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PM2.5 · Methodology & exposure guidance

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Methodology & Exposure Guidance

Most public air pollution messaging is built around AQI systems that combine multiple pollutants into one index and then attach broad advice such as “sensitive groups should reduce prolonged outdoor exertion.” That can be useful for alerts, but it is not always the best tool for making day-to-day decisions about personal particulate exposure. PM2.5 is one of the most consistently studied air pollution metrics in health research, and much of the strongest evidence on long-term and short-term risk is expressed directly in PM2.5 concentrations rather than AQI categories.

This framework therefore uses PM2.5 levels anchored to WHO guideline and interim target thresholds, then connects them to practical actions people can actually take: improving indoor filtration, using stronger purifiers, and using masks or respirators when needed. The goal is to move from general warning language to actionable recommendations tied to exposure reduction. In other words, instead of only saying air is “bad” or “moderate,” this page asks: given a PM2.5 level, what combination of realistic measures is likely to move exposure closer to a safer range?

Assumptions used in this guidance framework include a typical day divided across home, work or school, transit, and outdoor time, along with approximate reduction factors for indoor filtration, purifiers, masks, and respirators.

Exposure recommendation calculator

Enter the PM2.5 level, then step through the wizard to get a recommendation.

Step 1 of 7

Enter ambient PM2.5

Please enter a valid PM2.5 value of 0 or greater.

Select your target

Choose the exposure outcome you want to aim for.

Please select a target.

Home intervention preference

Choose the most complex home measure you already have or would be open to using.

Please select an option.

Work or school filtration level

Choose the level that best matches your work or school environment.

Please select an option.

Transit preference

This helps tailor the result where transit-related protection may matter.

Please select an option.

Outdoor protection preference

This helps contextualize recommendations involving outdoor exposure.

Please select an option.

Final finding

Reference table

WHO references use the 2021 WHO Air Quality Guidelines for PM2.5: annual guideline 5 μg/m³, daily guideline 15 μg/m³, and daily interim targets IT-4 25 μg/m³, IT-3 37.5 μg/m³, IT-2 50 μg/m³, IT-1 75 μg/m³.

Band 24-h PM2.5 (μg/m³) WHO reference To achieve Ideal exposure To achieve Acceptable exposure
Ideal 0–5 At or below the WHO's annual PM2.5 exposure guideline No special measure needed. No special measure needed.
Acceptable >5–15 At or below the WHO's daily PM2.5 exposure guideline
Home: HEPA air purifier
Work: Standard filtration
 No special measure needed.
Caution >15–25 At or below the WHO's daily PM2.5 exposure interim target 4
Home: High-capacity purifier with IAQ monitoring
Work: High-efficiency filtration
Home: Doors and windows closed
Work: Standard filtration
Elevated Caution >25–37.5 At or below the WHO's daily PM2.5 exposure interim target 3
Home: High-efficiency mechanical ventilation filtration
Work: High-efficiency filtration
Home: HEPA air purifier
Work: High-efficiency filtration
High >37.5–50 At or below the WHO's daily PM2.5 exposure interim target 2
Home: High-efficiency mechanical ventilation filtration
Work: High-efficiency filtration
Outdoor: Respirator
Home: HEPA air purifier
Work: High-efficiency filtration
Outdoor: Respirator
Very High >50–75 At or below the WHO's daily PM2.5 exposure interim target 1
Home: High-efficiency mechanical ventilation filtration
Work: High-efficiency filtration
Transit: Mask
Outdoor: Respirator
Home: High-efficiency mechanical ventilation filtration
Work: High-efficiency filtration
Outdoor: Respirator
Extreme >75 Above all the WHO's daily PM2.5 exposure targets Ideal exposure cannot be met.
Home: High-efficiency mechanical ventilation filtration
Work: High-efficiency filtration
Outdoor: Respirator

WHO reference note: The table uses WHO 2021 PM2.5 guideline and interim target thresholds as anchors for public communication. The band labels and recommendations are part of this framework and are not official WHO categories.

Footnotes

Calculation logic

The recommendation framework estimates average daily exposure using a weighted time model across home, work or school, transit, and outdoor time. A simplified structure is:

Daily exposure = (home hours × ambient PM2.5 × home factor + work hours × ambient PM2.5 × work factor + transit hours × ambient PM2.5 × transit factor + outdoor hours × ambient PM2.5 × outdoor factor) / 24

The working assumptions used in this framework are approximately 13.5 hours at home, 8 hours at work or school, 1.5 hours in transit, and 1 hour outdoors. Recommended measures are selected so that the resulting daily average exposure is brought as close as reasonably possible to the chosen target band.

Evidence-based thumb rules and assumed reduction factors

  • No special measure: assumed exposure factor 1.00, or 0% reduction relative to ambient conditions.
  • Doors and windows closed: assumed exposure factor 0.80, or about 20% reduction.
  • HEPA air purifier: assumed exposure factor 0.50, or about 50% reduction.
  • High-capacity purifier with IAQ monitoring: assumed exposure factor 0.25, or about 75% reduction.
  • High-efficiency mechanical ventilation filtration: assumed exposure factor 0.05, or about 95% reduction.
  • Standard filtration at work or school: assumed exposure factor 0.60, or about 40% reduction.
  • High-efficiency filtration at work or school: assumed exposure factor 0.10, or about 90% reduction.
  • Baseline transit assumption: assumed exposure factor 0.50, or about 50% reduction.
  • Mask in transit: assumed exposure factor 0.30, or about 70% reduction.
  • Respirator: assumed exposure factor 0.05, or about 95% reduction.
  • These values are simplified planning assumptions, not guarantees. Real-world performance depends on room size, air exchange, filter quality, fit, leakage, wear time, and user behavior.
  • PM2.5 is often the pollutant most closely tied to major air pollution health burden estimates and is commonly used in epidemiologic risk studies.
  • Indoor exposure usually tracks outdoor PM2.5 imperfectly; filtration, air cleaning, building tightness, and occupant behavior can substantially reduce indoor concentrations.
  • Portable HEPA purifiers can materially reduce indoor PM2.5, but performance depends on room size, clean air delivery rate, placement, filter condition, and run time.
  • Higher-capacity purification and tighter indoor control become more important as ambient PM2.5 rises because small percentage differences translate into larger exposure differences.
  • Well-fitted respirators generally reduce particulate exposure more reliably than loose or poorly fitted masks, especially at higher pollution levels.
  • At very high ambient PM2.5 levels, even strong home and workplace controls may not fully achieve an ideal daily exposure target, which is why the framework treats some conditions as not fully correctable.

Important limitation

This is a decision-support framework for communication and planning. It does not replace local public health guidance, occupational standards, clinical advice, or building-specific measurements. Real-world performance varies with air exchange rates, purifier sizing, fit and adherence, time spent in each microenvironment, and the accuracy of the PM2.5 input itself.