Clearly, the COVID-19 pandemic, caused by the SARS-CoV-2 virus, has had sustained, far-reaching, and devastating impacts on global health, economies, and innumerable aspects on daily life. Six months after it began to widely proliferate, we are still learning about the virus with thousands of scientific papers and a large number of clinical trials underway. However, we can minimize the spread of the virus if what we DO know about the virus and essential concepts regarding infectious disease are appropriately applied.
We know that the virus is primarily spread through mucous droplets expelled by infected individuals, although 35-50% of those can be asymptomatic or pre-symptomatic. It can also be spread through contact with particles on surface, though this appears to be less prevalent. When expelled by an infected person, large particles generally drop to the floor, however, small particles can spread over considerable distance and may remain airborne and infectious for up to three hours depending on factors such as humidity, air exchange/velocity, movement of people, and the opening and closing of doors and windows. Density of particles and distribution of particle size varies significantly depending on, for instance, whether an infected person coughs, sneezes, or is shouting or singing. Spread of the disease is exacerbated in indoor areas with high occupant density such as nursing homes, hospitals, prisons, indoor music venues, meat packing plants, and churches. These have lead in some cases to “superspreader events.” In one example, 53 individuals of the 61 attending a choir practice became infected, and two died.
In the case of SARS-CoV-2, the reproductive rate or average number of new infections caused by each patient is 2-5, and, 10% of the infected persons cause approximately 80% of the cases. This can be caused by direct exposure to large droplets (e.g., being coughed or sneezed on by an infected person) as well as prolonged exposure to small particles. Key contributors to the spread are being indoors and prolonged exposure. Hot or cold temperatures that drive people indoors can exacerbate the problem, particularly when it is combined with close proximity to other (potentially infected) people.
There has been some controversy over the use of face coverings such as masks and face shields to reduce airborne transmission of SARS-CoV-2. Early reluctance to require the use of masks was largely related to their scarcity and in order to ensure their availability to medical professionals. Furthermore, while use of masks can be helpful, and they are generally recommended for this virus by infectious disease experts, they are just one piece of a “hierarchy of controls” that must be applied to control the spread of the virus. Avoiding or preventing exposure, such as staying at home, avoiding other people, and social (physical) distancing (i.e., maintaining at least six feet between individuals) are some of the most effective measures since these reduce exposure which leads to lower transmission.
Of course, complete social isolation is not always possible or desirable. Where workplaces are concerned, key additional considerations include surface cleaning and the use of ventilation-related measures. Which controls are most appropriate depends on the facility type and relevant risk factors. The hierarchy of controls helps us to focus on the most effective controls first.
While viral particles can be spread through the heating, ventilation and air conditioning (HVAC) system, these systems can also be configured to minimize the spread by paying attention to several important operational aspects. Ventilation systems should be optimized to supplement response strategies in accordance with the hierarchy of controls based on risk and building design. Essential aspects include the following:
- Ventilation rates and directional airflow - Outside air delivery is the key, and air changes per hour should be optimized according to room size and number of occupants. Within the facility, air should move from clean to dirty areas by negatively pressurizing air flow to reduce the flow from areas that may have more of the airborne virus, and by optimizing the delivery rate of outside or “fresh” air. Meeting or exceeding ASHRAE standard 62.1 (Ventilation for Acceptable Indoor Air Quality) is considered a best practice minimum.
- Air filtration - Enhanced air filtration practices can provide additional value, particularly when there is not a large volume of outside air per person being delivered to the occupied space. In the U.S., ASHRAE Minimum Efficiency Reporting Values (MERV) are generally used to indicate filtration effectiveness. MERV-13 or higher or HEPA filtration are considered best practices. At MERV-13, 70% of small particles and 90% of large particles will be filtered out (whereas HEPA approaches 100% for both sizes - but most systems will not be able to accommodate HEPA filters unless designed for it). Single space, portable HEPA filtration can be helpful - particularly in small, contained areas. In addition to high filtration efficiency, it is also critical to ensure proper seating of and/or sealing around filters to minimize air bypass.
- Air purification - Ultraviolet light can be used to inactivate microorganisms and limit their ability to grow and multiply; however, due to the speed that the air flows through the HVAC, the UV light will likely be ineffective. (It can be used for surface decontamination but is hazardous to skin and eyes so safety goggles and gloves are essential if people are exposed to UV light) Other novel air purification approaches include photocatalytic oxidation, bipolar ionization/corona discharge, ozone, vaporized hydrogen peroxide, pulsed xenon/pulsed UV, near and far UV. These approaches currently lack scientifically rigorous, peer-reviewed studies and can pose their own hazards.
- Temperature and relative humidity control - Dry air ( <40% RH) causes the droplets to evaporate more quickly and stay airborne longer. However, humidification has not been proven to be particularly effective at reducing viral load for SARS-CoV-2. Furthermore, the high temperatures needed to inactivate the virus are not compatible with personal comfort (one study found that temperatures of over 130 ºF for more than an hour was needed to inactive the virus). Therefore, increasing outdoor/clean air ventilation as indoor and outdoor conditions permit is more practical than adjusting indoor temperature or humidity level directly. Avoid bypass energy recovery ventilation systems such as heat wheels that might cross-contaminate supply air.
Keep in mind that regardless of the ventilation efficacy, viral exposure can still occur when in proximity to infected individuals - always consider the full hierarchy of controls. Furthermore, a few other measures that may be considered include:
- Minimize exposure on elevators: limit occupancy (ideally to one person), and consider portable HEPA filters
- Install plexiglass barriers as needed to enhance physical separation (some consideration may need to be given to the impact of barriers on air flow, particularly for floor-to-ceiling/complete separation)
- Small or contained office areas - utilize portable HEPA filters to enhance filtration
The article is based on a review of scientific literature, as well as guidance from the following individuals and organizations:
- Dr. Mark Cunningham-Hill, MB ChB FFOM FACOEM
- Centers for Disease Control and Prevention (CDC)
- American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE)
- Federation of European Heating, Ventilation, and Air-Conditioning Associations (REHVA)