Melanoma in Aircrew and Defence Members: A Narrative Literature Review

Introduction

Melanoma is a malignant skin tumour with the potential to metastasise¹ and lead to premature death. While classical risk factors, such as a tendency to sunburn and a phenotype of pale skin, hair and eye colour, are well described, the literature also reports several groups with a higher incidence of melanoma. These include aircrew and Defence Force members. The importance of exploring an association between occupational factors and an elevated risk of melanoma is critical. If found, this may have implications for informing workers – past, present and future – about risks they may face. There are implications concerning selecting workers for specific jobs and developing and improving methods to protect them from melanoma. In addition, there are implications related to the monitoring for the occurrence of disease and actions to be undertaken if melanoma does occur. Conversely, if a link between melanoma and occupation is not found, such information can reassure those involved. This literature review aims to explore the published literature on melanoma in occupational groups, focusing on aircrew and Defence Force members.

Materials and methods

This work presents a narrative scoping literature review on melanoma focusing on aircrew and Defence Force members. This style was chosen to permit the location and inclusion of as much information as possible through not being limited to a particular research question. However, as narrative reviews are vulnerable to limitations in several areas, including reproducibility, the search method is outlined here. Using terms related to melanoma and aviation, PubMed was searched in June 2022 (Table 1).

Table 1: Search terms used for melanoma in aircrew and Defence Force members

The title and abstract (where available) for each paper in English were reviewed. The full text was obtained (where available) for all papers that discussed an element of melanoma concerning aviation or an occupation or hobby considered relevant (for example, one that discussed occupational exposures such as oils that aircrew may be exposed to). Additional articles were reviewed while preparing the literature review until Dec 2022, amounting to over 200. Data was extracted from articles and arranged into the themes presented in this review. Articles included original research articles, yielding empirical data^2–4 and review articles.⁵ Article reference lists were also reviewed, and any considered to contain relevant information based on the title review were obtained and included.

Consistent with a narrative literature review, a quality assessment of articles was not performed. Findings are presented in three themes: Occupation and melanoma, Melanoma and aircrew, Melanoma and Defence Force members.

Results

Occupation and melanoma

‘Since exposure leading to skin cancer takes place over a long period of time (in most cases several decades), differentiation between occupational exposure and recreational exposure is problematic.’⁶

Determining a relationship between melanoma and occupation is not a simple process and is further complicated by the increasing incidence of melanoma, which occurs with age.⁷ As skin cancer has not been regarded as a traditional occupational condition, it is likely that its incidence as a potential occupational disease is under-reported.⁶

While UVR is the most accepted risk factor for melanoma,¹ and recognised by the International Agency for Research on Cancer to be carcinogenic to humans,⁸ it is not universally accepted that workplace exposure leads to the development of melanoma. For example, a 45-year historical cohort study from five Nordic countries reported that occupation provided only a modest input to the relative risk of melanoma, with a lowered risk seen for outdoor workers.⁹ In this work, the highest risk of melanoma for males was seen in dentists, with others at risk, including Defence Force personnel. However, the local context, including the relatively low UVR levels in the Nordic climate⁹ and other confounders, including socioeconomic status, are relevant and may differ markedly from the Australian setting.

Beyond UVR, the literature also reports many other examples of associations between non-solar factors and melanoma development. These include various chemical compounds (e.g. polychlorinated polycyclic aromatic hydrocarbons, benzene and polychlorinated biphenyls [PCBs]),¹⁰ non-solar radiation, including ionising radiation, and trauma.¹¹ A literature review focusing on non-solar occupational risk factors for melanoma highlighted a wide range of industries of concern, from the petroleum, printing, electronics and electrical, to chemical, clothing and metal industries. Workers in biomedical research, farming and agriculture, and aircrew and radiation workers have also been shown to have increased rates of melanoma.¹⁰ Subsequent reports have highlighted many other occupational and hobby groups as potentially associated with an increased risk of melanoma, including firefighters,^12–14 police force members,^13,14 and again, pilots and aircrew.⁵

While the relationship between occupation and an elevated risk of melanoma is challenging to determine, the importance of investigating this for the health of the vast array of workers potentially at risk is evident.

Melanoma and aircrew

The incidence of cancers in aircrew has been the focus of numerous studies over many years. Several have reported greater than expected levels of some cancers, including melanoma and propose and explore possible reasons for this. Several highlight potential occupational exposures that aircrew face, including some of the hazards outlined in the previous section. Exposure to ionising cosmic radiation¹⁵ is considered in many. The fuselage of an aircraft does not completely shield against ionising radiation,¹⁸ (including gamma rays and neutrons from galactic and solar radiation sources^16,17) which is present at cruising altitudes¹⁹ and results in doses above those experienced by the general population. Aircrew likely have the highest radiation exposure of any occupation.¹⁸ The radiation levels aircrew are exposed to vary with many factors, including the type of flight and number of hours flown,²⁰ the latitude, the altitude and the time of flight,¹⁷ with greater exposures occurring during flights at high altitudes and closer to polar regions.²¹ Other variables, such as solar flares, also affect exposure.²¹ Personal conventional dosimeters unfortunately do not detect all components of radiation exposure,^20,22 so they have limitations in recording aircrew exposures. While program dose estimation software is reported to be accurate in providing an estimate of individual exposure doses,²² authors have shown that calculated data may underestimate the radiation dose crew are exposed to, owing to significant variations in solar flares.¹⁷

Aside from ionising radiation, the other potential health hazards that aircrew may be exposed to as part of their work include altered sleep patterns,²¹ alternation to circadian rhythm (though others argue against this),²³ electromagnetic fields from instruments in the flight deck, chemicals including jet fuels and engine emissions, pesticides and previously also, passive smoking.²⁰ A combination of these factors may also be relevant. For example, aircrew are often exposed to ionising radiation and disturbance in circadian rhythm together,²⁴ contributing to each challenging to determine.

Of particular relevance to melanoma, several recent works have measured UVR levels in the flight deck during flight. While UVB and UVC radiation were not detected, UVA was seen to be present in several aircraft,^25,26 though at lower levels than at the ground.²⁶ However, Sanlorenzo and colleagues’ earlier work contrasts with some of these findings, as they report that pilots flying at 30 000 feet for just under an hour would receive an equivalent UVA dose to that received from 20 minutes in a solarium. They note this could be even greater when flying over snow or thick cloud due to the reflection of a majority of the radiation²⁷ and pose a real risk to the crew.

Determining whether the multitude of varying risk factors that aircrews face translate into the subsequent disease development is critical to investigate.

As outlined, several studies have reported on the occurrence of melanoma in aircrew, with an increased incidence seen compared with that of the general population or particular subgroups. Studies have employed various data collection methods, including self-report only in surveys or interviews,²⁸ or data from cancer registries.^29,30

Several published works, including systematic reviews and metanalyses, have reported pilots and cabin crew to have approximately twice the incidence of melanoma compared with that of the general population.^5,15,31 Concerningly, Miura and colleagues’ systematic review also found that pilots were twice as likely to die from melanoma compared with the general population.⁵ A limitation of many of the included studies relates to the age and data sources – most being from decades ago, largely from the Northern Hemisphere and with male data often predominating. In contrast, a more recent study on Australian commercial pilots for 2011–16⁴ did not find an increase in rates of invasive melanoma compared with the general population and only a modestly raised incidence of melanoma in situ. This was suggested to relate to increased surveillance with the lack of variation of melanoma on the body site compared with the general population arguing against an occupational cause.⁴ The contrasting findings of this study to most previous ones may be explained by the higher baseline of UVR exposure in Queensland, Australia, where the greatest proportion of cases resided.

Melanoma and Defence Force members

Similar to reports with aircrew, some authors have reported a higher incidence of melanoma in members of the defence forces.³² Again, some works have found particular subgroups (e.g. older members)³³ to be at greater risk for melanoma and two^32,34 found Air Force members to have higher rates compared with members of the other services (though it is noted one study did not require histological confirmation of melanoma).³⁴ Compared with civilian workers, members of the defence forces may face extended and more extreme working conditions and a reduced ability to use sun-protective methods as designed or recommended,³⁵ which may lead to an increased risk of disease.

However, as with melanoma incidence in aircrew, not all studies of melanoma in Defence Force members have shown them to have elevated rates.¹³ In other studies, though a small increase in melanoma incidence may be observed,³⁶ no significant or excess occupational disease was found. Durmishi and colleagues found that defence personnel in Albania self-reported an increased number of risk factors for melanoma (including a history of melanoma) compared with the general population, which may, therefore, confound occupational factors.³⁷ Of the few Australian studies, one reported male Department of Veterans’ Affairs clients to have a statistically significantly increased risk of melanoma but not of increased risk of death from melanoma,³⁸ and another showed melanoma incidence was elevated in Australian male Vietnam veterans compared with the Australian community.³⁹ In contrast, a case-control study of white males from Queensland did not find evidence of an association between previous military service and an increased risk of melanoma.⁴⁰ The pattern of sun exposure and subsequent risk of melanoma is relevant and has been considered in some studies. For example, non-significantly higher mortality from melanoma was seen in US male Army World War II veterans who had been prisoners of war (POWs) in Pacific settings compared with those who had been POWs in Europe or were not POWs.⁴¹ While many potential confounding factors were also present here, the authors argued that this difference in outcome reflects the impact of a high but short duration of UVR exposure in young adulthood and the development of melanoma years later.⁴¹ As with aircrew, a combination of potential exposure risks may be relevant, such as an increased incidence of melanoma and prostate cancer that was observed in a group of US Air Force Vietnam veterans who sprayed dioxin-based herbicides compared with national rates of cancer.⁴²

Within the defence forces, pilots and aircrew were reported to have higher (though non-significant) rates of melanoma compared with other officer groups as early as 1983.⁴³ Fixed-wing pilots and crews saw the highest incidence rates of melanoma in the US Military and an exponential increase in melanoma diagnosis from their 7th to 20th year of service, equating to a 16 times greater rate.⁴⁴ Defence force pilots are reported to be healthier compared to comparable defence members. They are unlikely to receive the same level of exposure to cosmic ionising radiation as commercial aircrews²¹ again, suggesting that other factors may be relevant if a difference is present. The incidence of melanoma in aircrew, compared with that in other US Air Force officers and the US general population, was the subject of a recent US Congress directive.⁴⁵ This review of Air Force fighter aviators who served between 1970 and 2004 found that male aircrew were approximately 25 per cent more likely to develop melanoma compared with both groups, equating to 33 additional cases of melanoma in a lifetime for every 10000 aircrew. Concerningly, similar to Miura’s study,⁵ the findings also showed that aircrew were more likely to die from melanoma compared to the male general population,⁴⁵ though at a lower rate.

Discussion

The findings from this literature review show that aircrew and Defence Force members may be at an increased risk of developing melanoma, though gaps in the literature are noted. Most data are decades old, and workplace practices and aircraft have changed in many aspects since much of this was collected. Further, data is mainly from the Northern Hemisphere, and there is comparatively less about females. Many factors potentially confound occupational ones, including recreational exposure to UVR, the leading risk factor for melanoma, which makes interpretation of associations challenging.

However, as occupational melanoma is an increasingly recognised condition,¹² it is essential to further our understanding of the impact of work on health. Currently, in our society, adults working full-time spend around one-third of their lives at work, meaning there is an increased opportunity for potential harmful risk factors to impact negatively on health. As such, this literature review calls for action in two areas.

The first is for further targeted, contemporary research. While prospective observational studies would be ideal for accurately measuring exposure related to outcomes and exploring causal inference, the study period required would be extensive, further delaying any required interventions. As alternatives, robust and carefully designed retrospective studies of aircrew and Defence Force members should be employed, comparing cases to controls and exploring other settings and contexts. Such epidemiological studies are powerful tools to inform public health measures, including changes to policy and practice where required and plans for some such studies are underway.

The second area of action relates to managing those potentially at higher risk of melanoma while further research is conducted. While the hierarchy of controls would focus first on eliminating known hazards, this is not always possible. For example, the nature of work in the defence forces, means limiting exposure to UVR is not always practical.⁴⁶ Defence members and aircrew should be encouraged to follow current advice around UVR exposure outside of the work context, and tailored occupational advice and education, which differs from population advice, is necessary for outdoor workers and those at increased risk of melanoma.⁴⁷ As others outline, such as in Gall et al.,⁴⁶ this should include increased education on sun protection, recognising suspicious lesions (and what to do if found). Optimising the use of sun-protective equipment, including uniforms, is also essential. Some authors suggest a work history that includes service in the defence forces, with its potential for unpredictability of work pattern, especially in the deployment context,³⁵ be considered a risk factor for melanoma, and so prompt more frequent skin examinations, skin cancer screening and early dermatologist involvement for this group.³⁵ Some authors argue that any period of Defence Force service is relevant to melanoma risk, even if the exposure period is short.³⁵

This review has several limitations. As highlighted, studies of aircrew and Defence Force members come largely from Northern Hemisphere settings, and data are primarily from several decades ago. Further, as a narrative review, this work does not seek to answer a specific research question but rather conducts a broad review of published works. The use of one database only may have resulted in missing relevant articles.

Conclusion

This narrative literature review summarises the literature on the risk of melanoma in aircrew and Defence Force members, suggesting that Defence Force members, particularly those working as aircrew, may have an increased risk of melanoma in specific contexts, though many confounding factors are recognised. This review highlights and calls for further work to explore melanoma incidence in these groups, particularly in the southern hemisphere context to address the gaps in the literature. Such work would have the potential to provide an evidence base to inform future practice changes if required, such as increased surveillance, attention to selection criteria for certain occupations or improvements in and tailoring of sun protection education. The ultimate aim is to improve knowledge, behaviours and outcomes concerning occupational melanoma. Optimising care and ensuring this is tailored to occupational settings for those who may be at increased risk of melanoma should proceed in the interim.