Indoor PM2.5 by Filtration Scenario
Full Results Table
| Parameter | Value | Notes |
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How to Use This Calculator
Choose a preset from clean rural air to severe wildfire smoke, or check Environment Canada's air quality monitoring for your current local reading and enter it directly. The preset levels are illustrative.
Infiltration rate reflects how leaky your building is. Filter rating applies to air entering through a mechanical ventilation intake, and the calculator treats it as applying to all infiltration air. Select whether a portable HEPA purifier is running.
Space volume is needed to calculate the effective air changes the purifier adds. If using a portable purifier, enter its smoke CADR from the product specification for an accurate reduction estimate.
The results show steady-state indoor PM2.5 compared with the WHO 24-hour and annual guidelines, plus a comparison across filtration scenarios showing how much each intervention helps.
PM2.5 in Canadian Homes: From Everyday Urban Air to Wildfire Smoke Events
PM2.5 has become the most closely watched air pollutant in Canada, driven by growing wildfire smoke exposure and a body of health evidence that keeps pointing to lower and lower safe thresholds. Understanding how outdoor PM2.5 translates to what you're actually breathing indoors, and what interventions actually move the needle, is central to both everyday air quality management and wildfire smoke response.
The Infiltration Factor Model
Indoor PM2.5 from outdoor sources is calculated using a mass balance model. At steady state, indoor concentration equals outdoor concentration times a penetration factor (0.8 here), times the infiltration rate, times the fraction that gets past any filter, divided by the infiltration rate plus the purifier's air changes plus a deposition loss (0.4 per hour here). The penetration and deposition values are planning assumptions. The ratio of indoor to outdoor is the infiltration factor. In this calculator a home with no filter beyond the basic 5% option at 0.3 ACH has an infiltration factor of about 0.33, and one at 1.0 ACH about 0.54, while a tight home at 0.1 ACH with MERV 13 on the intake has about 0.06, roughly a tenfold difference between the leakiest and tightest cases. See the air filtration calculator for the underlying filter efficiency math.
Health Canada and WHO Guidance
Health Canada's guidance on PM2.5 in residential indoor air stops short of a numerical limit, noting that studies have not found a level below which effects disappear. Its advice is to keep indoor concentrations as low as reasonably achievable, and below the outdoor level as a minimum. The World Health Organization's 2021 global guidelines are 15 µg/m³ as a 24-hour average and 5 µg/m³ as an annual average, reflecting accumulating evidence that health effects occur at lower concentrations than previously understood. This calculator compares indoor PM2.5 with the WHO values as a practical yardstick, and shows the reduction from outdoor levels as Health Canada's guidance frames it. There is no known safe threshold below which PM2.5 has zero health effect, so these values represent judgment calls about acceptable risk rather than a hard biological cutoff.
Wildfire Smoke Season Across Canada
Wildfire smoke has become the dominant driver of extreme PM2.5 events in Canada, and it can travel far from the fires themselves. The 2023 fire season carried smoke well beyond the fire regions, including across Ontario, Quebec, and into the eastern United States. Health Canada's wildfire smoke guidance names fine particles (PM2.5) as the main health risk and advises keeping windows and doors closed as much as possible, using the best filter your ventilation system can handle, and using a certified portable air cleaner. In the calculator's wildfire example, 80 µg/m³ outdoors, 0.2 ACH, and a 250 m³ space, indoor PM2.5 is 21.3 µg/m³ with no filtration, 18.1 with MERV 8, 7.5 with MERV 13, and 1.9 with MERV 13 plus a 250 CFM purifier. The Air Quality Index calculator converts PM2.5 readings to the AQHI scale used in Canadian public health messaging.
MERV 13 vs. Portable HEPA: Which Helps More?
Both interventions work, but they address different pathways. MERV 13 on a mechanical ventilation intake filters outdoor air as it enters the building, reducing the source at the point of entry. A portable HEPA purifier continuously recirculates and cleans room air regardless of how PM2.5 got in, whether through infiltration, cooking, or an open door. For homes without mechanical ventilation, portable HEPA is the primary practical lever. For homes with a furnace or HRV system, upgrading filtration on that system provides whole-building benefit, and the HRV/ERV selection calculator covers choosing the ventilator that carries that filter. A portable unit then adds targeted protection in bedrooms during sleep. In the example above, MERV 13 alone cuts indoor PM2.5 by about 65% compared with no filtration at the same air exchange, and adding the purifier takes it from 7.5 to 1.9 µg/m³.
Building Tightness and PM2.5
Infiltration rate is often the most underappreciated variable in indoor PM2.5. In this calculator a home at 0.1 ACH lets in about half as much outdoor PM2.5 as the same home at 0.3 ACH, because deposition removes part of what enters at both rates. This is one of several reasons why air sealing and weatherization work, usually done for energy efficiency, also provides meaningful indoor air quality benefits, and the blower door calculator covers measuring how leaky a home actually is. However, tight homes need mechanical ventilation to maintain adequate fresh air for CO2 and moisture control, and simply sealing a home without adding controlled ventilation trades one problem for another. See the CO2 level calculator for the ventilation side of this tradeoff.
Frequently Asked Questions
Health Canada's PM2.5 guidance for homes sets no numerical limit, because no apparent threshold for health effects has been found. It recommends keeping indoor PM2.5 as low as possible, and lower than outdoors as a minimum. This calculator compares indoor levels with the World Health Organization's 2021 global guidelines, which are 15 micrograms per cubic metre as a 24-hour average and 5 micrograms per cubic metre as an annual average. PM2.5 health effects don't distinguish between indoor and outdoor exposure, so the same yardsticks are useful for both.
Indoor PM2.5 from outdoor infiltration is calculated using an infiltration factor, which represents the fraction of outdoor PM2.5 that remains indoors at steady state, accounting for penetration through the building envelope, deposition onto surfaces, and any filtration. In this calculator an unfiltered home has an infiltration factor of roughly 0.25 to 0.54 across 0.2 to 1.0 ACH. A tight home at 0.1 to 0.3 ACH with MERV 13 filtration on the intake has roughly 0.06 to 0.12. Adding a well-sized portable HEPA purifier reduces it further. The penetration factor (0.8) and deposition rate (0.4 per hour) are planning assumptions, and real buildings vary.
This calculator credits MERV 13 with 65% removal of PM2.5 in the air passing through it, blending its efficiency across the 0.3 to 2.5 micron range. In a home with mechanical ventilation and MERV 13 on the intake, that cuts indoor PM2.5 by about 65% compared with the same home unfiltered at the same air exchange rate. Adding a portable HEPA air purifier sized for the room provides additional reduction, particularly useful in homes without whole-building mechanical ventilation. The wildfire example shows the steps: 21.3 µg/m³ unfiltered, 7.5 with MERV 13, and 1.9 with MERV 13 plus a 250 CFM purifier.
Yes. Keeping windows and doors closed during active wildfire smoke events significantly reduces the rate at which outdoor PM2.5 enters the building compared to open-window natural ventilation. Health Canada's wildfire smoke guidance advises keeping windows and doors closed as much as possible, using the best filter your ventilation system can handle, and using a certified portable air cleaner that filters fine particles. If the home has no mechanical ventilation and indoor PM2.5 levels remain high with windows closed, running a properly sized HEPA air purifier is the most effective intervention available to most households.
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