How Canadian Wildfire Smoke Affects Commercial Buildings, and What Facility Managers Can Do About It

On the worst morning of the July 2026 wildfire smoke event in southeastern Michigan, visibility outside our manufacturing facility was less than a quarter mile. The end of the road was difficult to see. The sky had taken on the dull, dirty appearance that has become increasingly familiar during major smoke events, and the odor was unmistakable as soon as someone stepped outside.

This was not a faint campfire smell drifting through the neighborhood. It was a widespread air-quality emergency.

Detroit was reported as having the worst air quality among major cities worldwide during the event. Michigan’s Department of Environment, Great Lakes, and Energy issued statewide alerts because Canadian wildfire smoke was driving PM2.5 concentrations to unhealthy levels. (Planet Detroit)

At 5:48 that morning, a commercial laser particle counter used outside our facility registered approximately 2.68 million particles in its 0.3-micron channel during the instrument’s sampling cycle. By approximately 8:30 a.m., the outdoor count had decreased slightly, to about 2.40 million.

Inside the manufacturing plant, we were measuring approximately 684,000 particles under the same general test method. In the front office areas, readings were approximately 391,000.

Those indoor numbers were still much higher than the levels we normally observe. However, they were dramatically lower than the air immediately outside the building.

That difference did not occur by accident. We kept the factory doors closed, continuously operated existing ceiling-mounted commercial air scrubbers, used HEPA-level final filtration with tackified paint-booth-style prefilters, operated the building’s HVAC equipment, and used both particulate and activated-carbon filtration in the office areas.

The experience reinforced an increasingly important lesson for facility managers:

A building does not automatically protect its occupants from wildfire smoke simply because everyone is indoors.

Wildfire smoke is not just an outdoor problem

Commercial buildings exchange air with the outdoors in several ways.

Mechanical ventilation systems intentionally introduce outdoor air. Exterior doors open for employees, deliveries, forklifts, customers, and material movement. Air enters through loading docks, louvers, wall penetrations, roof openings, window gaps, and imperfections in the building envelope. Exhaust equipment can also place portions of a building under negative pressure, causing replacement air to be drawn through whatever openings are available.

Even a mostly closed building is not an airtight container.

During ordinary conditions, outdoor ventilation is essential. It helps dilute carbon dioxide, odors, moisture, and pollutants generated inside a building. During a major wildfire smoke event, however, the outdoor air that normally provides dilution can become the dominant source of contamination.

This creates a difficult balancing problem. Facility managers must limit smoke intrusion without creating unacceptable indoor temperatures, process problems, pressure imbalances, or insufficient ventilation.

ASHRAE’s wildfire guidance recognizes this problem. Its recommendations include developing a smoke-readiness plan, reducing uncontrolled infiltration, evaluating outdoor-air operation, using MERV 13 or better filtration when the system can accommodate it, and supplementing central equipment with appropriate air-cleaning systems. (ASHRAE)

What is actually in wildfire smoke?

Wildfire smoke is a changing mixture of particles and gases produced by burning vegetation, buildings, soil, vehicles, plastics, coatings, and other materials encountered by the fire.

The greatest widespread health concern is usually fine particulate matter, commonly identified as PM2.5. These are particles with aerodynamic diameters of 2.5 micrometers or smaller.

Their small size allows them to remain suspended in the air, travel long distances, enter buildings, and penetrate deeply into the respiratory system. Wildfire smoke can aggravate asthma and other respiratory conditions and can also contribute to cardiovascular stress. Reported effects include burning eyes, coughing, wheezing, headaches, shortness of breath, and worsening heart or lung conditions. (Michigan.gov)

Smoke also contains gases and volatile organic compounds. This distinction matters because particle filtration and gas filtration do different jobs.

A high-efficiency particulate filter may substantially reduce airborne particles without eliminating the smoke odor. Activated carbon can adsorb some gases and odor-causing compounds, but it is not a substitute for effective particulate filtration.

This is why one filter material should not be expected to solve every part of the problem.

The biggest misconception: “It is basically a distant campfire”

The most dangerous misunderstanding may be the belief that wildfire smoke is little more than an unpleasant campfire smell.

A campfire is usually a nearby, visible source that people can move away from. A regional wildfire smoke event can affect millions of people and persist for hours or days. The smoke may have traveled hundreds or thousands of miles, undergone atmospheric changes, and accumulated across a large geographic area.

The odor can also be misleading.

A person may smell smoke at a relatively low particle concentration, or the odor may diminish while harmful fine particles remain elevated. Conversely, a building using effective carbon filtration may have little noticeable smoke odor even though particulate concentrations still need to be monitored and controlled.

Your nose is not a particle counter.

The absence of odor does not prove that the air is clean, and the presence of odor does not tell you the exact concentration of PM2.5.

What our measurements showed

Our outdoor particle count was approximately 2.68 million in the instrument’s 0.3-micron channel early that morning. The manufacturing area measured approximately 684,000, and the front offices measured approximately 391,000.

Expressed as simple relative differences:

• The manufacturing reading was approximately 74% below the outdoor reading.
• The office reading was approximately 85% below the outdoor reading.

These percentages should not be interpreted as laboratory-certified filter efficiencies. This was a real building under active operating conditions, with multiple filtration systems, air leakage, HVAC mixing, employee movement, and changing outdoor concentrations.

The readings also do not represent AQI. AQI is based on pollutant concentration and health-based breakpoints, while an optical particle counter reports particle counts within defined size channels. Different particle compositions, densities, shapes, humidity conditions, instruments, and sample durations can affect the results.

What the measurements demonstrate is more practical:

The building’s operating decisions and air-cleaning systems created a substantial separation between indoor and outdoor particle counts during an extreme event.

That is exactly what a smoke-response strategy should attempt to accomplish.

Why the indoor numbers remained higher than normal

The front-office particle count of approximately 391,000 was much better than the outdoor count, but it was still higher than what we normally see.

That initially caught us off guard.

The likely explanation was not failure of a single filter. It was the total contaminant load placed on the building.

If outdoor air is ten or twelve times worse than the conditions a facility normally experiences, even a relatively small amount of infiltration can matter. Outdoor air may enter through the mechanical system, door traffic, leakage, pressure differences, or other openings.

For example, imagine that only 5% of an air stream consists of highly contaminated outdoor air. If that outdoor portion contains an enormous particle concentration, constantly blending it into recirculated air can continue adding particles faster than might occur during an ordinary day.

Filtration then becomes a continuous removal process rather than a one-time cleanup.

The important question is not whether particles enter. In a working commercial building, some usually will. The question is whether the building removes them quickly enough to prevent indoor concentrations from approaching outdoor levels.

Why we kept the HVAC system running

A common reaction during smoke events is to shut everything down.

That may be appropriate for certain pieces of equipment, but simply shutting off an HVAC system is not automatically the best strategy. If a system provides useful recirculation through effective filters, turning it off can also eliminate one of the building’s primary particle-removal mechanisms.

AirNow recommends operating central systems so they continue filtering indoor air, using high-efficiency filters when the equipment can safely accommodate them, and selecting recirculation settings where appropriate. NIOSH similarly advises workplaces to close doors and loading docks, use recirculation or temporarily reduce outdoor-air introduction when safe and appropriate, and use HEPA-equipped air cleaners. (AirNow)

The correct response depends on the building.

A facility manager should understand:

• How much outdoor air the system introduces
• Whether the outdoor-air damper can be adjusted
• Whether exhaust systems create substantial negative pressure
• What filtration is currently installed
• Whether the fan can accommodate higher-efficiency media
• Whether the building still requires outside air for processes, pressure control, or code-related reasons
• Whether reduced outdoor air would cause heat, humidity, carbon dioxide, or contaminant problems indoors

Wildfire operation should be planned before the smoke arrives, preferably with the building’s HVAC contractor, engineer, controls provider, or test-and-balance professional.

MERV 13 and wildfire particles

AirNow and ASHRAE commonly recommend MERV 13 or better filtration for wildfire smoke, provided the HVAC system can safely support it. (AirNow)

MERV stands for Minimum Efficiency Reporting Value. Under the ASHRAE test method, filters are evaluated across particle-size ranges. MERV 13 media provides substantially better fine-particle capture than common low-MERV filters, particularly in the particle ranges relevant to smoke.

That does not mean a MERV 13 filter captures every particle in one pass. It also does not mean that every product labeled MERV 13 performs identically throughout its life.

Actual building performance depends on:

• The filter’s construction and media
• Air velocity through the filter
• Filter fit and bypass leakage
• The amount of recirculated air
• Runtime
• Filter loading
• Fan performance
• Outdoor-air introduction
• Door traffic and building leakage
• Placement of the filtration
• The number and location of supplemental air cleaners

Our own comparative testing has repeatedly shown strong performance from Camfil MERV 13 mechanical media. During the July smoke event, the filtration continued producing a meaningful indoor-to-outdoor reduction, although the indoor count was higher than we normally observe because the outdoor challenge was so extreme.

This is an important distinction. A filtration system can be performing well even when its downstream reading is higher than usual. The correct comparison is not only today’s indoor number versus last week’s indoor number. It is also today’s indoor concentration versus today’s outdoor concentration.

HEPA air scrubbers and prefiltration

Our manufacturing facility has used ceiling-mounted commercial air scrubbers for years. These units contain HEPA-level final filters, with tackified paint-booth-style media used as prefiltration.

The prefilter performs an important job. It captures larger dust, lint, fibers, and industrial debris before those contaminants reach the more expensive final filter. This helps preserve the HEPA filter’s capacity for smaller particles and can extend its useful service life.

The concept is similar to staged filtration used in many industrial systems:

1. A coarse or tackified prefilter captures heavier contaminants.
2. A higher-efficiency final filter removes finer airborne particles.
3. Recirculation repeatedly moves room air through the system.

A filter’s single-pass efficiency is only part of the equation. An air cleaner also needs sufficient airflow and runtime to process a meaningful portion of the room’s air.

A very efficient filter moving little air may have less room-level effect than expected. A somewhat lower-efficiency filter moving a larger volume of air repeatedly may, in some applications, produce a strong cumulative reduction. The best design considers both efficiency and airflow.

Carbon filtration handles a different part of smoke

The difference in odor between our offices and the factory was striking.

Outside the offices, the smell was obvious. Inside the offices, activated-carbon filtration substantially reduced it.

This did not mean carbon had removed all particulate matter. Carbon media primarily helps adsorb certain gases and volatile compounds that pass through ordinary particulate filters. ASHRAE specifically distinguishes gas-phase filtration from particle filtration and notes that odor is associated with gaseous compounds rather than simply serving as a measure of particle concentration. (ASHRAE)

For a more complete wildfire strategy:

• MERV-rated or HEPA filtration addresses particles.
• Activated carbon or other appropriate gas-phase media addresses certain gases and odors.
• Source control and building operation reduce how much smoke enters.
• Monitoring helps determine whether the strategy is working.

Carbon media also has finite capacity. Once adsorption sites become occupied, performance decreases. Carbon filters should remain sealed before they are needed because exposure to ambient air can begin using their capacity even when no wildfire event is occurring.

Facilities in smoke-prone regions should consider keeping properly packaged replacement carbon media available rather than waiting until smoke is already overhead.

Final-stage filtration near occupied spaces

A central HVAC filter is important, but it is not the final point in the air’s journey.

After central filtration, air may travel through ductwork, dampers, VAV boxes, lined ducts, flexible connections, plenums, and supply diffusers. It may also mix with air introduced through leakage or mechanical ventilation.

This creates an opportunity for final-stage filtration, meaning filtration near the point where supply air enters an occupied room.

Final-stage filtration does not eliminate the need for proper central filtration. It provides another layer of control, particularly in spaces where cleanliness, occupant sensitivity, dust reduction, or smoke protection is a priority.

Airotrust was developed around this principle. Its magnetic frame allows appropriately selected filter media to be positioned at a ceiling supply diffuser. In our office testing during the smoke event, the system helped maintain a significant reduction from outdoor particle levels despite employee traffic and continued HVAC operation.

The engineering question is not simply, “Can a filter be placed over a diffuser?”

The correct questions are:

• What is the diffuser face velocity?
• How much effective filter area is available?
• What is the media’s pressure drop at that velocity?
• How does airflow change after installation?
• Does the system maintain acceptable room delivery?
• Is the installation secure?
• How will filter condition be monitored?
• How often must the media be changed?

Any filter installation should be evaluated as part of the system rather than judged only by assumption.

Why lower face velocity matters

Filter ratings are obtained under defined laboratory conditions. The same media can behave differently when used at a different face velocity.

Face velocity describes how quickly air approaches and passes through the exposed filter area, usually expressed in feet per minute.

At a central HVAC filter rack, a large volume of air may be pushed through a relatively constrained area. At a ceiling diffuser, airflow is spread across the diffuser face, and the velocity through a full-face filter may be much lower.

Lower face velocity can provide several potential benefits:

• Lower pressure drop across the media
• Longer particle residence time within the filter structure
• Reduced likelihood of some captured material being disturbed
• More filter area relative to the local airflow
• Improved practical performance from certain media

This does not mean lower velocity makes every filter perfect. Filtration mechanisms vary by particle size, fiber construction, electrostatic charge, media depth, and other factors. However, velocity is a fundamental operating condition and should not be ignored when evaluating either capture or resistance.

Closing the doors was an operational decision, not a complete solution

We kept our factory doors closed throughout the day. They were opened only once to pre-stage deliveries and bring product into the building.

That decision significantly reduced uncontrolled smoke entry, but it came with tradeoffs.

Manufacturing buildings often rely on open doors for heat relief. Wildfire events frequently coincide with hot, dry weather. A facility without sufficient cooling may face two hazards at once:

• Heat stress if doors remain closed
• Smoke exposure if doors remain open

There is no universal answer. The employer must evaluate both risks and develop a plan that protects employees from each.

Possible actions include adjusting production schedules, reducing strenuous work, increasing cooling, using filtered recirculating air cleaners, establishing cleaner break areas, staging deliveries to reduce door-open time, providing appropriate respiratory protection where required, and pausing operations when conditions cannot be controlled safely.

The worst response is to accept severe exposure without examining alternatives.

“Other factories had their doors open” is not a safety plan

As I drove through our industrial park, many plants had their doors wide open while employees continued working.

That may have been necessary in some cases because of heat. It may also have been the result of limited equipment, limited preparation, or a belief that nothing meaningful could be done.

But “it is what it is” should not be the end of the discussion.

Facility managers cannot control where a wildfire starts or which direction the wind blows. They can control many aspects of how their own building responds.

They can measure conditions. They can stock filters. They can examine outdoor-air controls. They can repair door seals. They can reduce unnecessary openings. They can add recirculating filtration. They can establish cleaner zones. They can coordinate deliveries. They can communicate with employees. They can determine in advance when production should be reduced or stopped.

Creativity matters in an emergency, but preparation is better.

A practical commercial-building wildfire checklist

Before smoke season

Determine what filters are installed, confirm their sizes, document their ratings, and identify suitable higher-efficiency replacements. Have the HVAC system evaluated before increasing filter resistance. Purchase replacement particulate and carbon filters before regional demand spikes.

Identify outdoor-air intakes, exhaust equipment, loading docks, frequently opened doors, and major leakage paths. Determine which controls can be adjusted during an event and who is authorized to adjust them.

Inspect portable or ceiling-mounted air cleaners. Confirm airflow, filter condition, electrical requirements, maintenance procedures, and room coverage.

Acquire a reliable indoor-air monitor or establish access to professional particle-counting equipment. Understand what the instrument measures and what it does not.

Develop written operating thresholds using official AQI information, indoor measurements, employee symptoms, temperature conditions, and process requirements.

When smoke arrives

Monitor AirNow, state environmental agencies, local alerts, and actual conditions around the facility. Keep unnecessary exterior openings closed. Consolidate deliveries and material movement when possible.

Continue useful recirculating filtration. Adjust outdoor-air intake only after considering code requirements, process safety, occupancy, carbon dioxide, pressure relationships, and heat.

Run supplemental HEPA or high-efficiency air cleaners continuously where appropriate. Install carbon media when odor and gaseous contaminants are a concern.

Reduce indoor sources of particulate matter when practical. Activities such as smoking, combustion, certain cooking processes, sweeping, and unfiltered vacuuming can add to the indoor contaminant burden. CDC guidance advises minimizing avoidable indoor pollution sources during smoke events. (CDC Stacks)

During the event

Compare indoor readings with outdoor conditions. Trends are often more useful than a single number.

Inspect prefilters and final filters more frequently. An extreme smoke event can load filters faster than their normal service schedule suggests. AirNow advises facilities and homeowners to expect more frequent replacement when filters become heavily soiled during smoke conditions. (AirNow)

Communicate with employees. Explain what is being measured, what controls are operating, what symptoms should be reported, and where the cleanest indoor areas are located.

Do not assume that a lack of complaints means conditions are acceptable. People vary widely in sensitivity, and some may hesitate to speak up.

After the smoke clears

Continue filtration long enough to remove contaminants that entered and settled inside the building. Inspect filters, clean appropriate surfaces, and review monitoring data.

Document what worked and what did not. Record outdoor conditions, indoor readings, equipment settings, door-opening practices, complaints, production changes, filter replacements, and lessons learned.

Then update the response plan before the next event.

What responsible leadership looks like

Air quality is about health, but it is also about trust.

Employees notice whether leadership pays attention to the environment in which they are being asked to work. They notice when a company measures conditions, explains what is happening, changes operations, installs equipment, closes doors, maintains filters, and looks for better solutions.

They also notice when severe smoke is dismissed as an odor everyone simply has to tolerate.

At STORCH, air quality became our highest operational priority during this event. As president and CEO, I am responsible for helping protect the people who show up every day to keep our company operating.

We could not make the wildfire disappear. We could not make the outdoor air safe. We could, however, refuse to accept outdoor conditions as the unavoidable indoor conditions of our plant.

That distinction matters.

The larger lesson for commercial buildings

Wildfire smoke is no longer a problem that Midwest and East Coast facilities can regard as rare or limited to western states.

Smoke can cross borders, states, and entire regions. A fire far from a facility can still affect employee health, production decisions, HVAC operation, deliveries, schools, offices, hospitals, warehouses, and manufacturing plants.

The buildings that perform best will not necessarily be the newest or most expensive. They will be the buildings whose owners understand their airflow, maintain their equipment, monitor actual conditions, prepare replacement media, and respond before indoor air has already deteriorated.

A commercial building cannot be made perfectly airtight, and no filter captures every contaminant.

But meaningful reductions are possible.

Close the avoidable openings. Control outdoor-air introduction intelligently. Keep useful recirculation operating. Use effective particulate filtration. Add gas-phase filtration where appropriate. Protect high-priority occupied spaces. Measure the results. Adjust the strategy based on evidence.

During the July 2026 smoke event, the air outside our facility reached particle-count levels unlike anything we had previously measured. Yet our indoor readings remained substantially below the outdoor readings because multiple layers of control were working together.

That is the central lesson:

Wildfire smoke may be unavoidable outdoors. Allowing it to enter and accumulate indoors without a plan is not.

Frequently Asked Questions

Does staying indoors protect people from wildfire smoke?

It usually reduces exposure compared with remaining outside, but the degree of protection varies greatly. Smoke can enter through ventilation systems, doors, loading docks, leaks, windows, and pressure-driven infiltration. A closed building with effective filtration can offer substantially better protection than a building with open doors or inadequate filtration.

Is MERV 13 enough for wildfire smoke?

MERV 13 is widely recommended as a practical minimum for reducing fine smoke particles when the HVAC system can safely accommodate it. It will not capture every particle in one pass. Building-level results depend on filter fit, airflow, runtime, recirculation, leakage, and contaminant loading. (AirNow)

Does activated carbon remove PM2.5?

Activated carbon is primarily used for gases, volatile compounds, and odors. It should generally be paired with an appropriate particulate filter rather than treated as a replacement for one.

Should an HVAC system be shut off during wildfire smoke?

Not automatically. If the system is recirculating indoor air through effective filters, shutting it off may reduce particle removal. Outdoor-air intake may need to be reduced during severe smoke, but ventilation, heat, humidity, pressure, process requirements, and applicable codes must also be considered.

Can an indoor particle counter tell me the AQI?

Usually not directly. Many handheld counters report the number of particles within size channels, while AQI for smoke is generally based on PM2.5 mass concentration. The instruments can still be valuable for comparing locations and trends when used consistently.

How frequently should wildfire filters be replaced?

There is no fixed interval that applies to every building. Inspect filters during and after severe events. Replacement may be needed sooner because smoke can load media rapidly. Consider pressure drop, visible loading, airflow, manufacturer guidance, monitoring results, and operating conditions.

Why can I still smell smoke after installing a good particulate filter?

The odor may be caused by gases and volatile compounds that pass through particulate media. Properly selected activated-carbon or other gas-phase filtration may be needed in addition to the particle filter.

What is final-stage filtration?

Final-stage filtration places filter media near the last point before supply air enters an occupied space. It can supplement central filtration and help address contaminants introduced or remaining downstream of the primary filter.

Should factories keep loading doors closed?

During severe smoke, minimizing door-open time can substantially reduce infiltration. Facilities must also manage heat, process ventilation, vehicle exhaust, and production requirements. Deliveries can sometimes be consolidated or staged to reduce the number and duration of openings.

What should a facility do first?

Understand the existing HVAC system and filtration. Identify outdoor-air and infiltration pathways, acquire appropriate filters, prepare supplemental air cleaning, establish monitoring, and create an operating plan before the next smoke event.