Airlines - A Breath of Fresh Air

The air we breathe onboard an airliner is a 50-50 mix of re-circulated filtered air and fresh air, the latter bled from the engines

At a time when the world is looking at issues related to pollution of the environment from a global perspective, it might seem churlish to seem to expound on the quality of air inside the cabin of an airliner. At risk here are cockpit and cabin crew, who fly on a regular basis and the airline passengers on a breathe-as-you-fly basis. If the debate over hazardous cabin air quality is getting more vociferous, passenger safety concerns and litigious individuals should keep the pot boiling in the years to come. However, it cannot be denied that addressing these issues is important, if not vital to the survival of the airline industry.

Many decades ago, the threat to crew and passengers from the ionising radiation during high altitude flights seemed very real. Add to this, the broadly predictable and periodic solar flare activity to this list and the aviation fraternity saw chronic ailments and cancers looming large among their kin. Fortunately, while theoretically a lot was expected to happen to the frequent flier, no serious harm to the human body has been actually detected so far.

In the 1970s, the previously unacknowledged ozone levels in the passenger cabin became a major issue on some but not on all high-altitude, longrange flights. The passengers and crew reported discomfort in the respiratory system such as nasal congestion, cough, chest pain and breathlessness. Irritation in the eyes, headache and fatigue were some of the other symptoms observed. Studies by the National Aeronautics and Space Administration (NASA) and the Federal Aviation Administration (FAA) established ozone gas as the cause and in 1980, standards for ozone concentrations in the cabin were established. Catalytic ozone convertors were also installed into the system to control the ozone levels in the cockpit and passenger cabin. The problem was seemingly solved at that point of time.

What is it that can be universally acceptable as far as the air quality of the cabin is concerned? The wide consensus revolves around keeping the air in the passenger cabin similar to an office or home environment. Air quality criteria and standards for buildings attempt to meet those of the US National Institute of Occupational Safety and Health (NIOSH), the American Society of Heating Refrigerating and Air-conditioning Engineers (ASHRAE), and the American Conference of Government Industrial Hygienists (ACGIH). Comparing these criteria with those for airplane cabin air quality shows that the airplane is far ahead.

Control of the airplane cabin atmosphere needs a different approach. Continuous improvement in air-conditioning and pressurisation systems may help improve matters, but could also throw up newer, unexpected hazards. In fact, apprehending the misuse of their standards, ASHRAE Technical Committee formed a new subcommittee for aircraft in 1994. Consisting of flight crews, manufacturers, component suppliers, government regulators and health experts, its stated mission was to examine in detail all possible causes of symptoms reported by flight attendants and passengers.

According to a CNN report, in August 2009, a British study for the House of Lords determined that fume events occurred in one out of every 2,000 flights. In the US, airlines are required to report fume events to the FAA and in 2008 there were 108 such events registered. Following exposure to a fume event, while one cabin attendant reportedly complained of debilitating migraine headache, tremor and blind spots in the eyes, the airplane manufacturer, Boeing, stood by the safe and healthy quality of their cabin atmosphere while acknowledging that the potential for occasional bleed air contamination had been known for many years. Needless to add, Boeing dismissed any responsibility for the complaint made by the cabin attendant.

The CNN report also referred to a study of 27 British pilots who reported difficulty in information processing and degraded reaction capability possibly as a consequence of breathing contaminated air. The author of the study also confirmed that the pilots “appeared to under perform on tasks requiring attention, speedy information processing, low reaction time and macro-level decision-making.” How the author of the study excluded bias of long-term use of sugar substitutes, fatigue, motivation and plain ageing/age related factors, is not known, but it could not have been easy.

Cabin Air-Conditioning System

With the attainment of higher altitudes by passenger aircraft, cabin pressurisation became a physiological necessity. With cabin pressurisation and air-conditioning, came the risk of slow or rapid de-pressurisation. The latter also referred to as explosive decompression and the possibility of contaminated air in the cabin.

From a layman’s point of view, the air we breathe onboard an airliner is a 50-50 mix of recirculated filtered air and fresh air, the latter bled from the engines. The filters are meant to keep out bacteria, viruses and are similar to those used for regulating the quality of air in intensive care wards of hospitals. The bleed air has the potential to bring in fumes containing known and unknown toxic products that could possibly be released by the engines even while functioning normally. A part of the ambient air captured by the compressor in the engine is hot and at a high pressure is cooled, filtered and fed into the cabin. Air circulation in the passenger cabin follows well-designed patterns of air entry and exit. As the flow pattern is radial, it restricts the circulation of air to a row or two seats at the most. There is, therefore, no risk of a passenger in the back rows being exposed to contaminated/infected air from the forward section.

A modern cabin air-conditioning system provides approximately 20 cubic feet per minute of air per passenger. Half of this is fresh air drawn from the outside and the other half consists of recycled air. A complete cabin air change will occur every 2-3 three minutes. In an hour, there would be 20-30 change cycles. The new Boeing 787 Dreamliner has an air-conditioning system that draws its inspiration from the ambient air and not engine bleeds. Clearly such an innovation will automatically exclude hydrocarbon residues or other toxic substances entering the passenger cabin from the engines.