Cosmic Ionizing Radiation: A DNA Damaging Agent That May Underly Excess Cancer in Flight Crews

Abstract

In the United States, the Federal Aviation Administration has officially classified flight crews (FC) consisting of commercial pilots, cabin crew, or flight attendants as "radiation workers" since 1994 due to the potential for cosmic ionizing radiation (CIR) exposure at cruising altitudes originating from solar activity and galactic sources. Several epidemiological studies have documented elevated incidence and mortality for several cancers in FC, but it has not yet been possible to establish whether this is attributable to CIR. CIR and its constituents are known to cause a myriad of DNA lesions, which can lead to carcinogenesis unless DNA repair mechanisms remove them. But critical knowledge gaps exist with regard to the dosimetry of CIR, the role of other genotoxic exposures among FC, and whether possible biological mechanisms underlying higher cancer rates observed in FC exist. This review summarizes our understanding of the role of DNA damage and repair responses relevant to exposure to CIR in FC. We aimed to stimulate new research directions and provide information that will be useful for guiding regulatory, public health, and medical decision-making to protect and mitigate the risks for those who travel by air.

Keywords: DNA damage; DNA repair; cancer; cosmic ionizing radiation; flight; flight attendants; pilots.

Figure 1

Components of CIR. Approximate proportions of the main components of CIR that occur in the atmosphere at aircraft altitudes and temperate latitudes. However, the composition of radiation inside the aircraft is expected to differ [43].

Figure 2

Exposure to cosmic ionizing radiation (CIR) that occurs at the cruising altitudes of aircraft. The heliosphere, Earth’s atmosphere, and the magnetic field provide some protection (approx. 90%) from exposure to CIR. On an average, circumpolar flights (shown by the green plane shape) operate at aviation cruising altitudes of 35,000 feet or above. The effective dose rate from exposure to the remaining CIR that passes through the stratosphere (including the peak level of approx. 65,000 feet known as the Regener–Pfotzer maximum) is directly proportional to the cruising altitude of the aircraft. Figure created in BioRender.com accessed on 1 July 2024.

Figure 3

Regulatory oversight on exposure to cosmic ionizing radiation in flight crews. Timeline of considerations and amendments made by various regulatory agencies for exposure to occupational radiation in flight crews.

Figure 4

Radiosensitivity of human tissues and organs. Cell proliferation is generally inversely related to sensitivity to radiation in human tissues (this figure is adapted from [115], with a modification on the left to reflect the relative frequency of cell division in the various tissues). We note that there are exceptions to these generalizations; for example, while most adult neurons are post-mitotic, some neurogenesis occurs even in adulthood [118,119,120,121].

Figure 5

Proposed model relating air travel, DNA damage and repair, and cancer risk. FC can be exposed to various DNA-damaging agents during air travel, such damage arising directly (dashed arrow) from flight-related exposures (radiation, physical, and chemicals) and damage arising indirectly (solid arrow) from biological processes that are disrupted during flight (metabolism, respiration, oxidative damage, inflammation, circadian rhythm disruption). Unrepaired DNA damage due to inhibition of or defects in the DNA repair mechanisms (dotted arrow) leads to genomic instability in cells that escape from cell death. Mutations can lead to the initiation of cancer.