airborne COVID-19 transmission risk reducing HVAC MERV-13 filtration ventilation rates Wells-Riley equation

Reduce the Risk of COVID through Ventilation and Filtration Changes to Your HVAC Systems

The COVID-19 pandemic will likely be with us for months to come, and as more businesses and schools attempt to return to normal operations, creating safe buildings with healthy air is more important than ever. Using the Wells-Riley equation, we can estimate the probability of an individual with COVID-19 infecting a healthy individual inside of a building via the HVAC system.

Lowest Cost Measures to Reducing COVID-19 Infection

Before undergoing changes to your HVAC systems, the most vital aspects of creating a safe environment for your building includes occupants wearing masks, social distancing, and staying home when sick. These are the most economical ways to reduce the probability of infection for every individual in the building. Simply requiring masks can decrease the risk of infection up to 75%.

The Wells-Riley Equation

To calculate the risk associated with the airborne transmission of SARS-CoV-2 (the virus which causes COVID-19), we use the Wells-Riley equation. The Wells-Riley equation was originally developed to model measles outbreaks in schools but has been used widely to model other diseases as well. ASHRAE also uses this equation for their HVAC recommendations. Our purpose using this equation is to help building operators understand the contribution of the HVAC system to the overall risk of spreading the disease in buildings and the utility of different strategies in decreasing that risk.

For details on the equation, you can read more in our whitepaper on ‘Reducing the Risk of COVID-19 in Indoor Environments using the Wells-Riley Equation’ and download our calculator (an Excel workbook) which allows you to estimate risk using different filtration levels and ventilation rates.

For additional information on mitigating the spread of COVID-19, you can read more in our blog post on reopening buildings safely, download our free guide with our top three recommendations for safe building operations, and read more about our COVID services for healthy building operation.

1. Upgrade Air Filters to MERV 13

After behavioral measures, the next most economical option to reducing the spread of COVID-19 is upgrading your air filters to at least MERV-13. For many existing installations, upgrading to MERV-13 could reduce the probability of infection by half. It is important to note that some systems are not designed to be compatible with high efficiency filters. For systems where MERV-13 is unobtainable, moving to the highest compatible MERV rating can still contribute to a reduced risk. Upgrading beyond MERV-14 will not see a significant improvement in safety.

air filter outside air airborne COVID-19 transmission risk reducing HVAC MERV-13 filtration ventilation rates Wells-Riley equation2. Increase Outside Air Rates

Another recommendation to reduce COVID-19 infection is to increase the outside air rate of your HVAC system. Typical air handlers in most buildings use about 20% outside air, while recirculating 80% of the air in the zones. However, increasing the outside air ventilation rate, while helpful, can become prohibitively expensive, and may mean that your system can’t meet loads on very warm or very cool days. This measure also becomes less important if high efficiency filters can be installed. Using our calculator, you can see the impact of different ventilation rates and risk vs other mitigation strategies.

Reducing the Risk of COVID-19 Infection in Indoor Environments

As always when making changes related to safe building operation, your building and occupancy specifics need to be considered. The approach to increasing filtration and ventilation rates is best used to validate modifications to the HVAC system as one part of a comprehensive safety plan which includes shortened school days, working from home, and social distancing which are all very important in reducing the spread of the virus. If you would like help assessing your HVAC operations for reducing the risk of infection, please call us or drop us a note on the website.

Due to the generalized form of the Wells-Riley equation and the ongoing research into the coronavirus, the numbers presented here, in our whitepaper and calculator should be taken as rough estimates.

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