Two things first.
- Far from settled, causes of cancer continue to be vigorously debated ( ).
- Not radiation per se, rather is considered damaging to tissues.
With respect to the cancer risk from ionizing cosmic radiation, this answer briefly summarizes
- Broad outlines of the dispute between competing models.
- As an illustrative example, epidemiological data that suggest flight personnel, a unique population group routinely occupationally exposed to higher levels of ionizing cosmic radiation, may have a two-fold increased risk for melanoma though surprisingly, cosmic ionizing radiation may not be the relevant risk factor.
Ionizing Radiation & Cancer Risk: Brief Consideration Of Competing Models
The relationship between dose and frequency of ionizing radiation on one hand and cancer on the other is neither straightforward nor settled, and is even a matter of considerable controversy and dispute.
A mainstay through much of the 20th century, the LNT () model presumes a strictly linear relationship between substance (radiation, potential toxin) dose and toxicity, higher the dose, higher the risk of damage, with no dose considered damage-free.
Some trenchant, even downright cutting, analyses suggest (2,, 4, ) that unquestioning acquiescence to the LNT model has been a travesty when defining cancer risk as well as for toxicology and public health policy in general.
The article () by in particular is a highly entertaining foray into the genesis of the LNT in the mid-20th century, how it was contrived as an attempt to explain the mechanism for evolution and how though it failed in that purpose early in its life, it got a second wind ‘in service of and application for environmental risk assessment‘.
OTOH stands the concept of(from the Greek word meaning ‘to stimulate’), which favors a biphasic response, where the possibility exists that sometimes lower doses might even confer health benefits while doses above a certain threshold would be increasingly damaging.
The principle of hormesis holds that homeostatic feedback mechanisms in complex organisms allow for over-compensation in response to low doses of various stressors, a result of inevitable time lags that different tissues and organ systems take to communicate with each other (6).
A modification of the hormesis model expands the idea further, ascribing beneficial effects to a hormetic zone with doses both below and above it considered toxic (see below figures that compare LNT, Threshold and Hormesis models from a guest post by Dr. William Sacks on the blog,, maintained by and a modified version of the hormesis model from ).
In the radiation field, the hormesis idea is called( ), that contrary to the assumption of a strictly linear relationship between radiation dose and cancer potential, below a certain threshold, low radiation doses might reduce cancer incidence while doses above that threshold would increase it. Mechanisms by which low radiation doses might help stave off cancer include some largely mutually exclusive possibilities such as
- More effective or appropriate activation of DNA and other repair enzymes.
- Potentially pre-cancerous cells more effectively triggered to commit suicide.
- Immune system activated more effectively or differently to be able to kill off pre-cancerous cells more efficiently.
All that said, as far as I’m aware, consensus statements from major scientific organizations currently do not accept radiation hormesis ().
Ionizing Radiation & Cancer Risk: Brief Consideration Of Empirical Data On A High-Risk Group
Going from theory to empirical data, unsurprisingly, higher the altitude, higher the level of cosmic radiation (). Frequent fliers, particularly flight personnel, are thus an obvious target population to study the relation, if any, between the effect of dose and frequency of exposure to ionizing cosmic radiation and risk for cancer.
As anticipated, many epidemiological studies on varying numbers of individuals (pilots, cockpit crew, flight attendants, other frequent fliers) and of varying quality have examined the link between cancers and flying (12). Problem is it’s difficult if not impossible to separate out the role of occupational versus equally relevant confounding lifestyle factors, which could serve to either increase or decrease cancer risk.
- For one,
disturbance, an integral component of flying could by itself increase cancer risk.
- As epidemiological studies probe the deleterious effect of night shift work on health, circadian rhythm disturbance is becoming a well-known risk factor for cancer in its own stead ( ). For example, a 2018 meta-analysis of 61 studies concluded (see below from 14).
‘confirmed the positive association between night shift work and the risks of several common cancers in women. We identified that cancer risk of women increased with accumulating years of night shift work, which might help establish and implement effective measures to protect female night shifters.’
- For another, flight personnel tend to be healthier compared to the general population with lower rates of hypertension, obesity and smoking, and higher levels of physical activity ( , ). This decreases their cancer risk.
While such confounding factors make it difficult to separate signal from noise, severalhave found increased risk for various skin cancers among flight personnel. A 2015 meta-analysis considered to be the largest thus far as well as among the most robust of such studies examined a total of 19 studies with a total of 266431 participants and concluded (see below from )
‘Pilots and cabin crew have approximately twice the incidence of melanoma compared with the general population. Further research on mechanisms and optimal occupational protection is needed.’
More importantly, both this study () as well as a previous one (17) link increased melanoma risk not to ionizing radiation but rather to ultraviolet light, specifically to UVA. However, while UVA could go some way in helping explain higher risk for pilots, it does not do so for cabin crew. An accompanying editorial explains how ionizing cosmic radiation is unlikely to be relevant as well as how implicating UVA as the relevant risk factor also fails to fully explain (see below from ).
‘Passengers and crewmembers are exposed to higher levels of radiation depending on cruising altitude, latitude, and solar activity.4 Pilots and flight crew receive an additional annual dose of 2 to 9 mSv [milli], 5 which is significantly below the current exposure limit of 20 mSv per year according to the International Commission on Radiological Protection. 3 For comparison, a single chest x-ray provides roughly 0.02 mSv and a computed tomographic scan of the chest, abdomen, and pelvis provides approximately 8 mSv. The lifetime risk of dying of cancer among US adults is approximately 220 in 1000 and would be expected to increase modestly to approximately 223 in 1000 after a 20-year career as an airline crewmember (assuming 80 mSv of aggregate radiation exposure). 4 In addition to the fact that this modest dose of cosmic radiation should have little effect on cancer risk more generally,there is also no known relationship between even relatively high doses of ionizing (cosmic-type) radiation and melanoma. 5 Thus, it seems unlikely that cosmic radiation exposure is relevant in the observed increase in melanoma in this population…
Episodic UV exposure is also associated with an increased risk of developing melanoma, whereas frequent low to moderate UV exposure is more associated with risk for nonmelanoma skin cancers. As discussed by Sanlorenzo et al, 3 a pilot’s exposure to UV-B (280-320 nm) radiation through the windshield is minimal because it is blocked by glass, but more than half of UV-A (320-380 nm) radiation penetrates glass. 3 The additional UV-A exposure for pilots offers a possible explanation for their increased incidence of melanoma but does not account for the increased risk for the cabincrew.’
Bottom line, it appears flight personnel are at higher risk for melanoma while the exact reason(s) still remain unclear.
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