Definitions, frequent flyer. “Some frequent flyers may fly at a frequency that is of the order of magnitude of a typical aircraft crew (e.g. 500 h a year)”. I am not sure that the value 500h a year represents frequency. Suggested change: “Some frequent flyers may reach a cumulative, annual flight time that is of the order of magnitude of a typical aircraft crew (e.g. 500 h a year)”.
Definitions, occasional flyer. “A person who travels by air from time to time, generally for personal purposes or professional duties”. The second part of the sentence is obsolete. Everybody fly for either personal or professional purposes. Suggested change: “A person who travels by air from time to time, reaching an annual flight time distinctly below that of a typical aircraft crew. ”
Definitions, Excess relative risk. “Excess relative risk (ERR) is the difference between a risk factor and a specific outcome. For example, ERR could be the difference between the proportion of subjects in a population with a particular disease who were exposed to a specified risk factor and the proportion of subjects with that same disease who were not exposed”. This definition is not clear to me. Moreover, ERR does not describe the difference, but the ratio. Why not use the following definition: “The excess relative risk (ERR) is an epidemiological risk measure that quantifies how much the level of risk among persons with a given level of exposure exceeds the risk of non-exposed persons. Excess relative risk is expressed as relative risk (RR) minus one.”
Table 1. It would be interesting and useful to mention the energy spectrum of neutrons coming from cosmic radiation.
2.5. Exposure of aircraft crew. It would be interesting and useful to mention the number of hours per year that this cohort spends in the air. I know that the maximum is 1000, but are there differences between airlines in the avarage value?
Figure 6 and Table 3. Where do the differences between the countries come from? The flight time or the flight routes? It would be useful to provide some explanation.