Saturday, November 7, 2009

Transmission of Disease in Aircraft

I was surprised to learn how many persons involved in infectious diseases and the aircraft industry attendees at the National Academy of Sciences/Transportation Research Board Symposium Washington, DC, Sep 18-19, 2009: Research on the Transmission of Disease in Airports and Aircraft, believed that because air change rate is high in aircraft (some 18 times that in buildings), ventilation is not a problem and in fact is better than in office buildings or classrooms when it comes to dealing with pathogen air pollutants.

Air quality measurements and ventilation theory both refute this! The problem with this belief is that air change rate is high in aircraft, not because ventilation rate per person is high which is what governs bioeffluent exposure both in theory and in measurements, but because occupancy density (the number of people per unit volume of space) is high.

The fact that human bioeffluent concentrations vary as ventilation rate per person, rather than air change rate, is not just theory. Measurements of bioeffluent VOC concentrations in aircraft are more than double those in office buildings where the fresh air ventilation rate per person is more than double that of airplanes (see Table 1 below). This higher VOC concentration in aircraft occurs in spite of the fact that the fresh air change rate in offices is some 18 times lower than in passenger aircraft.

Similarly, the recirculation rate per person in offices is 8 times that in passenger aircraft. With office HVAC systems using MERV 13 filters removing some 80% of 0.3 micron and larger particles, pathogen concentrations where source strengths are similar when measured will be some 4 times higher in aircraft (20 CFM/p free of 0.3 micron particles and larger in aircraft air supplies versus 84 CFM/p free of these particles in office air supplies). Further, the high occupancy density in aircraft versus buildings and classrooms makes VOC and pathogen exposure dose (area under the concentration vs time curve) for the same source strength even higher (see Figure 1 below).

The above measurements and calculations do not include the proximity impact on transmission rate. This factor will increase exposure the closer the seating.

Thus the risk of in flight infectious disease transmission between aircraft passengers and crew [e.g. incidences of febrile illness(colds, flu..), TB...], ) is estimated to be at least five times higher than in offices. This is especially noteworthy during current concerns about an influenza-type pandemic (i.e. H1N1).

Douglas S. Walkinshaw, Ph.D., P.Eng.
ASHRAE Fellow and Member of the ISIAQ Academy of Fellows.
Indoor Air Technologies Inc.
ECHO Air, Inc.
VEFT Aerospace Technology Inc..

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