ASHRAE Journal - April 2010 - 72

period in the office, there will be 17 virus particles inhaled and up to one infection. The location of persons most likely to be infected will be determined by proximity to the infected person, susceptibility, air current patterns, and aisle or not seat location. The ratio of virus inhalation for the two exposure settings is shown in Figure 2. It can be seen that for exposures over two hours, Ve for the aircraft must be doubled and V tripled to provide an office building equivalent level of protection for the same activity level of occupants. However, breathing rates on aircraft typically will decrease as the flight duration increases and more people are dozing. If the infected person also dozes, this could result in up to one-third reduction in average breathing rate and virus inhalation as the flight progresses. This commensurately decreases the aforementioned V and Ve multiples required to reduce virus inhalation rates in aircraft by a four times rather than a six times reduction factor to provide office building level equivalent protection. Other pathogens aerosolized by passenger movement also would decrease as fewer passengers move about the cabin. A leveling off of airborne viable bacterial abundance after midflight has been noted by La Duc, et al.27 To date, the role of HVAC system design and operation to help control infectious disease transmission has been mainly limited to the use of HEPA filtration and air purification in the main recirculation system and, in the case of health-care facilities, isolating infectious patients in separately ventilated, depressurized rooms. The effectiveness of HEPA filters in trapping airborne cold virus was tested in a study of passenger aircraft, where some flights had no recirculation and some had 50% recirculation through HEPA filters.28 This study found no difference in the rates of transmission of upper respiratory illness symptoms, runny nose and colds, in two-hour flights. Although the flight durations were short, this suggests air filtration can be a useful measure in the prevention of these illnesses. In a related development, the aircraft cabin air supply that is used to ventilate and pressurize the cabin, is also used to filter the air from around the occupants using the flow entrainment created by the high velocity airflows out of the gaspers. This technique effectively doubles the cabin air filtration rate and increases the ventilation effectiveness without adding local air circulation fans or using extra energy.29 It is time for ASHRAE to begin the multidisciplinary research and development necessary to identify infectious disease mitigation measures for critical settings, and set criteria that will effectively limit airborne infections of most concern. These would include: • Design exposure periods to be used for different settings; • Design occupancy densities to use for different settings; • Design humidity conditions to use for different settings; • Occupant breathing rates to be used for different settings; • Pathogen dose criteria to be used for pathogens of airborne concern; • Coughing and breathing aerosol size ranges, dispersion distances and settling rates for different settings for each pathogen of concern;
72	 ASHRAE	Journal	

110 Number of Influenza Virus Inhaled by Group 100 90 80 70 60 50 40 30 20 10 0 0 60 120 180 240 300 360 Exposure Duration, Minutes 420 480 Aircraft Passengers Office Workers

Figure 1: Number of influenza virus particles inhaled by office worker and aircraft passenger groups exposed to the exhaled breath (coughing not included) of one infected person for the same “at rest awake” activity level in each.
35

30 Virus Inhalation: Aircraft/Office

25 Ratio of Virus Inhalation: Aircraft Passengers/Office Workers

20

15

10

5

0

0

30

60 90 120 Exposure Duration, Minutes

150

180

Figure 2: Ratio of virus inhalation by aircraft passengers to office workers versus exposure time for the same number of infected persons and “at rest awake” activity level in each group.

• Viability time limits for pathogens of concern; • Total pathogen-clearing airflow per person to be provided to the breathing zone in proportion to airflow effectiveness (Ve) in diluting and removing pathogens in the breathing zone; • Ve for each occupant’s breathing zone to be used for different spaces and diffuser types and arrangements; and • Removal/immobilization rates per unit airflow to be used for each pathogen of concern for the various air filters and purifiers available. Regarding pathogen aerosol size range, settling rates, and viability time limits, the higher the OD the sooner the equilibrium
ashrae.org	 	 April	 2010
ASHRAE Journal - April 2010

Table of Contents for the Digital Edition of ASHRAE Journal - April 2010
