Wildfires and Infections: Maintaining Good Indoor Air Quality

How is the air quality inside your offices or other facilities? Have you spoken recently with your building or facilities manager about your heating, ventilation, and air conditioning (HVAC) systems?

Proper ventilation and air filtration can lower indoor concentrations of both infectious disease particles and particulate matter from wildfires, protecting your employees, customers, and visitors.

Particulate matter from wildfire smoke—material less than 2.5 micrometers in diameter (PM 2.5)—affects outdoor workers more than indoor workers. However, some smoke and particles can seep into indoor spaces.

There’s no federal wildfire smoke standard, but some state plan states have their own standards for protecting outdoor workers from wildfire smoke. State requirements are triggered by air quality index (AQI) levels and include administrative and engineering controls, providing respirators (N95, N99, N100, R95, P95, P99, or P100) for voluntary use, and requiring respirators in the highest levels of particulate matter, such as an AQI for PM2.5 exceeding 500.

In indoor spaces, properly maintained HVAC systems can mitigate building occupants’ particulate matter and wildfire smoke exposure. Maintenance and possible upgrades can address both ventilation and air filtration.

Air filters in HVAC systems have Minimum Efficiency Reporting Value (MERV) ratings that range from MERV 2 to 16 and high-efficiency particulate air (HEPA). MERV 13 or higher filters can remove airborne particles from outdoor air introduced into a building’s systems, including particles from desert dust storms, vehicle traffic, and wildfires, as well as infectious disease particles in air recirculated throughout a building.

Portable air cleaners or purifiers with HEPA filters can further reduce the concentration of PM 2.5 in a room.

Free-standing, plug-in portable air cleaners with HEPA filters can also capture infectious airborne particles, like those that cause COVID-19, influenza, and respiratory syncytial virus (RSV), in the rooms where they’re placed.

After smoke from Canadian wildfires moved into the Midwestern and Northeastern United States this summer, the Healthy Buildings project at Harvard University’s T.H. Chan School of Public Health released a fact sheet on wildfire smoke and electrostatic air filters. The group recommended the use of electrostatically charged air filters, such as MERV 13 or higher, in HVAC systems. Because smoke particles can diminish the filter charge, Healthy Buildings also recommended changing air filters after a wildfire smoke event diminishes.

ASHRAE Standard

When the COVID-19 pandemic struck in 2020, employers had little information on controlling infections in indoor spaces. There were suggestions and recommendations at the time, some developed to address the hazards posed by Legionella bacteria that can cause Legionnaire’s disease, a bacterial pneumonia.
Now you have a consensus indoor air quality standard for all airborne disease hazards.

The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) developed aControl of Infectious Aerosolsstandard (ASHRAE 241-2023), establishing requirements for reducing the risk of disease transmission caused by exposure to infectious aerosols. Standard 241 is a consensus industry standard but not a formal American National Standards Institute (ANSI) standard.

Unique aspects of the ASHRAE’s Standard 241, which covers aspects of air system design, installation, operation, and maintenance, include:

• Filtration and air cleaning technology requirements such as testing to ensure system operation doesn’t degrade air quality in other ways, like elevating ozone levels;
• Equivalent clean airflow rate providing flexibility in the methods and equipment used to achieve pathogen-free airflow—technologies that include volume of outdoor air, filtration of indoor air, and air disinfection by technologies like germicidal ultraviolet (GUV) light;
• An infection risk management mode (IRMM) that applies during identified periods of elevated risk of disease transmission; and
• Assessment and planning requirements that result in the development of a building readiness plan.

The Standard 241 committee remains intact and will continue to improve sections of the standard.

Earlier this year, the Centers for Disease Control and Prevention (CDC) updated its “Ventilation in Buildings” guidelines to recommend aiming for five or more air changes per hour (ACH) of clean air to help reduce the number of infectious particles in the air.

The CDC recommended that building owners and managers take 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 recommended upgrading filter efficiency in HVAC systems to MERV 13 or better when compatible with existing equipment. The CDC also suggested using portable or built-in HEPA fan/filtration systems (“air cleaners” or “air purifiers”), as well as ultraviolet germicidal irradiation (UVGI), also called GUV, as a supplemental treatment to inactivate airborne viruses, such as SARS-CoV-2.

In its workplace guidance on COVID-19 control and prevention, the Occupational Safety and Health Administration (OSHA) recommends adjusting HVAC systems to introduce additional outside air and/or increase air exchange to introduce fresh air and refers employers to the CDC’s Ventilation in Buildings recommendations.

Last summer, the Lancet COVID-19 Commission’s Task Force on Safe Work, Safe School, and Safe Travel offered recommendations for building owners to address SARS-CoV-2 infection risks in schools and workplaces in its report “The First Four Healthy Building Strategies Every Building Should Pursue to Reduce Risk from COVID-19.” Strategies included verifying that building systems are performing as designed, increasing outdoor air ventilation, upgrading air filtration, and deploying portable air cleaners where needed.

Verifying building systems will address HVAC system deficiencies such as imbalanced airflow, system operation mismatch with occupied hours, damper malfunction, and systems control malfunctions.

Increased outdoor air ventilation can be achieved through the number of HVAC system air exchanges or open windows, which can dilute concentrations of infectious particles in an indoor space.

Respiratory Infections and Wildfire Smoke

Better air filtration can reduce infectious disease risks in indoor spaces but can also offer occupants protection from wildfire smoke and its particulate matter.

The New York Times recently reported on indoor air monitoring installed at Skidmore, Owings & Merrill, a Manhattan architecture and design firm, in its offices at 7 World Trade Center in the wake of the COVID-19 pandemic. The monitors kept employees informed of the office’s air quality when the smoke from Canadian wildfires moved into the Northeastern United States. The Environmental Protection Agency (EPA) offers guidance on finding low-cost pollution monitors that can help you improve indoor air quality in your facilities.

These low-cost monitors may be referred to as air sensors, air quality sensors, air quality monitors, air pollutant monitors, or air pollutant meters or detectors. The EPA also has more detailed information on its website about air sensor technology that may be used to detect airborne pollutant concentrations, activate equipment like an air cleaner, or trigger safety devices like carbon monoxide or smoke alarms. 

The Mechanics of Aerosol Transmission

Airborne diseases include COVID-19, influenza, and RSV, as well as measles and tuberculosis.

Those infected release particles and droplets of respiratory fluids containing bacteria and viruses when they exhale through breathing, coughing, exercising, singing, or speaking. When COVID-19 first emerged, public health experts thought it spread through large respiratory droplets, prompting recommendations of 6 feet of “social distancing.”

Infectious particles or very fine droplets can continue to spread through the air in the room or space and can accumulate in spaces with poor ventilation.

Physical distancing can help as part of a layered approach to controlling COVID-19 infections that also includes vaccination, cleaning and disinfection, hand hygiene, and wearing face masks.

Remember, however, that ventilation and air filtration can reduce the concentration of infectious particles in an indoor space. Steps to achieve adequate ventilation and air filtration might include:

• Bringing in as much fresh air as possible by increasing the HVAC system’s outdoor air intake and reducing recirculated air, as well as opening windows or other sources of fresh air where possible when weather permits.
• Ensuring that all HVAC systems are fully functional and determining whether demand-controlled ventilation has been adjusted or disabled, which would hinder the flow of maximum outdoor air.
• Using HVAC system filters with a MERV rating of 13 or higher where feasible. (Some systems may not support or need adjustment to support higher-filtration filters.) Ninety percent of particles are captured in filters with a MERV rating of 13.
• Inspecting filters and seals monthly, eliminating gaps around filters that allow nonfiltered air to recirculate, changing filters, and cleaning the system as needed.
• Ensuring that exhaust air isn’t pulled back into the building’s HVAC air intakes or open windows.
• Turning off ceiling fans or adjusting them to pull air up rather than down to reduce infectious particle dispersal.
• Considering the use of portable HEPA-equipped air cleaners or purifiers to increase clean air, especially in higher-risk areas.
• Ensuring that exhaust fans in restrooms are fully functional, operating at maximum capacity, and set to remain on.

When and where wildfire smoke is a concern, you’ll want to limit the intake of outdoor air and rely more heavily on air filtration and the use of portable air purifiers.

Portable air cleaners can offer you air filtration when outdoor air ventilation isn’t possible or preferable due to air pollution, outside temperature and humidity, or wildfire smoke.

Portable air cleaners can supplement HVAC systems’ air filtration and ventilation, especially in indoor spaces, where it’s difficult to achieve adequate ventilation. Portable air cleaners can add a layer of protection from both infectious disease particles and wildfire smoke and its particulates.

Your building manager or facilities engineer may be able to help you with your building’s systems, or you may want to procure the services of an HVAC consultant.

You should ask key questions, such as:

• How many ACHs of outside air are brought into the building or produced by filters in the HVAC equipment, and where does the outside air enter the building?
• What is the MERV rating of the filters used in the building, how often are filters changed, and can the building’s current systems function with filters rated MERV 13 or higher?
• Have building renovations changed conditions in some of the rooms since the building’s HVAC systems came online?

Indoor air quality is an important consideration during both respiratory disease outbreaks and wildfire smoke events.