Airborne infection control in commercial HVAC

Infectious respiratory aerosols, the small particles people exhale when they breathe, talk, cough or sing, can stay suspended in indoor air for minutes to hours and travel well beyond close contact distances. HVAC systems cannot eliminate transmission, but they strongly influence how quickly those aerosols are diluted, captured or inactivated. Since 2020, owners of offices, schools, retail and assembly buildings have asked what their systems can realistically do, and the industry now has a dedicated standard to answer that question.

This guide covers the layered measures available, how ASHRAE Standard 241 organizes them, and practical steps for non-healthcare commercial buildings. Hospitals and other healthcare facilities follow CSA Z317.2 and their own infection prevention requirements, discussed briefly at the end.

How HVAC affects airborne risk

Risk from long-range airborne transmission in a room depends on how many infectious particles are released, how long people stay, and how fast those particles are removed from the breathing zone. HVAC systems control the last factor. Particles are removed by:

• Dilution with clean outdoor air
• Filtration of recirculated air, either centrally or with in-room air cleaners
• Inactivation by germicidal ultraviolet light or other technologies
• Deposition on surfaces, which HVAC does not meaningfully control

HVAC has little effect on close-range transmission between people in conversation, which is why ventilation measures complement, rather than replace, other public health measures.

Equivalent clean airflow

ASHRAE Standard 241, Control of Infectious Aerosols, published in 2023, introduced the concept of equivalent clean airflow. Rather than specifying outdoor air alone, it adds together every route that delivers pathogen-free air to the breathing zone:

• Outdoor air supplied to the space
• Recirculated air multiplied by the single-pass efficiency of the filter treating it
• Clean air delivery from in-room air cleaners, based on tested clean air delivery rate (CADR)
• Effective inactivation from germicidal UV systems

The standard sets minimum equivalent clean airflow per occupant by space type for an infection risk management mode. That mode is intended to be activated when public health authorities or owners decide risk is elevated, not necessarily to run year-round. The flexibility is important: an owner can meet the target with the most cost-effective combination of measures rather than simply flooding a building with outdoor air, which is expensive and often impossible in a Canadian winter. Standard 241 also requires planning, documentation and assessment of the measures in a building readiness plan.

The layered approach

Ventilation

Outdoor air dilutes contaminants and should first meet the design rates in ASHRAE Standard 62.1 or the locally adopted code. Steps that improve effective ventilation include:

1. Verify minimum outdoor airflow by measurement, not damper position alone.
2. Correct economizer and outdoor air damper faults.
3. Extend ventilation run times before and after occupancy to flush spaces.
4. Review demand-controlled ventilation setpoints so they do not reduce outdoor air below code minimums during occupancy.

Increasing outdoor air beyond design is limited by coil capacity, heating and freeze risk, humidity and energy cost, so it is rarely the whole answer in Canada.

Filtration

Upgrading central filtration is often the most cost-effective step. MERV 13 filters capture a large share of particles in the size range of respiratory aerosols, while MERV 8 filters capture far fewer. Before upgrading, check that the fan can handle the added pressure drop, that filter racks seal properly (bypass around a filter defeats its rating), and that coils are not starved of airflow. See filtration, MERV ratings, HEPA and pressure drop.

Portable and in-room air cleaners

HEPA-based portable air cleaners add clean airflow to a specific room without modifying the central system. They are useful in classrooms, meeting rooms and clinics where central upgrades are not practical. Size them using their tested CADR and the room volume, place them so airflow is not blocked, and consider noise, which often leads occupants to turn units down.

Germicidal ultraviolet

Upper-room UVGI fixtures irradiate the air above occupants' heads and rely on room air mixing to carry particles through the UV zone. In-duct UV inactivates organisms as air passes, though a single pass at typical duct velocities gives limited dose. Both require careful design for safety and effectiveness. See UV germicidal irradiation in air systems.

Airflow patterns and humidity

Good mixing avoids stagnant zones where aerosols accumulate. Displacement and underfloor systems can remove contaminants efficiently when properly designed. Maintaining winter RH in the range of roughly 40 to 60 percent is sometimes recommended because very dry air may favour survival of some viruses, but humidification in cold Canadian climates must be balanced against condensation risk on windows and in the envelope.

Comparing options

Technologies to evaluate carefully

Ionization, plasma, photocatalytic oxidation and other electronic air cleaning devices are widely marketed. Their performance in real buildings is less well established than filtration or UVGI, and some can produce ozone or other byproducts. ASHRAE guidance recommends selecting devices with evidence of effectiveness under realistic conditions and confirming that they do not generate harmful byproducts, for example through UL 2998 validation for zero ozone emissions. Treat manufacturer claims based on small test chambers with caution.

Healthcare and higher-risk spaces

Healthcare facilities in Canada are designed to CSA Z317.2, which specifies air change rates, filtration, pressure relationships and other requirements by space type. Airborne infection isolation rooms are kept negative to adjacent spaces with high air change rates and dedicated exhaust. Those requirements go well beyond what Standard 241 asks of commercial buildings, and are covered in isolation rooms, negative and positive pressure and our healthcare HVAC work.

Practical steps for owners

1. Have outdoor air, supply air and filter performance measured and documented by a qualified balancing contractor.
2. Repair damper, actuator and economizer faults identified in the process.
3. Upgrade to MERV 13 or the highest efficiency the system can handle.
4. Identify high-occupancy rooms that fall short and add portable air cleaners or UVGI.
5. Write the measures into a building readiness plan with clear triggers for switching modes.
6. Train operators on how to activate and deactivate enhanced modes.

Key takeaways

• HVAC reduces long-range airborne risk through dilution, filtration and inactivation; it has little effect at close range.
• ASHRAE Standard 241 adds outdoor air, filtered air, air cleaners and UV into a single equivalent clean airflow target.
• Verifying design ventilation and upgrading filtration are usually the first and most cost-effective steps.
• Portable HEPA units and upper-room UVGI fill gaps in specific rooms.
• Electronic air cleaners need independent evidence and byproduct checks before use.
• Healthcare facilities follow CSA Z317.2, which sets more stringent requirements.