On May 12, the Centers for Disease Control and Prevention (CDC) updated its “Ventilation in Buildings” guidelines to include a discussion on “How much ventilation is enough?” to help prevent airborne disease transmission.
“When possible, aim for 5 or more air changes per hour (ACH) of clean air to help reduce the number of germs in the air,” the CDC advised building owners and managers.
On May 15, the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE), a professional society that sets heating, ventilation, and air conditioning (HVAC) standards, released a draft of a standard for maintaining healthy indoor air quality (IAQ). ASHRAE is accepting comments on its draft 241P-202X Standard, Control of Infectious Aerosols, through May 26.
The draft ASHRAE Control of Infectious Aerosols standard would provide minimum requirements for HVAC-related measures to reduce the risk of transmission of COVID-19, influenza, and other airborne viruses in homes, hospital offices, and schools during periods of high transmission risk.
CDC’s Ventilation in Buildings
In addition to a five ACH ventilation recommendation, changes to the CDC’s ventilation and air filtration guidelines include the following:
- Addition of “Key Strategies,” a simplified summary of the CDC’s most important recommendations;
- Updated minimum filter recommendations—Minimum Efficiency Reporting Value (MERV)-13 filters or better;
- Updated guidance on post-occupancy flushing of building air;
- More information on upfront, maintenance, and energy cost considerations for ventilation strategies;
- A frequently asked questions (FAQs) on do-it-yourself (DIY) air cleaners;
- Updates to all FAQs to include a concise answer, followed by more detail; and
- Updated discussion on whole-room ultraviolet germicidal irradiation (UVGI) (also called Far UV).
The CDC recommended that building owners and managers take the following four steps to improve air circulation in buildings:
- At a minimum, ensure existing HVAC systems provide at least the minimum outdoor air ventilation requirement in accordance with ventilation design codes.
- Increase the introduction of outdoor air beyond code-minimum requirements.
- Use fans to increase the effectiveness of open windows.
- Rebalance or adjust HVAC systems to increase total airflow to occupied spaces when possible.
The CDC’s recommendations for improving air cleanliness included:
- Upgrading central HVAC filter efficiency to MERV-13 or better. Increased filtration efficiency, when compatible with an existing HVAC system, is helpful when ventilation options, such as enhanced outdoor air delivery, are limited.
- Inspecting HVAC systems to ensure ventilation systems operate properly and are up to date on maintenance; air filters are properly sized and within their recommended service life; and filters fit housing and racks appropriately to ensure air flows through, rather than around, the filter.
- Using portable or built-in high-efficiency particulate air (HEPA) fan/filtration systems (“air cleaners” or “air purifiers”).
- Using UVGI, also called GUV, as a supplemental treatment to inactivate airborne viruses, such as SARS-CoV-2.
The CDC acknowledged that DIY air cleaners, when built and used correctly, can provide a temporary intervention for reducing COVID-19 transmission, but it cautioned that DIY air cleaners are only appropriate during emergencies or for short-term use, such as when obtaining commercial air cleaners isn’t possible, not as a permanent, long-term solution.
A common DIY design is a cube made of MERV-13 (or better) filters as the cube’s sides and bottom and a 20-inch box fan attached on top. The EPA is studying the use of DIY air cleaners to reduce wildfire smoke indoors.