vick said:
Hi everyone, I am a long time lurker on this board, and I am QF platinum.
Just thought I would correct NM here. The cabin air is about 50% recycled, mainly in order to save fuel. It is passed through filters, so viral transmission through the recycled air is unlikely.
Vicki
Welcome to AFF Vick. I have done a little more research into this matter and dug up some interesting facts. My original comment is out of date and based on very old (and inefficient) systems found on aircraft like the DC9. Modern aircraft do indeed use some proportion of recycled air in the cabin.
A 20 year old (1986) US congressional study into the health affects of cabin conditions on travellers and cabin crew determined that the rate of flow of
outside air into modern airliner cabins varies from a minimum of 7 cubic feet/min per economy class passenger, to 50 cfm/first class passenger. These rates vary significantly between airliners and operators. The minimum rate to meet the oxygen needs for people is about 0.24 cfm/person.
A flow rate of 7cfm/passenger (the lowest possible with current equipment design) results in approx 50% recycled, filtered air. So 50% is a worst case scenario. Note that 50% is similar to commercial building air-conditioning, so the time spent in the airport terminal is likely to result in more exposure to recycled air than the time spent in the cabin. Its just that we don't usually spent 14+ hours at a time in the terminal!
The study mentioned above found that the flow of outside air into the cabin for a 747-200 with 318 passengers and using 3 ECUs set to "normal" flow was 17.1 cfm/passenger (in economy cabin), and 10.6 cfm/passenger when set to 50% flow. These are significantly higher than the minimum of 7cfm/passenger mentioned above. On a 767 with optional filters installed, the flow rate was even higher at 19.1 cfm/passenger.
A 1980 study by NASA shows that a DC-10 could experience about 1% fuel saving if the ventilation from of outside air was reduced from 18cfm/passenger to 8 cfm/passenger.
The use of high-pressure water separators in the modern ECUs results in very dry air from the bleed system. The lack of moisture in this air results in less heating affect during pressurisation, and as a result the pressurised air is often still below 0 deg C. To heat this pressurised dry bleed air, it is mixed with recycled cabin air before being fed to the cabin air inlet ducts.
The recirculated air is taken from different places on different aircraft, but most modern aircraft (eg 747, 767, 777 etc) take it from near the ceiling, while the vented air leaves the cabin through slotted grills at floor level. This is based on the assumption that contaminants are generally heavy particles (at least compared with the low humidity air) and are more concentrated at floor level, while the air at the top of the cabin contains less contaminants.
The proportion of recycled air is higher on newer aircraft due to the need to further heat the very dry air that results from the newer high-efficiency high-bypass engines and high pressure water removal process. However, recycled air filtering treatment has also improved over time.
Environmental Tobacco Smoke (ETS) is no longer a problem on most flights. It was determined that 7 cfm/passenger was insufficient to remove the hazardous affects of ETS. This required about double the flow rate. As can be seen from the NASA study noted above, by banning smoking from flights, the airflow can be reduced and fuel savings achieved. So there was more than just health issues as an incentive for airlines to ban smoking, or at least to support the government regulations.
The low relative humidity in the cabin, generally less than 20% and can be as low as 2% depending on the circumstances, may cause passengers some discomfort after prolonged exposure. After 3 to 4 hours of exposure to 5-10% relative humidity, passengers may suffer from dryness of the eyes, nose and throat. The use of high-pressure water separators in the modern ECUs results in even lower cabin humidity.
Of course there is a trade-off. High rates of outside air intake result in lower concentrations of carbon dioxide, carbon monoxide and other contaminants. But it also results in lower relative humidity and increased ozone when operating at certain altitudes.
Of more concern to me is the low relative humidity and exposure to cosmic radiation (for very frequent travellers). The low humidity is an efficiency issue since moisture in the air adds weight to the aircraft and hence more lift is required to maintain flight resulting in higher fuel burn. It is this low humidity that causes discomfort on long flights.