One particulate-matter reading can produce five different index values. This guide explains why and shows exactly what the portal calculates.
An Air Quality Index (AQI) turns a pollutant concentration into a simpler number, colour, and health category. Instead of asking everyone to interpret a PM2.5 concentration in micrograms per cubic metre (µg/m³), an index can show that the air is in a low, moderate, or high-concern band.
There is no single worldwide AQI. Countries and organizations choose their own pollutants, averaging periods, breakpoints, scales, colours, and health messages. An index value of 50 in one method therefore does not necessarily mean the same thing as 50 in another.
The VisiblAir Cloud Portal calculations described here use particulate matter measured by your sensor. They are useful for viewing local trends, but they are not a replacement for an official AQI issued by a government monitoring network.
For every method, the portal first averages all valid PM2.5 samples stored for that sensor during the preceding 24 hours. It then converts that rolling average into the selected regional index.
This “position between two breakpoints” step is called linear interpolation. In simple terms, if the concentration is halfway through a concentration band, the index is placed halfway through the corresponding index band.
PM2.5 is the normal input for all five choices on AQI-compatible sensors. The U.S. implementation also contains a fallback path intended for PM10 when no positive PM2.5 sub-index is available.
For a 24-hour PM2.5 average of 25 µg/m³, the portal calculates:
| Portal method | Result | Displayed scale |
|---|---|---|
| United States | 78 | 0–500 |
| Canada | 1 | 0–10+ |
| Europe | 42 | 0–100 |
| China | 36 | 0–500 |
| India | 42 | 0–500 |
The sensor measurement did not change. Only the conversion method changed.
The portal uses a 0–500 scale and PM2.5 breakpoints from the previous U.S. EPA AQI. Its portal colour bands are green (0–50), yellow (51–100), orange (101–150), red (151–200), purple (201–300), and maroon above 300.
| 24-hour PM2.5 (µg/m³) | Portal index |
|---|---|
| 0.0–12.0 | 0–50 |
| 12.1–35.4 | 51–100 |
| 35.5–55.4 | 101–150 |
| 55.5–150.4 | 151–200 |
| 150.5–250.4 | 201–300 |
| 250.5–350.4 | 301–400 |
| 350.5–500.4 | 401–500 |
Implementation note: the EPA updated its PM2.5 AQI breakpoints in 2024. For example, the current Good upper breakpoint is 9.0 µg/m³ rather than the portal's 12.0 µg/m³. The portal value should therefore not be described as the current official U.S. AQI.
Canada officially reports an Air Quality Health Index (AQHI) on a 1–10+ scale. The portal uses only the PM2.5 term from the traditional Canadian AQHI equation:
round[(1000 ÷ 10.4) × (e(0.000487 × PM2.5) − 1)]
The portal colours are green through 3, yellow from 4 to 6, red from 7 to 9, and purple at 10 or above.
Implementation note: this is a simplified, PM-only indicator. The official Canadian AQHI normally combines ozone, nitrogen dioxide, and fine particulate matter and uses shorter rolling averages. Canada can use a separate PM2.5-focused approach during wildfire smoke events, but that is not the same calculation as the portal's rolling 24-hour PM term.
The portal's Europe option is a 0–100, PM2.5-only calculation based on legacy Common Air Quality Index (CAQI)-style daily bands.
| 24-hour PM2.5 (µg/m³) | Portal index | Portal colour |
|---|---|---|
| 0–15 | 0–25 | Blue |
| 15–30 | 25–50 | Green |
| 30–55 | 50–75 | Yellow |
| 55–110 | 75–100 | Red |
| Above 110 | 100 | Red |
Implementation note: this is not the current European Environment Agency Air Quality Index. The current European index uses hourly data, up to five pollutants, and six named categories rather than a numeric 0–100 scale.
The portal uses a 0–500 scale with the PM2.5 breakpoints from China's HJ 633—2012 method.
| 24-hour PM2.5 (µg/m³) | Portal index |
|---|---|
| 0–35 | 0–50 |
| 35–75 | 51–100 |
| 75–115 | 101–150 |
| 115–150 | 151–200 |
| 150–250 | 201–300 |
| 250–350 | 301–400 |
| 350–500 | 401–500 |
Implementation note: China's Ministry of Ecology and Environment replaced that method with HJ 633—2026 on March 1, 2026. The portal still reflects the earlier PM2.5 bands, so it is not the current official Chinese AQI.
The portal uses the Indian National AQI's PM2.5 bands on a 0–500 scale, with the highest concentrations capped at 500.
| 24-hour PM2.5 (µg/m³) | Portal index | Portal colour |
|---|---|---|
| 0–30 | 0–50 | Green |
| 31–60 | 51–100 | Blue |
| 61–90 | 101–200 | Yellow |
| 91–120 | 201–300 | Orange |
| 121–250 | 301–400 | Red |
| Above 250 | 500 | Maroon |
Implementation note: India's official National AQI can use eight pollutants and reports the highest available pollutant sub-index. The portal uses only its local PM2.5 measurement and simplifies the final band.
Choose the method most familiar to the people reading the dashboard, usually the regional method used where the sensor is installed. For internal comparisons, consistency matters more than converting back and forth: use the same method for every sensor and throughout the period you are studying.
For public health decisions, alerts, or regulatory reporting, consult the official air-quality service for your location. The portal AQI is best used as a clear view of your own sensor's particulate-matter trend.
The portal averages your sensor's last 24 hours of PM data, places that average between the selected method's breakpoints, and shows the resulting whole-number index. The index is easier to read than raw PM, but its meaning always depends on the method selected.