What Does an Orange Air Quality Alert Mean? Canada's AQHI System Explained
What Is the AQHI?
The Air Quality Health Index, or AQHI, is Canada's scale for communicating the health risk from air pollution in real time. It was developed by Environment and Climate Change Canada together with provincial and health partners, and it is designed to translate a mix of pollutants into a single number that ordinary people can act on. The index runs from 1 to 10 or higher, and it is published hourly with a forecast for the next several hours and the next day.
The Scale and What "Orange" Means
The AQHI divides air quality into four bands. A reading of 1 to 3 is Low risk, usually shown in blue or green. A reading of 4 to 6 is Moderate risk, often shown in orange or yellow. A reading of 7 to 10 is High risk, shown in red. Anything above 10 is Very High risk, shown in purple or dark red.
An "orange" alert — the Moderate, 4-to-6 band — means the air is somewhat polluted and could matter for sensitive groups. Sensitive groups include people with heart or lung conditions such as asthma or COPD, older adults, young children, pregnant people, and anyone exercising or working hard outdoors. For the general population, a Moderate reading usually calls for no specific action. For sensitive groups, the guidance is to reduce or reschedule strenuous outdoor activity — meaning if you have asthma and were planning a long run on a smoky afternoon, it is a good day to move it indoors or shorten it.
How the AQHI Is Calculated
The AQHI is driven by three pollutants: ground-level ozone (O3), fine particulate matter (PM2.5), and nitrogen dioxide (NO2). These are combined into a single number using coefficients derived from epidemiological research that links short-term exposure to these pollutants with premature mortality and hospital admissions. In other words, the index is explicitly health-weighted: it does not just report how dirty the air is, it reports how much that dirtiness is associated with people getting sick or dying.
This is different from the older Air Quality Index (AQI) used in the United States, which has more pollutant-specific breakpoints and can be harder to map directly to a single health risk. The AQHI's design is deliberately simple — one number, one risk band, one set of recommendations.
What Causes Orange-Alert Days
Several conditions push the AQHI into the Moderate band and higher. Wildfire smoke is the most visible cause, and it has become far more common. Canada's 2023 and 2024 fire seasons were among the worst on record, and smoke from large fires can drift hundreds or thousands of kilometers, pushing PM2.5 levels up across whole regions. Summer heat and sunlight also drive ground-level ozone: vehicle exhaust and industrial emissions cook in the sun, producing ozone that builds through the afternoon. Temperature inversions, where a layer of warm air traps cooler polluted air near the ground, can hold pollution in place for days. Industrial accidents or refinery upsets can cause localized spikes.
How to Check It and What to Do
The AQHI is published on Environment and Climate Change Canada's website and in most Canadian weather apps, updated hourly. At the Moderate level, sensitive groups should pay attention and adjust outdoor activity. At High (7 to 10) and Very High (10+), the guidance extends to everyone: reduce outdoor exertion, keep windows and doors closed, run a HEPA air purifier indoors, and — if you must be outside in heavy smoke — wear a well-fitted N95 or equivalent respirator, which can filter out much of the fine particulate matter that drives the risk. Cloth and surgical masks do not offer the same protection against PM2.5.
Why It Matters
Air pollution is one of the leading modifiable health risks worldwide, and exposure to fine particulate matter is linked to cardiovascular and respiratory disease. For Canadians, wildfire smoke has made "orange air quality days" a new normal in a way they were not a generation ago. Understanding the AQHI — and knowing that Moderate means "sensitive people, take it easy," while High and Very High mean "everyone, take precautions" — turns a colored alert on a weather app into an actionable decision about whether to go for that run, open the windows, or reach for a mask.