
Radiotherapy uses high (cell-killing) doses that, in general, involves localized exposures of diseased tissues and is experienced by a small proportion of the population, but lower diagnostic exposures of increasing numbers of people are occurring in economically developed countries. With the advancement of medical technology, new imaging techniques have been developed, including the use of ionizing radiation, e.g., computed tomography (CT) scanning. The availability of equipment, and also the frequency of use, has also increased in recent years. The American Cancer Society has provided effective doses for some imaging tests for comparison with doses from natural background radiation19 (Table 1). The use of imaging is also increasing for disease screening (e.g., lung cancer, coronary calcification).
HOW MEDICAL PROCEDURES HAVE INCREASED POPULATION EXPOSURE TO IONIZING RADIATION: FROM INFREQUENT EXPOSURE TO EXTENSIVE USE
The amount of radiation received through the use of some of these medical procedures is not negligible. For example, a healthy US woman, 70 years old, might have received an annual mammogram from 45 to 54 years and then an additional mammogram every 2 years, totaling 17 mammograms for breast cancer screening.20 The same woman might also undergo lung cancer screening if she fulfills inclusion criteria for such screening. In the United States, it is recommended that lung cancer screening should be performed annually starting at 55 for smokers of >30 pack/years,20 so she could have received at least 15 low-dose CT (LDCT) scans of the chest, but it is also true that ~1 in 4 screenings has a positive result21 requiring a follow-up usually consisting of further LDCT scans and other procedures such as a contrast-enhanced CT scan. Therefore, we can assume that this woman might have received around 20 LDCT scans. Overall, this diagnostic ionizing radiation is equivalent to ~15 years of exposure to average background radiation (assuming that no further imaging tests using radiation have been performed).
But a further aspect has to be considered for this woman, which is her actual exposure to natural background radiation. If she lives in Idaho, then according to NCRP Report 160,1 her average annual effective dose from residential radon would be about 1.1 mSv, whereas if she lives in Utah this dose is around 4.5 mSv—these are geographically averaged doses, and individual homes can be found with much higher levels of radon.
Table 2 shows the total effective doses received from different exposure scenarios, according to different radiation sources taken from NCRP Report 160 together with American Cancer Society estimations for medical procedures (Table 1). The average annual effective dose can range from around 2 mSv with low indoor radon concentrations and no medical imaging (Scenario 1) to 24.7 mSv for a woman living in a high radon concentration home (300 Bq/m3) and who undergoes medical screening for breast and lung cancer and has an additional pelvis CT scan (Scenario 5), a situation fulfilled by many in the United States nowadays. Despite this relatively large variation in the annual effective dose of ionizing radiation, what must be borne in mind is not a message of avoiding radiation-based medical procedures, with their associated health benefits, but an effort to mitigate those homes with high radon levels because on average in the United States more than half of natural background radiation comes from residential radon exposure.
The fact is, nonetheless, that although the radiation dose from a given medical procedure has been progressively reduced with improving technology, the frequency of using such procedures has increased and new procedures have been introduced, and in the end, the radiation dose received overall by the population is increasing.
POINTS FOR DISCUSSION
From the points raised above, it is clear that the average exposure to ionizing radiation is increasing, particularly due to the extensive use in economically developed countries of medical imaging procedures. Since the benefit of medical exposures generally outweighs the risks, effort should be invested in: (1) reducing overall exposure to indoor radon, and (2) introducing technological advances allowing the reduction of radiation exposure when medical procedures are needed. An additional point is the strong evidence of an interaction between tobacco smoke and radon—in fact, radon remediation measures benefit smokers more than nonsmokers.7 There is also evidence that smoking influences the risk from exposure to other types of radiation, such as those used in medical procedures. Further, the average age of the population in high-income countries is increasing, and therefore the probability of receiving medical procedures using ionizing radiation is higher, underlining the need for an appropriate use of these procedures. The average overall annual effective dose received by North Americans is more than double that received by the British (6.2 mSv compared with 2.7 mSv)—is the contribution from imaging in the United States medically justified or partly from over-use?
It is interesting to note how international bodies and administrations have progressively reduced the action levels for indoor radon,15 but it is comparatively recently that concerted efforts have been made to raise the awareness of health professionals regarding the potential risks from ionizing radiation used in medical procedures.22 Even so, there are homes in the United States with radon concentrations that are so high as to deliver to their occupants annual effective doses exceeding 100 mSv, and such dwellings need to be found and remediation measures undertaken. A recent General Safety Guide by the International Atomic Energy Agency23 requires the establishment of an appropriate reference level for public exposure to radon that in general will not exceed an annual average activity concentration due to 222Rn of 300 Bq/m3. Nevertheless, studies have observed that of the lung cancer cases attributable to indoor radon exposure, the greater proportion is below action levels because of the log-normal distribution of radon exposure and the linear no-threshold dose–response. This is the reason why the WHO established a lower target concentration of 100 Bq/m3, which ideally should not be surpassed.14
To conclude, medical imaging procedures are necessary on most occasions, but health professionals should bear in mind the potential risks associated with relatively high dose procedures or repeated imaging in making a balanced judgment on their use. This is even more pertinent nowadays with more screening procedures using ionizing radiation to detect diseases, and the risk–benefit balance of such procedures must be carefully thought through. Finally, and the most relevant, we should aim to increase the protection of the population from natural sources of radiation, predominantly radon. We have the knowledge and the techniques, and remediation measures are not expensive, so there is no justification for living in an environment with high radon exposure. This is all the more necessary since population exposure to ionizing radiation from medical uses is most likely to increase in the near future.
ABOUT THE AUTHORS
ALBERTO RUANO-RAVINA is Professor of Preventive Medicine and Public Health at the University of Santiago de Compostela and a Fulbright Senior Scholar. He has performed extensive research on the effects of indoor radon on lung cancer and was a member of the International Radon Project leaded by WHO. He is particularly interested on the effects of radiation on health linked to lung cancer screening.
RICHARD WAKEFORD is Professor in Epidemiology at the Centre for Occupational and Environmental Health at the University of Manchester, and specializes in radiation epidemiology and radiation risk assessment. He is a member of national and international expert advisory groups, including Committee 1 of the International Commission on Radiological Protection and the UK delegation to the United Nations Scientific Committee on the Effects of Atomic Radiation.
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