Germs, flying and the truth.
The formulas should be noted by practitioners and references reviewed to educate all on travel situations that affect all of us on a daily or periodic basis. The world has become a smaller place because of air travel, and evidence of continent-to-continent transmission has been well documented.
One point within your article should be clarified to readers. When comparing a HEPA filter and ASHRAE grade filtration, different standards of evaluation are used. A HEPA filter is evaluated per Recommended Practice--CC001.5: HEPA and ULPA Filters, as published by the Institute of Environmental Sciences and Technology. Every HEPA filter is qualified by the manufacturer, and the efficiency or penetration is published on the filter's label. Indeed, the minimum acceptable efficiency of a HEPA is at least 99.97% removal of 0.3-micron size particles.
An ASHRAE grade filter may be evaluated using ASHRAE Standard 52.2-2007, and the resultant value provides the specifier or user with a minimum efficiency reporting value (MERV). A MERV 14 filter has efficiency of approximately 48% when considering particles 0.3 micron in size. A MERV 13 filter has an efficiency of around 30% on 0.3 micron in size.
Hence, the comparison in your article of a HEPA filter at 99.97% and a MERV 13 filter at 80% at 0.3 micron will not provide an accurate result in the listed formula, as the efficiency of the ASHRAE filter is not correct, and the comparison will not be relative. When the proper comparison efficiencies are inserted into the formula, the chasm of performance differentiation widens. The critical value of HEPA filters in areas of high occupant density where long-term exposure is possible becomes very clear.
Reviewed by 2.04 Chair And Assigned Committee Members
The Author Responds
Thank you for your input to this discussion. I believe I should have said up to 75% of particles between 0.3 and 1 micron mass median diameter are removed by a MERV 13 filter operating at a typical building HVAC system face velocity, that the actual performance of various filters in removal of infectious aerosols remains to be determined, and that for this analysis an 80% influenza virus removal was assumed.
Fabian, et al., pointed out that 70% of the 67 to 8,500 particles/L in the breath of an influenza-infected person had diameters between 0.3 and 0.5 microns, with rarely any larger than 5 microns. This suggests that the use of MERV 13 filters will be helpful in controlling influenza exposures. How helpful, just as for HEPA filters, remains to be determined.
The point is that passing 80 cfm/p through a filter removing 30% of infectious aerosol particles is equivalent to 24 cfm/p passing through a filter removing 100%, so this flow rate and filter combination removes three times more infectious particles than passing 7.5 cfm/p through a filter that removes 100% of infectious aerosol particles.
In terms of the sensitivity of the analysis, if the MERV 13 filter removes 30% of the influenza virus particles and a HEPA filter removes 100%, then the group inhalation ratio in an aircraft cabin versus in an office after 3 hours of exposure for 15 cfm/p and 41 cfm/p virus-free ventilation air in the cabin and the office air, respectively, is 3.6. After 6 hours in each setting with an infected person present, the group inhalation is 3.1 times higher in the cabin than the office. Both ratios are higher than the virus-free ventilation ratios due to the higher aircraft occupancy density.
In terms of design (worst case) inhalation periods, office building bioeffluent concentrations typically peak at 11:30 a.m. and 3:30 p.m., and fall substantially over lunch hour, so a 6 hour design exposure period is probably appropriate. Nevertheless, using an 8-hour exposure to be conservative, there will be 32 viruses inhaled in the office for a 30% effective filter and 18 inhaled for an 80% effective filter.
For a single-aisle passenger plane, a worst-case design exposure period is probably 6 hours (73 viruses inhaled). For a twin-aisle aircraft, a worst-case 15 hour design period is appropriate for intercontinental flights (184 viruses inhaled). So for a 30% effective filter in the office and 100% in the plane, the design exposure is 2.3 times higher in the single-aisle plane than in the office and 5.8 times higher in the two-aisle plane than in the office. For an 80% effective filter in the office and 100% in the plane, the design exposure is 4.1 times higher in the single-aisle plane than in the office and 10.2 times higher in the two-aisle plane than in the office. Incidentally, my field measurements of HEPA filter performance in removing ETS particulate indicated a 96% average removal rate of 0.3 micron and larger particles, not 100%.
When measuring illness rates, actual ventilation and filtration rates are required. In the fall and early winter when influenza rates are typically highest, some building HVAC systems may be operating in free cool mode when outside temperatures are at or below 20[degrees]C, with outside airflow rates of 100 cfm/p, which is well above the 17 cfm/p minimum of Standard 62.1. This will further reduce infectious aerosol exposures in this setting.
I agree that contagion aerosol inhalation is a concern in high occupancy density spaces, and am currently investigating this concern in other spaces as well as entrainment filtration devices to increase ventilation effectiveness to better address the concern. For those who want to use the formulas for their applications, Equation 1 should read:
C = [N/(V x [V.sub.e])][1 - exp(-V x [V.sub.e] x t/v)]
Douglas S. Walkinshaw, Ph.D., P.Eng., Fellow ASHRAE, Bonita Springs, Fla.