Q: How effective are filters at blocking the virus?
A: Filters with a MERV rating of 11 or higher are very effective at blocking the virus. In a study by the Illinois Institute of Technology, researchers investigated airborne virus and bacteria movement in a residential HVAC system. MERV 8 filtration was no different than no filter in removing viral matter. MERV 11 and MERV 16 (similar to HEPA) were found to remove 85% and 95% of viral matter, respectively [5].

Q: How often should air ducts and vents be cleaned?
A: In most cases, duct cleaning is unnecessary if filters are changed regularly and the air handling units are cleaned and functioning properly. The key is prevention.  Increasing filters from today's MERV 8 to MERV 13 will improve duct cleanliness. Duct dust can harbor contaminants, but multiple studies have found that duct cleaning rarely improves indoor air quality and sometimes can make it worse [67]. Duct cleaning is appropriate if you have:

  • Persistent duct water damage
  • Slime or microbial growth observed in ducts
  • Debris build-up that restricts airflow
  • Dust discharging from supply diffusers
  • Offensive odors originating in ductwork or HVAC components.

Q: What are some things I should consider when restarting air handling units after a period of disuse? 
A: Contact HVAC professionals for basic system startup and maintenance (cleaning out cobwebs, checking filters, etc). Maintenance will be similar to restarting AC in the spring and heating in the winter. Increased ventilation air flow may loosen accumulated dust/dirt on duct walls of older systems. Cleaning ducts prior to increased fan ventilation can prevent the release of duct wall debris (and lower particulates in indoor air).

Q: What are some considerations when deciding whether to use a portable in-room air cleaner (with HEPA filters and UVGI) or mount one on the wall or ceiling? 
A: In-room air cleaners can be portable or fixed. According to a review of literature by the Ontario Health Technology Advisory Committee, fixed are preferable because they have a greater degree of reliability. The effectiveness of these cleaners depends on their correct placement. Some things to consider when deciding between wall mounted or ceiling mounted include airflow patterns, number of occupants, time of occupancy, and type of air cleaning technology used.  Consult with ventilation engineers, infection control experts, and industrial hygienists to install the air cleaner.

Other Questions

Virus transmission

Overall recommendations

Increasing airflow

Ultraviolet irradiation

Risk assessment

Have a question about building modifications during COVID-19? Contact SEDAC at info@sedac.org or 800.214.7954.

Interested in receiving a COVID-19 Readiness Building Assessment? SEDAC's COVID-19 Readiness Assessments give you the independent, expert advice you need to identify medium to high risk spaces and make building modifications to reduce the risk of virus transmission.