Ventilation for wildfire smoke events

Wildfire smoke has become a regular summer concern across much of Canada. Cities in British Columbia, Alberta, the Northwest Territories and, increasingly, Ontario and Quebec have experienced days or weeks of heavy smoke. The main health concern is fine particulate matter (PM2.5), which penetrates deep into the lungs. Buildings offer real protection, but only if their HVAC systems are prepared to keep smoke out and clean the air that gets in.

This guide covers how smoke enters buildings, practical HVAC strategies during a smoke event, and how to prepare before the season starts.

What is in wildfire smoke

Smoke is a mixture of fine and ultrafine particles, carbon monoxide, nitrogen oxides, and many volatile and semi-volatile organic compounds. Most of the particle mass is in the fine range, well under 2.5 µm, which is why PM2.5 is the standard indicator. Filtration addresses particles well. Gases and odours are harder: they pass through particle filters and require activated carbon or similar sorbent media for meaningful removal.

Environment and Climate Change Canada and provincial agencies publish the Air Quality Health Index (AQHI) and special air quality statements during smoke events. Health Canada guidance encourages keeping indoor air as clean as possible during smoke events, including through filtration and portable air cleaners.

How smoke gets inside

• Outdoor air intakes deliver smoke directly into the supply air, reduced only by the filters in the air handler.
• Infiltration through the envelope, doors and openings carries unfiltered air in, especially if the building runs negative.
• Filter bypass around poorly sealed racks lets smoke skip the filter.
• Exhaust-driven depressurization, for example from kitchen or washroom exhaust without balanced make-up air, increases infiltration.

Strategy during a smoke event

The objective is to reduce indoor PM2.5 while still providing safe ventilation for occupants.

Filtration

Filtration is the most important tool. MERV 13 filters capture a large fraction of smoke-sized particles, and MERV 14 to MERV 16 do better. MERV 8 filters, still common in older buildings, capture relatively little of the fine fraction. Before upgrading, confirm:

1. The air handler fan can deliver design airflow with the higher pressure drop.
2. Filter racks are sealed and filters fit tightly, because bypass can undo the upgrade.
3. Spare filters are on hand, because heavy smoke loads filters quickly and pressure drop climbs fast.

See filtration, MERV ratings, HEPA and pressure drop for details.

Outdoor air

Reducing outdoor air to the minimum required for occupants is common practice during heavy smoke, along with disabling economizers. Outdoor air should not normally be shut off entirely in occupied buildings: occupants still generate CO2 and other contaminants, and buildings still need pressurization to limit unfiltered infiltration. ASHRAE guidance for wildfire smoke allows a temporary reduction of outdoor air during smoke events within a planned approach, and some jurisdictions accept temporary reductions during declared events. Confirm with the authority having jurisdiction where code minimums are in question, and resume normal ventilation as soon as outdoor air quality improves.

Pressurization

Maintain a slight positive building pressure so that air leaks out rather than in. When outdoor air is reduced, exhaust fans may need to be reduced proportionally to avoid depressurizing the building. See building pressurization and infiltration control.

Recirculation and fan operation

Run supply fans continuously during occupied hours, and consider extended or continuous operation during heavy smoke, so recirculated air passes through filters repeatedly. Each pass removes more particles.

Portable air cleaners

Portable HEPA air cleaners add clean air in specific rooms, particularly useful in buildings with low-efficiency central filtration, unit ventilators, or no central air handling. Size them by clean air delivery rate (CADR) for the room volume. Avoid devices that intentionally generate ozone.

Gas-phase filtration

For buildings serving vulnerable populations, such as hospitals, long-term care homes and schools, adding activated carbon filters in air handlers can reduce smoke odours and some gaseous pollutants. Carbon adds pressure drop and has a limited capacity, so it is usually a deliberate design choice rather than a temporary fix.

Summary of event actions

Monitoring

Indoor and outdoor PM2.5 monitoring turns guesswork into decisions. Optical particle sensors are inexpensive and, while their absolute accuracy varies, they show trends and the ratio of indoor to outdoor concentrations clearly. Integrating sensors into the BAS allows operators to trigger a smoke mode automatically when outdoor PM2.5 exceeds a threshold and to confirm indoor levels are falling. Monitor CO2 at the same time to confirm reduced outdoor air is not causing poor ventilation. The IAQ testing guide covers instruments.

Preparing a smoke readiness plan

The time to prepare is before smoke season. A smoke readiness plan should include:

1. An inventory of air handlers, their filter types and the maximum filter efficiency each can accept.
2. Confirmation that filter racks are sealed and in good repair.
3. A stock of higher-efficiency filters and, if needed, portable air cleaners.
4. A written smoke mode sequence in the BAS: outdoor air to minimum, economizer disabled, exhaust adjusted, fans continuous.
5. Triggers for entering and exiting smoke mode, based on AQHI or measured PM2.5.
6. Identification of clean air rooms for vulnerable occupants.
7. Operator training and a post-event procedure for filter replacement and system return to normal.

Implementing and testing the smoke mode sequence is a task suited to control system verification.

Key takeaways

• PM2.5 is the main health concern in wildfire smoke, and filtration is the most effective defence.
• Upgrade to MERV 13 or higher where fans can handle it, and seal filter racks.
• Reduce outdoor air to minimum and disable economizers during heavy smoke, but maintain ventilation and positive pressure.
• Portable HEPA air cleaners help in rooms with weak central filtration.
• A written, tested smoke mode sequence and filter stock prepared before the season make the response fast and reliable.