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Save Lives: Become a Mechanical Engineer

There is enough solid science available to back up the claims that controlling IAQ can, in fact, save lives.

We focus on the airborne

germ du jour — influenza — in order to illustrate how your decisions on controlling the indoor air in your clients’ buildings can be critical to health and life preservation.

Since the influenza virus lives happily in pigs and large birds like ducks and chickens, for as long as we’ve lived near them we’ve swapped mutated flu viruses that can also infect us

humans. As a result, influenza epidemics have swept across the world for thousands of years. Recorded flu pandemics started in 412 B.C., when Hippocrates described its impact in his book Of the Epidemics, and the first accepted pandemic by experts was in 1580.1 There have been seven “modern” documented flu pandemics in the last 350 years: 1729, 1781-2, 1830-3, 1889-90, 1918-19,1957-58, and 1968, and now we are recording 2009-10.

How You Spew Flu

If you’re infected with the flu, each time you breathe, cough, sneeze, sing, or talk, you spew out airborne mucus/saliva droplets filled with viruses called virions.2 A critical factor in efficient airborne human flu infection is the impact of low grain moisture indoor environments, which force the mucus/saliva shell surrounding the virions to evaporate

faster, creating “droplet nuclei.” The small droplets that become airborne droplet nuclei are created in less than a second and can stay airborne and survive for hours or days within indoor air spaces.3,4

Here's How Virons Act in the Air

Because it is so small, once a virion is launched, dries out, becomes a droplet nuclei, and is airborne inside a building, the following forces work together to keep it afloat for hours until it plates out on a surface or is inhaled by one of us:

People movement
People breathing, sneezing, coughing, talking, and singing
Heat plumes from people
HVAC fan created air currents
Air temperature thermal differentials
Stack effect” air movements

The science of in-room air mixing clearly demonstrates just how air movements allow airborne virions to catch a ride on these invisible currents just as fish do on ocean currents. If you had electron microscope glasses, you’d see a cavalcade of invisible virions, bacteria, fungal spores, and other debris floating effortlessly on these invisible

currents around you right now. I use a laser particle counter instrument to document these microbial objects, and I regularly find one million airborne particles 0.3 microns in size indoors. Sometimes I find five million or more.

Figure 1:

Airborne Influenza Transmission Conditions - Droplet Nuclei Creation and Survival

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Now, for the first time you can advise your clients about the impact that indoor relative humidity and temperature can have not merely as a comfort issue, but as an occupant health issue.

Humidity is Toxic to Airborne Flu

The connection between low grains and airborne flu survival and transmission becomes even stronger when you convert earlier airborne survival rate experiments done in the 1950s, ’60s, and ’70s. I converted those results into grains of moisture, and low grain air infection rates synch up with earlier airborne influenza survival experiments, which saw high flu survival in low grain air conditions.

The Mt. Sinai scientists were part of a long line of scientists who never connected relative humidity at different temperatures with variable levels of grains of moisture. It’s no wonder they were baffled by low airborne survival at 70° and 50% rh, because you have a whopping 55 grains of moisture. Whereas at low temperatures (40° to 45°) and 50% rh, airborne flu virions were surviving due to a low 20 grains of moisture environment.

Airborne Transmission "Zones"

When I compiled the airborne guinea pig data along with the earlier airborne virion survival experiments, I found that indoor conditions above 45% rh and 70° (50 grains) lower airborne flu virions survival times in order to infect less people. I constructed a graph with four different transmission condition zones to reflect these results. The green zone has 50 plus grains of moisture, and thus lower airborne virion survival/transmission. The orange zone has “medium” airborne virion survival/transmission rates (26 to 49 grains), with infection

increasing within the “high” yellow zone (11 to 25 grains). The red zone has the most ideal conditions for airborne flu survival/transmission, with conditions below 10 grains of moisture (Figure 1).

AIrborne Virus Science Has Landed in the Real World

IWhen I compiled the airborne guinea pig data along with the earlier airborne virion survival experiments, I found that indoor conditions above 45% rh and 70° (50 grains) lower airborne flu virions survival times in order to infect less people. I constructed a graph with four different transmission condition zones to reflect these results. The green zone has 50 plus grains of moisture, and thus lower airborne virion survival/transmission. The orange zone has “medium” airborne virion survival/transmission rates (26 to 49 grains), with infection

The Dew Is In Your Corner

Now, for the first time you can advise your clients about the impact that indoor relative humidity and temperature can have not merely as a comfort issue, but as an occupant health issue. I recommend that you humidify indoor spaces to 45 grains or more with 50

grains and above putting your clients in the green low-survival/ transmission zone. This can also reduce the drying out of occupant’s mucus membranes, which will keep these important virus trapping mechanisms in their fully functional state. I’ve taken readings in schools and buildings with 20% rh levels in the winter, which at 68° is a low 20 grains of moisture. These levels put those school occupants at great airborne virion

Welty is a specialist in airborne and infectious diseases and HVAC energy efficiency and president of Green Clean Air (Reston, VA). He uses computer modeling to design airborne infection control systems using UV light and MERV filters. Reach him by e-mail at steve@GreenCleanAir.com.

Cited Works

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Microbiology, Vol. 91, 2001.

2. Gerone, P.J. et al., “Assessment of experimental and natural viral aerosols,” Bacteriological

Review, Vol. 30, 1966.

3. Xie, X. et al., “How far droplets can move in indoor environments – revisiting the Wells

evaporation-falling curve,” Indoor Air, Vol. 17, 2007.

4. Harper, G.J., “Airborne micro-organisms: survival tests with four viruses,” Journal of

Hygiene, London, Vol. 59, 1961.

5. Harper, G.J., “The influence of environment on the survival of airborne virus particles

in the laboratory,” Arch Gesamte Virusforsch, Vol. 13, 1963.

6. Hemmes, J.H., “Virus Survival as a Seasonal Factor in influenza and Poliomyelitis,” Nature, Vol. 188, 1960.

7. Dr. Peter Palese is the co-author of the influenza chapter (orthomyxoviridae species

viruses) in the latest edition of Fields Virology, the accepted reference authority and medical

textbook on viruses.

8. Palese, Peter, et al., “Influenza virus transmission is dependent on relative humidity and

temperature,” PLoS Pathology, Vol. 19, 2007.

9. Palese, Peter, et al., “Transmission of Influenza viruses via aerosols and fomites in the

Guinea pig model,” The Journal of Infectious Diseases, Vol. 199, 2009.

10. Blachere F.M., et al., “Measurement of airborne influenza virus in a hospital emergency

department,” Clinical Infectious Diseases, Vol. 48, Feb. 2009.

11. Alford, Robert H., “Human Influenza resulting from Aerosol Inhalation,” Proc Soc Exp

Exp Biol Med, Vol. 122, 1966.