While Pinkman gathers his references, I looked again at the HEPA filters. What we have discussed above is that the use of appropriate filters and correct recirculation of air in the plane reduces the risk of infection. Although the safety of HEPA filters in protection against viruses has been questioned, (Gammaitoni I, Nucci MC. Using a mathematical model to evaluate the efficacy of TB control measures. Emerg Infect Dis. 1997;3: 335-42), a more serious concern is the absence of legislation obliging their use in most countries.
HEPA filters were found not to be used on 15% of flights carrying more than 100 passengers in the USA, and that figure is considerably higher in small planes that undertake local flights (United States General Accounting Office. Aviation safety: more research needed on the effects of air quality on airliner cabin occupants. [Cited 2005 Sept 26] January 2004, Washington, DC. Available from:
http://www.gao.gov/new.items/d0454.pdf). I just downloaded this, and the actual paragraph is:
Several technologies are available today that could improve cabin air quality, (e.g., increasing cabin humidity and pressure or absorbing more cabin odors and gasses); however, opinions vary on whether FAA should require aircraft manufacturers and airlines to use these technologies. GAO found that one available technology, high-efficiency particulate air (HEPA) filtering, was strongly endorsed by cabin air quality and health experts as the best way to protect cabin occupants' health from viruses and bacteria in recirculated cabin air. While FAA does not require the use of these filters, GAO’s survey of major U.S. air carriers found that 85 percent of large commercial airliners in their fleets that recirculate cabin air and carry more than 100 passengers already use these filters. However, the use of HEPA filters in smaller commercial aircraft that carry fewer than 100 passengers is much lower. The cost to retrofit the smaller aircraft to accept the HEPA filter, if it were made mandatory, could be expensive.
OK, the smaller (<100 pax) planes are not likely to be doing extended flights, but that still leaves the 15% to think about.
I wonder if this figure is reflecting the USA fleet composition in 2004, and now in 2013, planes with air recirculation and without HEPA filters have been retired?
And I wonder what it means elsewhere in the world? In Asia/Europe/Australasia, I would guess that the advent of LCCs with fleets of sparkling new 737NGs and A32x aircraft has meant that almost all aircraft >100 pax have HEPA filters, but that is only a guess and is open to someone more knowledgeable to comment.