ASHRAE Journal - April 2010 - 71

Officials sometimes cite misleading information. For example, aircraft ventilation systems, as in hospitals, use HEPA filters, and air-change rates are 18 times higher than in buildings.
At a recent international symposium jointly organized by the U.S. Transportation Research Board (TRB) and the National Academy of Sciences (NAS), experts in infectious diseases and aircraft ventilation systems discussed the movement of potentially infectious particles between rows fore and aft of the source.16 Two studies (following up on earlier studies, including work by Boeing17–19) found that these air velocities, and the turbulence induced at the airflow boundaries, disperse particle and gaseous contaminants from a single source in two ways: past others in the same row and in other rows in measurable quantities six or more rows forward and backward. The contaminant flows in the area immediately surrounding the source are relatively chaotic, and then more ordered several rows away.20,21 At the symposium, several field investigations found that aisle seats and those near lavatories were most prone to disease transfer. Mathematical modeling used to investigate how the SARS virus was transmitted as far as seven rows away in the Air China flight 117 from Hong Kong to Beijing in 2003, found that the wake created by occupant movement in the aisles can carry an airborne contaminant this distance and when the movement stops the contaminant is distributed to the passengers seated in the adjacent aisle seats.22 A study by Fabian23 found between <3.2 to 20 influenza virus RNA copies per minute (up to 1,200 viruses per hour) in the exhaled normal at rest breath (tidal breathing) of infected persons, indicating that sneezing and coughing are not the only potential source of infectious aerosols. Seventy percent of the 67 to 8,500 particles/L in the breath had diameters between 0.3 and 0.5 microns, with rarely any larger than 5 microns.23 By way of comparison, Duguid reported 6,200 cold viruses per hour emitted by an infected person at rest.24 Combined, the symposium findings demonstrated that no systems or measures are in place to prevent the airborne spread of infectious agents over several rows, and that infectious disease transmission within an aircraft cabin occurs before airborne pathogens are directed to the HEPA filters or exhausted outdoors. Humidity also can play a role. Research published in 2007 indicates that lower levels of relative humidity (RH) such as that in aircraft cabins shortly into cruising flight, increases the potential for influenza and possibly other respiratory infections when a source is present.25 In contrast, higher levels of RH may favor the survival and spread of the common cold.26 Relevant HVAC engineering calculations also serve to correct misperceptions about risk of disease transmission on aircraft. Although aircraft air change rates are higher than in office buildings, ventilation and recirculation flow rates per person are lower and engineering equations indicate that airborne 0.3 micron and larger pathogen concentrations will be at least four times higher in passenger aircraft equipped with HEPA filters
April	2010	

than in typical office environments with MERV 13 filters, for the same pathogen emission rates and a uniformly mixed system. Because passenger aircraft cabin ODs are 20 to 40 times higher than in office buildings, their pathogen concentrations will reach peak equilibrium values sooner, with the result that time-weighted exposure ratios will be at least five times higher than in offices, depending upon exposure duration, when the same number of pathogens are emitted in each. In another comparison, aircraft cabin occupancy densities are more than three times higher than classroom ODs. Here are the calculations: Using the equation for average pathogen concentration C in a uniformly mixed system at time t in a space C = [N/(V × Ve)][1 – exp(– Vt /v)] (1) where C = bioeffluent pathogen concentration in the space at time t N = rate of bioeffluent pathogen generation/person in the space V = total ventilation air supply rate with no pathogens Ve = efficiency of supplying the ventilation air to each occupant’s breathing zone v = spatial volume/person And using v and V values typical of aircraft cabins and offices, v = 32 ft3/p for the aircraft cabin v = 1,430 ft3/p for the office V = 15 cfm/p for the aircraft cabin (based on ASHRAE Standard 161 and 100% influenza filtration by the HEPA filters) V = 84 cfm/p for the office (based on 20 cfm/p outside air and 80% virus filtration by MERV 13 filters for 80 cfm/p recirculation air) And, using the average Fabian influenza generation rate: N = 11 influenza virus generated per minute continuously in the exhaled breath of one influenza infected person, not including coughing generation And, using: Ve = 1 for both settings Solving Equation 1, and incorporating at rest awake inhalation and exhalation rates of 0.28 cfm/p (0.13 L/s), the numbers of influenza virus particles inhaled by office and aircraft groups exposed to the exhaled breath of one infected person versus exposure time are shown in Figure 1. It can be seen that after eight hours in the aircraft cabin, there will be 98 influenza virus particles inhaled by previously uninfected persons, and up to nine infections for a 10 virus particle dose criterion. By comparison, for the same exposure
ASHRAE	Journal	 71
ASHRAE Journal - April 2010

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