The airborne transmission of coronavirus has been a widely debated topic among medical professionals and researchers. At first, the prevention guidance focused on avoiding large droplets when infected persons sneeze or cough, while keeping surfaces and objects disinfected. However, there is now scientific evidence that airborne COVID-19 infection is possible, in the form of microscopic particles that are called aerosols. These particles can reach high concentrations in poorly ventilated spaces, increasing the risk of infection.
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At first, guidance from the World Health Organization (WHO) focused on preventing infection by large droplets and surfaces, and airborne transmission was not considered a significant risk. However, the latest guidance from the WHO recognizes the threat of COVID-19 infection by exposure to aerosols.
With the risk of airborne COVID-19, indoor air quality becomes critical in buildings. ASHRAE has provided detailed guidance on how to improve IAQ and reduce the risk of infections. Specific guidance is provided for each building type, but in general they focus on:
- Increased ventilation with outdoor air, with zero air recirculation when possible.
- Upgrading air filters to medical grade, which means MERV 13 or better.
- Air and surface disinfection with Ultraviolet Germicidal Irradiation, or UVGI.
ASHRAE Guidance To Control Airborne COVID-19
The infection mechanisms of the new coronavirus are not fully understood. However, airborne transmission seems more likely in building interiors, especially if they have high occupancy and deficient ventilation.
- When there is a large number of occupants, the chance of having COVID-19 positive persons is higher.
- Deficient ventilation means that viral particles are not removed from the air effectively. Since this increases exposure to the virus, infections are more likely.
Occupancy can be reduced with stay-at-home orders, instructing companies to use teleworking as much as possible. This infection risk is reduced for the simple reason that collaborators are not interacting in person.
Ventilation improvements depend on the existing HVAC installation. Some systems can be easily reconfigured, while others may need component upgrades to provide adequate ventilation. In general, ASHRAE recommends increasing the outdoor air supply as much as possible, while reducing or eliminating air recirculation. Outdoor air does not normally contain coronavirus, and an increased air supply can dilute air pollutants. With reduced recirculation, outdoor air can displace indoor air that has been accumulating viral droplets.
Filter upgrades and UVGI can provide additional protection against COVID-19 infection: medical-grade filters can capture a high percentage of air particles, while UV light can kill germs by destroying their DNA. However, building owners must use the right products: filters should have at least a MERV 13 rating, and UVGI systems must use UV-C light specifically.
Finding the Best Air Quality Measures for a Building
To improve indoor air quality, the best starting point is a professional assessment of the property. Depending on the type of ventilation system and the HVAC configuration, the most effective measures will vary. For example, HVAC systems that are controlled by a Building Automation System (BAS) can be easily reconfigured, while systems without smart controls will need manual adjustments.
Increasing the outdoor airflow is beneficial for air quality, but this may be limited by weather conditions. For example, if the outdoor air is very hot or cold, a high airflow may overwhelm the air conditioning or space heating system. This may cause indoor temperatures that are unsuitable for extended occupancy.
Upgrading air filters is only recommended after a fan inspection, or you can end up installing filters that restrict airflow. Instead of improving air quality, this will end up having a detrimental effect. Professional guidance is also necessary when deploying UVGI, since ultraviolet light can cause eye or skin damage with incorrect use.