Re-Opening and Ventilation Adjustments

We have assisted many different types of employers with their re-opening plans in light of the COVID infections. One question that comes up often is what adjustments should be made to the HVAC system. This article briefly discusses issues with ventilation and COVID.

According to the CDC, “Current data suggests person-to-person transmission most commonly happens during close exposure to a person infected with the virus that causes COVID-19, primarily via respiratory droplets produced when the infected person speaks, coughs, or sneezes. Droplets can land in the mouths, noses, or eyes of people who are nearby or possibly be inhaled into the lungs of those within close proximity. Transmission also might occur through contact with contaminated surfaces followed by self-delivery to the eyes, nose, or mouth. The contribution of small respirable particles, sometimes called aerosols or droplet nuclei, to close proximity transmission is currently uncertain. However, airborne transmission from person-to-person over long distances is unlikely.”

There is also information in the literature that direct contact such as touching may pose a path of transmission as well the possibility of the virus being suspended in air from fecal matter during flushing of toilets with the lid left open.

The size of the droplets being created from coughing, sneezing, talking, etc. have been reported to be in the 5 to 10 micron range, are relatively heavy, and do not stay suspended in air for long.  Particles that land on surfaces and desiccate will become smaller and may become suspended.  If that occurs, the particles can travel a distance. What distance and whether the virus could be entrained in the ventilation system is questionable.

Airborne transmission of the virus is being debated.  As you note from the CDC quote above, mention of airborne particle transmission over long distances is unlikely.  WHO also states COVID is “primarily transmitted between people through respiratory droplets.”  The scientific literature has limited articles describing infections of people in buildings that were more than 6 feet.  A study about a restaurant in China where several people became infected from one patron is quoted often but information indicates that the restaurant was poorly ventilated.

Other studies seem to indicate the possibility of disease transmission by ventilation, but the room size where this seems to be discussed generally are small areas.  Most studies reported in larger HVAC systems do not show ventilation as a mode of transmission.  For example, the study on the Diamond Princess Cruise Ship that reported more than 100 cases among the passengers concluded, “the ship central air conditioning system did not play a role…Most transmission appears to have occurred through close contact and formites.” [Xu 2020]

ASHRAE’s Position Paper on infectious aerosols [April 2020] states, “transmission of SARS-CoV-2 through the air is sufficiently likely that airborne exposure to the virus should be controlled.”  They also state, “ventilation and filtration provided by heating, ventilating, and air conditioning systems can reduce the airborne concentration of SARS-CoV-2 and thus the risk of transmission through the air.”  ASHRAE makes the following recommendation for non-healthcare buildings:

  • Increase outdoor air ventilation to 100% (open outdoor air dampers)
  • Use MERV-13 or higher air filtration
  • Keep systems running 24/7 if possible
  • Add portable room HEPA or high-MERV air cleaners
  • Add duct or air handling unit mounted upper room and/or portable UVGI in connection to in-room fans in high density spaces, e.g., waiting rooms [UVGI is ultraviolet germicidal irradiation]
  • Maintain temperature and humidity as applicable to the infectious aerosol of concern [several publications have recommended maintaining humidity in the 40-60% range. ASHRAE does not recommend an indoor temperature and humidity to control aerosol transmission]
  • Bypass energy recovery ventilation systems that potentially leak exhaust air back into the air supply.

I believe these recommendations are good to follow, but the basis does not seem to be justified in the literature.  There are a few papers in the literature that suggest that the infectious aerosol can be picked up and drawn through the HVAC system, and supplied to another room where some can become infected, however, the information seems quite specific to the area being studied.  That being said, in light of the pandemic and in an abundance of caution, we suggest following the ASHRAE guidelines.  There are studies that recommend the rate of ventilation (in air changes per hour) based on an analysis of aerosol dilution.  This will not be discussed in this newsletter article.  The topic of ventilation and its impact on the spread of the COVID virus remains an interesting one that we’d be happy to discuss in detail.