Canada's Air Quality Health Index (AQHI) Explained

Last updated May 29, 2026 · 6 min read

Canada took a different path from almost everyone else. Instead of reporting the concentration of the single worst pollutant, the Air Quality Health Index (AQHI) blends three pollutants into one number that estimates the combined short-term health risk of the air you're breathing. The result is a 1-to-10+ scale tied to mortality and hospital-admission research, not to a regulatory limit. This guide explains how it's built and how to read it.

What the AQHI is

The AQHI is a health-risk index, not a pollution measurement. It combines three pollutants — ground-level ozone (O₃), nitrogen dioxide (NO₂), and fine particulate matter (PM2.5) — using a formula derived from Canadian studies linking those pollutants to daily mortality and morbidity. The number you see is reported on a 3-hour moving average of all three, so it reflects recent exposure rather than a single instantaneous reading.

The four risk categories

The index has no fixed upper bound — during severe wildfire smoke, Canadian cities have reported AQHI values well into the dozens, all of which are simply communicated as "10+" / Very High.

The formula

Unlike the breakpoint-table approach used almost everywhere else, the AQHI is computed directly from a continuous equation. The published formula is:

AQHI = (1000 / 10.4) × [ (e^(0.000537 × O₃) − 1) + (e^(0.000871 × NO₂) − 1) + (e^(0.000487 × PM2.5) − 1) ]

where O₃ and NO₂ are 3-hour-average concentrations in ppb and PM2.5 is the 3-hour average in µg/m³. The result is rounded to the nearest integer (a value below 1 is reported as 1).

Each term captures the excess mortality risk associated with that pollutant; summing them and rescaling produces a single risk number. Because the relationship is exponential, the index climbs faster as concentrations rise — which is why severe smoke can push it past 10 quickly.

Why Canada uses a risk index instead of a concentration index

The design choice is deliberate. A concentration-based index like the US AQI answers "how dirty is the air?" The AQHI answers "how much is today's air likely to affect my health?" By blending pollutants rather than reporting only the worst one, it captures the reality that ozone, NO₂, and particulates act together on the cardiovascular and respiratory systems. The trade-off is that the AQHI number can't be back-translated into a single pollutant concentration — it's a composite by construction.

How AQHI compares to the US AQI

The scales aren't convertible, but the action thresholds line up closely in practice:

• AQHI 1–3 (Low) ≈ US AQI 0–50 (Good)
• AQHI 4–6 (Moderate) ≈ US AQI 51–150 (Moderate to Unhealthy for Sensitive Groups) — the at-risk action band
• AQHI 7–10 (High) ≈ US AQI 151–200 (Unhealthy)
• AQHI 10+ (Very High) ≈ US AQI 201+ (Very Unhealthy and worse)

Putting it to work

1. If you're at risk, AQHI 4 is your watch level. "At risk" includes people with heart or lung conditions, older adults, children, pregnant people, and outdoor workers.
2. Use the symptom test. ECCC's guidance leans heavily on "how do you feel?" — if you're symptomatic at any level, ease off, regardless of the number.
3. During wildfire season, expect 10+. When smoke arrives, the index saturates the top of the scale; check the forecast AQHI and plan exertion for cleaner hours.

Primary sources: ECCC — Air Quality Health Index · ECCC — Understanding AQHI messages