Hong Kong's Air Quality Health Index (AQHI) Explained
Hong Kong reports air quality with its own Air Quality Health Index — a 1-to-10+ scale that, like Canada's, expresses the health risk of the air rather than the concentration of a single pollutant. Despite sharing a name (and the "AQHI" abbreviation) with Canada's index, it's a different formula. This guide explains how Hong Kong's works.
What the AQHI is
Adopted in late 2013, Hong Kong's AQHI is reported on a scale of 1 to 10 and "10+", grouped into five health-risk bands. It is calculated from the combined added health risk of four pollutants — nitrogen dioxide (NO₂), sulphur dioxide (SO₂), ozone (O₃), and particulate matter (PM2.5 or PM10, whichever poses the greater risk) — based on their 3-hour moving-average concentrations.
The five risk bands
| AQHI | Health risk | Added health risk | In plain English |
|---|---|---|---|
| 1–3 | Low | 0–5.6% | No need to change normal activities. |
| 4–6 | Moderate | 5.7–11.3% | People with heart or respiratory illness who feel symptoms should consider reducing outdoor exertion. |
| 7 | High | 11.3–12.9% | People with heart or respiratory illness should reduce outdoor exertion; children and the elderly too if symptomatic. |
| 8–10 | Very High | 12.9–19.4% | The general public should reduce outdoor exertion; at-risk people should avoid it. |
| 10+ | Serious | 19.4%+ | Everyone should reduce or avoid outdoor exertion; at-risk people should stay indoors. |
How it's computed
For each pollutant, the EPD estimates the percentage increase in the risk of hospital admissions associated with the past three hours' concentration, then sums them:
Added Health Risk (%) = %AR(NO₂) + %AR(SO₂) + %AR(O₃) + %AR(PM)where each term has the form
%AR = [ e^(β × C) − 1 ] × 100, with a pollutant-specific coefficient β. The summed risk is then mapped onto the 1–10+ band scale above.
Because it blends pollutants into a single risk figure, the AQHI — like Canada's — can't be back-translated into one pollutant concentration. Hong Kong publishes it separately for general stations and for roadside stations (Causeway Bay, Central, Mong Kok), where traffic NO₂ is far higher.
Hong Kong vs Canada: same name, different index
Both are 1–10+ health-risk indices built from added-mortality/morbidity research, and both round above 10 to "10+". But they use different pollutant sets and coefficients: Canada's AQHI combines O₃, NO₂, and PM2.5, while Hong Kong's adds SO₂ and lets PM10 or PM2.5 stand in for particulate. Don't assume a "5" means the same thing in Toronto and Hong Kong.
Putting it to work
- If you have heart or lung disease, AQHI 4 is your watch level.
- Check the roadside reading if you'll be walking busy streets — it's routinely worse than the general index.
- Use the symptom test — ease off outdoor exertion if you feel effects, whatever the number.
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Download for iOSPrimary sources: Hong Kong EPD — Air Quality Health Index · GovHK — Air Quality Health Index