What happens when it gets really hot and the AC is blasting on full?
In much of the United States, the malls and restaurants are reopening to the public. Some of those states get really hot in the summertime. Sarah Goodyear, a writer and host on The War On Cars, posted an interesting tweet:
Question for epidemiologists and other smart people: In Sun Belt states like Arizona and Texas, where summer is an indoor, air-conditioned season because of very hot weather, will the spread of the virus be facilitated? 1/x— Sarah Goodyear (@buttermilk1) May 21, 2020
It is a really interesting question. We noted in an earlier post that the Federation of European Heating, Ventilation and Air Conditioning Associations (REHVA) in Europe had warned that the coronavirus could stay airborne for some time, and travel long distances. Architect Justin Bere noted that "it recommends avoiding central recirculation during SARS CoV-2 episodes and closing the recirculation dampers, even if there are return air filters. As the REHVA guidance says, these don’t normally filter out viruses." Bere explained:
Recent research indicates that large droplets from sneezing can travel much further than 2 meters, even if there are no air movements. Small particles (<5 microns), generated by coughing and sneezing, may stay airborne for hours according to the REHVA guidance and can travel long distances. A Coronavirus particle is only 0.8 to 0.16 microns diameter so there could be many virus particles in a 5-micron droplet floating around in the air.
They have been studying the problem in Canada too. Professor Brian Fleck told the National Post that "this has been on people’s radar for quite a while. Somebody on a different floor sneezes …The particle can stay airborne long enough to go all the way through the system and then pop out in somebody else’s office."
There are various ways that the risk can be lessened, including use of filters that catch a greater number of those particles, and drawing more fresh air into a system... But each of those changes carries a cost. Adding more fresh air can require additional heat or air conditioning. Heavier filters means more energy is needed to push the air through them.
But it doesn't get as hot in Canada as it does in Arizona. Engineer and Professor Ted Kesik told TreeHugger that "we shall be greatly challenged retrofitting our existing buildings to eliminate dilution ventilation systems." This is especially a challenge in the heat of a southern summer, where the difference between inside and outside air can be 40°F in Arizona or Texas. In the Southeast, there is also a lot of humidity with the heat. That's why the air is recirculated; the amount of energy needed to condition a mall's worth of outside air would be ridiculously high.
ASHRAE, the American Society of Heating, Refrigerating, and Air-Conditioning Engineers, had a look at the issue of the coronavirus and issued a statement in late April:
Transmission of SARS-CoV-2 through the air is sufficiently likely that airborne exposure to the virus should be controlled. Changes to building operations, including the operation of heating, ventilating, and air-conditioning systems, can reduce airborne exposures.
They issued technical guidelines in a position document on infectious aerosols:
Infectious aerosols can be disseminated through buildings by pathways that include air distribution systems and interzone airflows. Various strategies have been found to be effective at controlling transmission, including optimized airflow patterns, directional airflow, zone pressurization, dilution ventilation, in-room air-cleaning systems, general exhaust ventilation, personalized ventilation, local exhaust ventilation at the source, central system filtration, UVGI, and controlling indoor temperature and relative humidity. Design engineers can make an essential contribution to reducing infectious aerosol transmission through the application of these strategies.
That's fine, the engineers know what to do with new buildings. But what about existing ones? Here, they make some recommendations, and I try to add an explanation in italics.
Non-healthcare buildings should have a plan for an emergency response. The following modifications to building HVAC system operation should be considered:
• Increase outdoor air ventilation (disable demand-controlled ventilation and open outdoor air dampers to 100% as indoor and outdoor conditions permit). Essentially, stop recirculation and bring in 100 percent outside air. When it gets very hot, outdoor conditions will probably not permit.
• Improve central air and other HVAC filtration to MERV-13 (ASHRAE 2017b) or the highest level achievable. MERV-13 is not quite HEPA filter but it is pretty good. The reason they add "highest level achievable" is because better filters add more air resistance, and the system may not actually have the power to push air through the MERV-13 filter.
• Keep systems running longer hours (24/7 if possible). More energy consumed.
• Add portable room air cleaners with HEPA or high-MERV filters with due consideration to the clean air delivery rate (AHAM 2015).
• Add duct- or air-handling-unit-mounted, upper room, and/or portable UVGI devices in connection to in-room fans in high-density spaces such as waiting rooms, prisons, and shelters. We have discussed UV filters before. UV-A and UV-B devices don't do all that much in open spaces, and UV-C is dangerous to humans so it can only be installed in ducts or in a place up high where it doesn't shine on people.
• Maintain temperature and humidity as applicable to the infectious aerosol of concern. We have noted before that "the higher the relative humidity, the more quickly the virus falls to the floor." You want to keep humidity between 40 and 60 percent.
• Bypass energy recovery ventilation systems that leak potentially contaminated exhaust air back into the outdoor air supply. This is because apparently heat and energy recovery wheels leak a lot.
All of these modifications are expensive, either in equipment or operating costs. All of these building owners and tenants have been bleeding money in the last few months. All of the companies making this equipment are going through the crisis, too. In short, it is probably safe to say it's not gonna happen, at least in the short term.
I have tried to get comments from engineers and experts, but the only one I have received so far is, "Yikes, I think that is a problem." I will add more comments as I receive them.
But I do believe that Sarah Goodyear has raised an interesting point. In my limited experience in Arizona in summer (two weeks in Scottsdale in July), I rarely saw anyone outside. And it's not even summer yet, but as one shopper in Arizona told NBC News after the mall opened, "We hit all the museums and this place because it’s hot."