by Sarah and Cara
As mechanical engineers who consult on heating, ventilating, and air conditioning (HVAC) systems, we’ve been closely following the evolving body of knowledge about how the SARS-CoV-2 novel coronavirus (the virus which causes COVID-19) spreads through the air. We thought some folks might be interested to know some of what we’ve learned, and how that’s affecting our thoughts on returning to the gym.
Some of the science so far
So far, we know that droplets in the air we breathe out (and in) are infectious to varying degrees depending on the size of the droplets – and that those droplets have the potential to be propelled for varying distances.
Relatively large and heavy droplets fall on and contaminate surfaces. This is a whole other topic, but it seems to be relatively well-known and understood. Also it can be controlled with frequent cleaning, so it’s less important from an engineering point of view. At the moment, our big concern (and the focus of this post) is with the smaller, lighter droplets known as aerosols.
Scientists and engineers take particular note of so-called “superspreading” events (such as the ones that were mentioned in Saturday’s post because they point to clues about how an infection is transmitted in a variety of real-world situations.
In the example of the choir in Washington State one mildly symptomatic person infected 52 of their 60 fellow choristers over the course of one or two 2.5 hour practices. Besides sitting close together, it is thought that the act of singing, itself, might have contributed to transmission, as aerosol emission has been correlated with loudness of vocalization.
The dance fitness classes in Cheonan, South Korea gives valuable insight into what factors which affect the risks of exercising indoors. Sports facilities are generally considered to represent a higher risk of transmission due to the warm, moist indoor air coupled with the turbulent air flow generated by intense physical exercise, which can cause more dense transmission of droplets.
Six instructors who were infected at a workshop went on to teach classes for about a week. Not all of them were necessarily even symptomatic. Secondary cases were identified from fitness dance classes with as few as 5 people in a ~60 square meter (~645 square foot) studio. Notably, an instructor who taught 7-8 person Pilates and yoga classes at one of the same facilities did not infect any of her students. Together, these us some insight as to how transmission risk might be mitigated in the short term for group fitness classes : very small class size, limits on movement to maintain physical distance, less aerobically- and movement-intense activities.
In the long term, engineers and building owners will have to address the significant concern that was raised by another notable case of a restaurant in Guangzhou, China one patron infected eight others who were sitting more than 6 feet / 2m away. It appears that air flow from the HVAC system helped carry infectious aerosols from one table to another.
The role of HVAC in controlling transmission
The possibility that a normal HVAC system can carry it through the air over distances greater than the current physical distancing guidelines is a major concern. While we don’t yet know for sure how infectious COVID-19 is in aerosol form, the Epidemic Task Force of the American Society for Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE, the leading industries standards organization) have stated: “Transmission of SARS-CoV-2 through the air is sufficiently likely that airborne exposure to the virus should be controlled.”
In hospitals, aerosols are controlled by continuously moving lots of air through infectious spaces to dilute them, adding lots of fresh air, careful airflow design, and HEPA filtration. All the air in a typical patient room is filtered about once every four minutes. While gym ventilation is actually comparable to that of hospitals – gyms need comparatively good ventilation to keep them from smelling bad – the circulating air may have little or no filtration. Even if a gym’s air handling equipment is modified with HEPA filters, to achieve that circulation the HVAC system draws air from one side of the space and blows it on the other side. Just like in the restaurant example above, air exhaled by someone will move through the breathing zone of those nearby.
When outdoor temperatures permit, it may be possible to make temporary changes such as opening existing windows and doors to encourage wind and buoyancy-driven natural ventilation in order to increase airflow and dilute contaminants in an existing fitness space. The openings need to be large to make a difference : a crossfit gym with a roll-up garage door and a back door propped open will be safer than a studio with a small open window. Openings on more than one side of the room gives better access to cross-breezes; openings high and low in the space will drive buoyancy flows, especially if the gym is warmer than the outside air. When natural ventilation is working well, the indoor air will smell like outside, and match the outdoor temperature and humidity levels, so the comfort of the occupants will vary accordingly.
Other approaches to improve airflow in gym spaces, such as redesigning the air distribution to direct fresh air directly onto each occupant, will be expensive and disruptive to install – and unfortunately, still not proven to be entirely effective against airborne infection. There are some HVAC solutions that will reduce the concentration of infectious aerosol droplets in the air in buildings, notably increasing outdoor air volumes; HEPA filtration; and UV lights that sterilize air above the heads of occupants. These solutions reduce, but do not eliminate, the risk of virus transmission.
What are the risks?
As gym patrons, we miss the motivation of exercising together with others, and access to equipment we don’t have at home. As we start to evaluate the risk of returning to indoor activity, there are a number of airborne infection risk factors which must consider in our decision making:
- Indoor exposure: Whether an office, a store, or a gym, shared indoor environments have inherent transmission risk. Each additional person occupying the space with us increases the risk.
- Extended exposure: Being the same place with specific other people for an extended period of time (15 minutes or more).
- Stationary exposure: Being in the same position relative to other individuals for an extended period of time, especially if the air conditioning system is blowing past the person next so that you are breathing their air.
- Increased respiratory droplet exposure: Intense aerobic activity, shouting and deliberate sudden exhalation reportedly increase the amount and spread of respiratory droplets. A low-intensity yoga class represents a lower risk than, for example, a Kiai (shout) filled karate class or high-intensity cardio class.
- Mechanical system efficacy and state of maintenance: Many gyms and other fitness spaces are tenants in older or repurposed commercial spaces which are not always in the good repair. HVAC systems lose effectiveness as they get older, and may distribute air poorly. Some owners may even shut off HVAC systems due to safety concerns but these actions could actually increase risk if they reduce the outdoor air flow into the space.
Controlling the risks
For any given hazard, there are many different possible ways to address or mitigate the associated risks. Those who have taken a workplace health and safety course may recognize this hierarchy which is commonly used to rank the effectiveness of the various controls.
Preventing infection using an engineering control – like the fresh air system inside a gym – or administrative controls such as cleaning – is necessarily less effective than substituting a lower risk activity – such as exercising outside in places where physical distancing can be maintained.
Is outdoors actually safer?
Both published research to date and epidemiological consensus appears to indicate outdoor activities are extremely low risk: a recent, not yet peer-reviewed study of infections in Chinese cities outside Hubei province in January and February showed that less than 1 in 300 outbreaks (only 1 out of 7000 individual infection events) could be traced to contact that occurred outdoors. B.C.’s provincial health officer has been quoted going so far as to say “the risk [of catching the virus] would be infinitesimally small if somebody walks [or runs] by you.”
The evidence is strong that for the foreseeable future, substituting parks, backyards or even gym driveways will be a reasonably safe way to enjoy exercise with others, while indoor workouts will remain high risk until either the risk of exposure to infection can be eliminated, or effective engineering controls can be implemented. We want to support our fitness spaces, and we are hopeful that the summer weather will allow everyone to use the outdoors to bridge the gap until it’s safe to be together inside again.
Cara is an active promoter and designer of sustainable buildings, specializing in multi-unit residential and municipal facilities, enjoys dancing, Jeet Kune Do, acroyoga and circus arts, and bikes to get places.
Sarah specializes in existing residential and commercial high-rises, and the systems that make them habitable. She spends far too much time poking around the guts of buildings and not nearly enough time on road bikes, sailing dinghies, or skis.
We’re both professional engineers.