A number of possible health hazards of air travel have been investigated.
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Infection
On an airplane, people sit in a confined space for extended periods of time, which increases the risk of transmission of airborne infections. For this reason, airlines place restrictions on the travel of passengers with known airborne contagious diseases (e.g. tuberculosis). During the severe acute respiratory syndrome (SARS) epidemic of 2003, awareness of the possibility of acquisition of infection on a commercial aircraft reached its zenith when on one flight from Hong Kong to Beijing, 16 of 120 people on the flight developed proven SARS from a single index case.
There is very limited research done on contagious diseases on aircraft. The two most common respiratory pathogens to which air passengers are exposed are parainfluenza and influenza. In one study, the flight ban imposed following the attacks of September 11, 2001 was found to have restricted the global spread of seasonal influenza, resulting in a much milder influenza season that year, and the ability of influenza to spread on aircraft has been well documented. There is no data on the relative contributions of large droplets, small particles, close contact, surface contamination, and no data on the relative importance of any of these methods of transmission for specific diseases, and therefore very little information on how to control the risk of infection. There is no standardisation of air handling by aircraft, installation of HEPA filters or of hand washing by air crew, and no published information on the relative efficacy of any of these interventions in reducing the spread of infection.
Air travel, like other forms of travel, may also increase the speed at which infections spread around the world.
Deep vein thrombosis
Deep vein thrombosis(DVT) is the third most common vascular disease next to stroke and heart attack. It is estimated that DVT affects one in 5,000 travellers on long flights. Risk increases with exposure to more flights within a short time frame and with increasing duration of flights. According to a health expert in Canada, even though the risk of a blood clot is low, given the number of people who fly, it is a public health risk. It is reported in 2016 that the average distance between seat rows has declined to 79 centimetres, from over 89 centimetres, while the average seat size has shrunk to 43 centimetres from 46 centimetres in the previous two decades.
Other possible health hazards
Other possible hazards of air travel that have been investigated include increased radiation from space, airsickness and chemical contamination of cabin air.