Over the last few months public health guidance to prevent the spread of COVID-19 has called on people to wash hands, stifle sneezes, wear masks and keep their distance — measures to contain particles expelled when we breathe, which are known to be the virus’s prime transmission route. As states reopen and people consider sharing indoor spaces again, experts suggest that ventilation will likely play a pivotal role in preventing the spread of the virus.
Aside from being too hot or cold in the office, most people don’t think about ventilation systems — Drexel College of Engineering researcher L. James Lo, PhD, is someone who does.
Lo studies how various air handling and ventilation systems can influence indoor air quality. He has provided his expertise in news stories in the Washington Post and NBC News looking at ventilation considerations for restaurants and political rallies. He is also involved in the testing of medical equipment designed to prevent airborne transmission of COVID-19 in hospitals. Lo recently shared some insight with the Drexel News Blog about how indoor air movement factors into preventing the spread of COVID-19.
Even if people are wearing masks inside and practicing safe social distancing, why is ventilation still an important consideration for preventing the spread of COVID-19?
Interestingly, the masks we wear — except for a properly worn N95 — are much better at preventing the virus spreading from a carrier than protecting yourself from breathing it in if it’s already in the air. This is because the masks can absorb larger droplets from a sneeze or a cough, but as the droplets evaporate and become smaller, the aerosol particles that remain are too small to be effectively stopped by most mask material.
Therefore, to reduce the amount of virus-laden small droplets/particles we could encounter, it is important to have ventilation bringing in outside air in order to dilute viral concentrations in the indoor air.
How might air handling considerations factor into room capacity recommendations?
While occupancy might seem to relate to amount of ventilation air, the key issue with capacity is about the ability to maintain social distancing. Even with a high ventilation rate, high occupancy, which likely means people are very close together, still poses an increased risk in terms of short distance person-to-person transmission.
CDC guidance suggests that building managers/owners: “Ensure ventilation systems operate properly and increase circulation of outdoor air as much as possible, for example by opening windows and doors.” What does “proper” operation likely mean in this instance?
Many HVAC systems are bringing in less outdoor air than they’re designed to exchange. This is typically an energy-saving measure, because it takes a lot to warm or cool outdoor air to the level necessary to maintain the comfort of the building’s occupants. And many systems are a little less-than-well-maintained — they could be missing a filter, or running with some broken components. So, “proper” here means operating as-designed.
But the CDC’s guidance here is purposely quite vague. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) recommends that systems bring outdoor air in at a rate of three exchanges per hour. This is about 3-5 times higher than most systems are designed for.
Many systems really aren’t going to get the air exchange rate necessary to adequately dilute the indoor air per the COVID-related recommendations. And if they can achieve that rate, it is likely that the heating and cooling equipment will not be able to keep up with the extra load. So, instead, the guidance is suggesting bringing in “as much outdoor air as possible” because what is “possible” varies widely between systems.
In terms of improving air quality, what are the primary differences between increasing circulation of outdoor air, air filtration and air purification technologies?
Increasing ventilation air, assuming outdoor air has no viruses, reduces the risk by diluting the indoor air. This means both replacing air that could be laden with viruses with fresh outdoor air, and diluting the possible viral concentration in the air, so it reduces your odds of breathing in viral particles.
Filtration uses filter or screen material to remove particles as air is forced through it — which would include virus-laden droplets. All HVAC systems have some form of a filter that cleans the air and protects the machinery from accumulating particles that could limit its function.
“Air purifier” is a catch-all term, which includes filtration, but also treatment systems like ultraviolet light or ionizing ozone. These portable air cleaner devices are becoming more popular, but in order to be effective, it needs to be placed close to the occupant. This would work for a single-occupancy office or a small classroom perhaps, but, as you can imagine, they wouldn’t be very practical for a large open office.
But there are also some maintenance challenges in using these air cleaners. The filters typically need to be replaced after several hundred hours of use, so how to replace and dispose the used filter — assuming it is potentially filled with viral particles — is an important question, especially for the staff who does the work.
Many hospitals and medical settings are designed specifically to prevent airborne spread of disease, what strategies do they employ? Would it be reasonable and/or effective to adapt any of them for non-medical indoor environments?
There are two strategies typically used in medical settings. The first is, again, high ventilation. The second is isolating rooms by creating a negative-pressure environment, so air won’t force its way out of a room and spread to other areas.
In non-medical settings, increasing ventilation is already being implemented and it is the only strategy that is really feasible in most cases. To create a negative-pressure zone it would likely require reinstalling the HVAC system so that rooms could be isolated. The goal of negative pressure is to create an imbalance of pressure in two adjacent rooms so that air is always rushing into the room with less, or “negative,” pressure to equalize the difference. This would keep airborne viral particles from exiting the room, but it is not usually a cost-effective solution outside of a medical setting.
Why might air management play a bigger role in preventing disease spread in an office or restaurant environment versus a grocery store — which has basically been the only place where people have been encountering others over the last few months?
In office and restaurant settings, people are stationary and grouped together for a long period of time. In those situations, if there is an infected person, the viral dose one can receive from a carrier could be very high, because of the continuous proximity to the carrier and the time spent there.
For spaces like grocery stores, people are moving around and not sticking together for a long period of time. Hence, managing air in static scenarios, like offices and restaurants, is more important as they are more likely to receive high dose of virus.
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