COVID-19 Ventilation Guidance and Response


According to the World Health Organization (WHO), COVID-19 is primarily transmitted through respiratory droplet particles 5-10 μm. These droplets typically fall to the ground within ±6 feet. Droplets smaller than 5 μm are referred to as “droplet nuclei” and are often caused by partial evaporation of the larger droplets. These droplet nuclei can become suspended in air and form an aerosol traveling longer distances and surviving for 3-7 days. Per the CDC, typical virus particle size in an aerosol is 0.05 to 0.15 μm. While airborne droplet nuclei are not believed to be the major means of the virus spread, there is sufficient likeliness of such transmission that efforts should be undertaken by building operations to reduce risk of the virus spread by reduction of the airborne concentration of SARS-CoV-2 with ventilation and filtration. (The symbol “μm” is a micron which is equivalent to 1/1000 millimeter. For reference, a human hair is about 100 microns in diameter.)

Typical respiratory droplet particle size and quantity from occupants are as noted below:

Sneezing releases 4500-1000000 organisms. 75% of droplets from sneezing are 10 micrometers, while less than 25% are 1-5 micrometers in diameter.

The most effective prevention for both airborne and droplet transmission is the combination of wearing masks and keeping a distance of greater than six feet between people. This combination reduces the chance of transmission very significantly – a current estimate of 50 to 100 fold.

In addition to requiring masks, Wooster is also following guidance on building ventilation systems—increasing fresh air and filtration and optimizing humidity levels as much as possible—to further increase safety.

Wooster’s Response

To help minimize the risk of virus spread within the building, Wooster Facilities staff working along with Consulting Engineer H. Lewis Hinkel, Jr., PE of HLH Engineering Consulting, LLC, of Wooster, OH, will, the best of our ability, follow recommendations and guidelines for proper building operations in specific consideration of the current COVID-19 pandemic as published by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and the Centers for Disease Control (CDC).

Throughout the pandemic, Facilities continues to perform preventative maintenance (PM) on the College’s ventilation systems, including cleaning air handling units, completing PM cycles on ventilation equipment (i.e. fans, pumps, chillers, boilers, filters, automation system controls and their calibration), and changing filters on a consistent basis. Temperature and pressure sensors have been tweaked along with schedules and set points. Facilities also has fully disinfected campus using EPA and CDC approved disinfectant and increased cleaning protocols including regularly disinfecting high touch surfaces.

We will operate our HVAC circulation/ventilation fans continuously and increase the amount of outdoor air brought into the occupied spaces by the HVAC system where applicable to filter and dilute contaminates within the indoor air. We will also be inspecting systems and restoring them back to their original design performance standard if any are found to have been modified.

We will improve the “air cleaning” effectiveness of the HVAC systems by replacing current air filters with a target for minimum air filter efficiency of MERV-11. MERV ratings range from 1-16. The higher the MERV rating on a filter, the fewer dust particles and other contaminants can pass through it. MERV is the Minimum Efficiency Reporting Value for an air filter in accordance with ASHRAE Technical Standard 52.2 with ratings from 1 to 16; with the higher the rating the better. MERV 11 filters will remove at least 85% of E3 particles, 65% of E2 particles, and 20% of E1 particles from the air. Thus, the air may need to pass through the filter numerous times to aide in very small particulate removal and thus the reason for continuous fan operation. (A typical commercial building HVAC system will circulate the air through a space between 5-8 times an hour.)

We are working with our supplier, Dayton Reliable Air Filters, to implement the majority of these filter replacements. Most of the equipment can be upgraded to MERV-11 air filters. Some smaller equipment can only be served by MERV-8 filters.

The spread of the virus by airborne transmission is greater in dry air, so is the ability of human beings to fight the infection (Harvard Medical School research). The ideal relative humidity range recommended by ASHRAE for unfavorable survival of microorganisms and for human comfort and health is 40%-60%. However, while this range is an ideal target, it must be considered as a general guideline as ASHRAE also cautions that such humidity may also cause adverse effects in some buildings and climates due to condensation and mold growth both within the occupied space and also concealed within the building wall cavities. Therefore, where feasible, the relative humidity will be targeted at between 40%-60% and we will pay attention to and balance the risk of virus spread for COVID-19 against the risk of creating mold.

Our HVAC/MEP systems are part of a holistic approach to combat the transmission of the COVID-19 pathogen SARS-CoV-2. Improved engineering controls along with proactive maintenance practices including a focus on preventative maintenance will improve the conditioned spaces on our campus as well as help to avoid transmission of the virus.

Please send questions to Associate Vice President of Facilities, Design and Construction Mike Taylor at or to our lead Consulting Engineer H. Lewis Hinkel at

For additional information and support documentation, please reference following articles: