Canadian Air Quality Health Index

Canada does not use the U.S. EPA AQI. Instead, Environment and Climate Change Canada publishes the Air Quality Health Index (AQHI): a 0 to 10+ scale derived from the combined health impact of ozone, NO₂, and PM_2.5 rather than the worst-of-many that EPA uses. The design origin paper by Stieb et al. explains the rationale.

The index is calculated from the relative health risks of three common pollutants taken together: ground-level ozone, particulate matter (PM_2.5/PM₁₀), and nitrogen dioxide. The result is a single number that is more directly interpretable as a health-risk indicator than the threshold-band-based EPA AQI. Bands: 1-3 Low risk, 4-6 Moderate risk, 7-10 High risk, 10+ Very High risk. Each band has explicit recommendations for the general population and for at-risk groups (children, older adults, those with heart or lung conditions). The calculation method is public.

On the Terrestream device, IAQ can be shown as AQHI+, the PM_2.5-only amendment Environment and Climate Change Canada uses where ozone and NO₂ are not measured: AQHI+ is PM_2.5 in µg/m³ divided by 10, rounded up. The device cannot compute the full AQHI because it measures neither ozone nor NO₂. A "moderate" AQHI of 5 carries different implications than an EPA AQI of 100; both numbers are accurate, they describe different aspects of the same air.

To use it, choose between US AQI and Canadian AQHI+ in the app's device settings; the device also switches to AQHI+ when you set its language to Canadian French. In AQHI+ mode the device uses the Canadian risk words (Low Risk, Moderate Risk, High Risk, Very High Risk). The app and web dashboard show outdoor air as the US AQI for every home, including homes in Canada. Health Canada also publishes residential indoor air guidelines (see Health Canada guidelines); the device does not change its bands to match them.