Tightslot, there are at least three possible reasons for CC not getting ill very often.
The first is that there is a lot of engineering in the commercial jet airliner environmental control system (ECS) to stop the bugs from being circulated. Mark in CA mentioned this. From Leder and Newman (reference at the bottom):
Although concern has been raised about air quality and spread of respiratory pathogens on aircraft, studies of ventilation systems and patient outcomes have suggested the dissemination of pathogens occurs rarely. This is because outside air entering the cabin at altitude is essentially sterile, heating/cooling further reduces microbial risks, HEPA filters remove microorganisms from recirculated air and the low humidity, high airflow rates, laminar airflow pattern and frequent air exchanges incorporated into the cabin ventilation and pressurization systems further minimize microbial contamination on board aircraft.
Secondly, the air circulation area within the cabin is limited. There is a common misconception amongst the travelling public is that if one person on board an aircraft has an infection, then all other passengers are at risk. Because air flow is generally from top of the cabin to the bottom, with little if any front to back flow, and because respiratory pathogens are diluted by frequent air exchanges, passengers at most risk are those in close proximity to the infected passenger, with minimal risk for others. So CC moving around the cabin are only in close proximity with a sick passenger for short periods at a time.
Op.cit.
The air enters the distribution pipework for delivery to the cabin. Cabin air is taken from below the floor of the aircraft to the overhead cabin ventilation system, which runs the length of the cabin. The ventilation system is usually designed so that air entering the cabin at a given seat row is exhausted at the same seat row. This limits the amount of air flowing in the fore and aft directions (i.e. towards the front and back of the aircraft, respectively), which also helps minimize infection risk.
Pinkman quoted the JEHR paper, and this supports both those points: it mentions that in that a passenger survey conducted by Zitter and coworkers suggests that cabin air recirculation has little impact on the incidence of colds.
The third reason could be differences between CC and passengers:
- is it possible that CC are less likely to go to work with upper respiratory tract infections whereas passengers might still travel even if they are slightly unwell,
- differences in stress and fatigue differences as Pinkman is suggesting,
- and/or for those of you that have read Pinkman's link to the JEHR paper on "Common cold transmission in commercial aircraft: Industry and passenger implications", it raises the real possibility that CC have developed an ability to resist infection while flying - specifically because CC developed improved resistance to the effect of exposure to dry air reducing the nasal mucociliary clearance. I have the two papers by Salah et al, and by Barry et. al. which are in the references to the JEHR paper. The Salah paper notes that there are "large inter-individual variations of the saccharin nasal transit time in our subjects, an observation already reported [21, 22], but the reasons for such large differences are still unknown." Basically some people are more affected by dry air, and their nasal mucociliary clearance system is reduced or stopped and that increases susceptibility to colds. Others are less affected, and I am wondering if this is a response learned over time and exposure by CC.
If you haven't read the JIEH paper, then let me extract a bit for you:
The natural human defence system against colds is known as the Mucociliary Clearance System, which consists of a layer of thin mucus that is kept in motion by beating cilia. This protective system traps viruses and bacteria and moves them from the nose and throat to destruction by acids in the stomach. However, when the air is dry, the mucus becomes too thick to be effectively moved by the cilia. This leaves more viruses and bacteria to cause upper respiratory tract infections. The typical relative humidity in aircraft cabins for flights over an hour is below 10% for most of the journey, often dropping to less than 5% on longer flights. It has been shown experimentally, using saccharin, that under these conditions the Mucociliary Clearance System either slows dramatically or stops (Barry et al, 1997, Salah et al, 1988). This would suggest that it is the low relative humidity in aircraft cabins that increases susceptibility to colds rather than a higher viral load in the air.
I guess after all this I can re-write my earliest post and say:
Look, it is not bugs in the cabin air that is the problem. Studies of airplane ventilation systems and patient outcomes have suggested the dissemination of pathogens occurs rarely. However the dry cabin air reduces the natural human defence system against colds, and your Mucociliary Clearance System either slows dramatically or stops. Fatigue and stress play a role. When you respond to pressure changes in the cabin by clearing the blocked eustachian tubes etc and all the grunting and snorting that goes with it, that forces virus laden particles back down the bronchus and oesophagus when you perform the valsalva maneuver to equalize. Looking at the graph of cabin pressure vs altitude, I think the problem time is on descent after hours of flying in very dry air. It is your own bugs being pushed around by varying cabin air pressure. Your defences against that have been given by Pinkman. And there are various ways of boosting the immune system.
REF: K. LEDER and D. NEWMAN Respiratory infections during air travel. Internal Medicine Journal 2005; 35: pp50–55.