Cabin air quality in aircraftBMJ 1994; 308 doi: https://doi.org/10.1136/bmj.308.6926.427 (Published 12 February 1994) Cite this as: BMJ 1994;308:427
- R Harding
Passenger safety during aircraft emergencies attracts considerable attention in the media.1 Fortunately, the dramatic is also the rare, so for most air travellers emergency considerations are academic. Comfort and wellbeing during normal flight are, however, of concern to all, and recently the quality of the environment in aircraft cabins has come under scrutiny. This has followed reports that nausea, headaches, and mucosal irritation are common; that poorly ventilated cabins may spread disease among passengers; and that environmental contaminants such as tobacco smoke may increase the risk of respiratory illness. Thus aircraft manufacturers devote much attention to national regulatory requirements for ventilation, pressurisation, and the composition and filtration of cabin air.2
Until the late 1980s about 0.57 m3 fresh air was delivered to the cabin per person per minute. In modern aircraft this figure has been halved although the total requirement remains the same: consequently, the rest is recirculated air. It was concern about contamination of recirculated cabin air with environmental tobacco smoke, and the increased awareness of the risks of passive smoking, that was partly responsible for the widespread ban on smoking in aircraft in the late 1980s,3,4 though pressure from passengers was also important. In 1989 the United States Department of Transportation commissioned a study into environmental tobacco smoke and other potential pollutants of cabin air - carbon dioxide, microbial aerosols, and ozone.5
Raised concentrations of carbon dioxide in closed environments result mainly from human metabolism so that the presence and removal of carbon dioxide are functions of the number of people present and the ventilation rate. The concentration of carbon dioxide can therefore be used as an indication of the efficiency of ventilation: as the fresh air component falls so the carbon dioxide concentration may be expected to rise. Not surprisingly, therefore, the Department of Transportation study, which looked at 92 randomly selected flights, found that carbon dioxide concentrations were substantially higher than those associated with comfort (<=1000 ppm) and averaged 1565 ppm on flights on which smoking was allowed and 1756 ppm on those on which it was not allowed.5 Other studies have confirmed these findings, although the concentrations have remained well below the time weighted average limit of 5000 ppm recommended by the American conference of governmental industrial hygienists and the absolute upper limit set for commercial aircraft of 30 000 ppm set by the Federal Aviation Administration.
Similarly, concentrations of microbial aerosols never approached those normally associated with a risk to health, as might be expected with the use of modern high efficiency particulate air filters, which are designed with the most difficult particle size (0.3 £m) as a target and have a goal of 91.0-99.9% effectiveness. Ozone concentrations also remained well below the Federal Aviation Administration's three hour exposure standard of 0.10 ppm. Other studies, however, have shown concentrations well in excess of this standard, and catalytic ozone converters are now required if the flight profile suggests that the mandated limits for ozone will be exceeded. As well as being annoying, environmental tobacco smoke has been cited as the cause of in flight headaches; eye, nose, and throat irritation; and breathing problems. In a recent paper, however, the authors argued that coexisting factors such as a low relative humidity, high ozone concentrations, and even hypoxia could equally well be to blame and that evidence to the contrary was not available.6 Of more serious concern is the possibility of a link between environmental tobacco smoke and lung cancer.7 Studies, however, have not so far supported a relation between environmental tobacco smoke (as measured by levels of carbon monoxide and respirable suspended particulates) and chronic ill health of any kind. Furthermore, although increased ventilation and filtration may improve air quality, the volume and flow of circulating air are already high, and segregation of passengers who smoke seems to be reasonably effective in reducing complaints about environmental tobacco smoke during flights. Of course, a total ban on smoking on all aircraft would instantly improve matters subjectively, and such measures are being considered.
Finally, even with the approximate halving in the volume of fresh air entering the cabin, there remains more than enough oxygen for human consumption (estimated at less than 0.003 m3 per person per minute) at a pressure equivalent to that at an altitude of 2440 m, which is the pressure normally maintained in the cabins of commercial aircraft. Nevertheless, even this reduction in pressure leads to a small desaturation of haemoglobin in healthy people. So although the National Academy of Sciences has deemed such a pressure-altitude to be “generally adequate to protect the travelling public,”8 pressurisation to below this is to be encouraged so that those with compromised cardiorespiratory physiology are further protected from the effects of even the mild hypoxia experienced at normal cabin altitudes.9
Thus, the general malaise associated with air travel is probably multifactorial in nature, with the quality of cabin air undoubtedly contributing. But other considerations such as the motion of the aircraft, jet lag, and even the difficulties associated with airport procedures cannot be ignored. It is reasonable to conclude that environmental tobacco smoke and other cabin contaminants do not represent a great threat to health.