Building Immune Systems in Restaurants

RISK REDUCTION | 27

Path Three (Aerosolized Airborne) Transmission

MERV 13 or 14 Filters Higher efficiency filtration removes more suspended particles from the recirculated air within a building. As shown below, a MERV 11 filter will capture 50% of the particles of 0.5 microns in size. On the other hand, a MERV 14 filter will capture 85% of those same size particles. Most existing air handling systems can move appropriate air flows with either MERV 13 or 14 filer replacements.

The CDC notes SARS-CoV-2 is primarily transmitted by person-to-person contact and by contact with virus-laden droplets expelled through coughing and sneezing (i.e., PATH 1 and PATH 2 noted above). However, it is known airborne transmission caused infections of SARS-CoV-1 in the 2003 outbreak. It is likely COVID-19 is also transmitted via this transmission path. The good news is ventilation systems can influence the transmission of droplet nuclei in infectious aerosols. American Society of Heating, Refrigeration and Air- Conditioning Engineers or ASHRAE's Epidemic Task Force has issued guidance on managing the spread of SARS- CoV-2 regarding the operation and maintenance of HVAC systems in buildings. ASHRAE officially opposes any advice not to run commercial HVAC systems (35) . ASHRAE asserts that keeping air conditioners on during this time can help control the spread of the virus. It is recommended systems are re-programmed to operate 24/7 if they are not already doing so. ASHRAE also recommends the employment of HVAC filters and other strategies that help to reduce virus transmission while removing other air contaminants (35) . However, some strategies to reduce aerosol airborne dissemination of viruses at the air handling system may not be possible without impacting existing HVAC system capacities. Modifications to existing HVAC systems may not be possible if there is a goal to keep space design temperatures and relative humidity the same as the pre-pandemic goals. Possible modifications to HVAC systems that will help reduce PATH 3 transmission risk include the following: Increased Ventilation ASHRAE recommends the continued operation of all HVAC systems (35) . Outside air for ventilation should be increased to as much as the HVAC system can accommodate and still maintain acceptable indoor conditions during occupied hours. Special flushing control sequence should be implemented to operate the HVAC system with a maximum outside airflow of two hours before and after occupied times.

Figure 25. Minimum efficiency curve for several filter ratings (41)

Pressurization and Air Flow (41) ASHRAE recommends systems maintain equal pressures on all the floors in multi-floor buildings while maintaining a slightly positive pressure as compared to outside. Within spaces, pressurization can be provided to maintain airflow from low-risk areas to high-risk areas (like restrooms). In addition, it is recommended return air registers be located away from entry points to create a lower risk area when individuals enter a space. Return air should be provided in every space even if those spaces are open to each other. This will minimize the risk of virus freely flowing from one infected space into another uninfected space.