AQHI vs AQI: Why the Two Air Quality Numbers Disagree
AQHI and AQI use different pollutants, formulas and averaging. See how each is built, how they translate through PM2.5, and which one to read.
Updated 2026-09-11
AQHI and AQI answer the same question, whether the air is bad right now, but they are built from different data and different math, so they rarely agree. Canada's AQHI combines three pollutants in an exponential formula and tops out around 10. The US EPA's AQI uses piecewise-linear breakpoint tables across six pollutants and runs to 500. A reading of AQHI 4 is not AQI 4, and it is not AQI 40 either.
This guide opens up both formulas, shows how the converter on this site bridges them through PM2.5, and gives a reference table of equivalent ranges so you can read either number with the right expectations.
Two indices, two different jobs
Both scales exist to protect health, but they were designed with different priorities. The AQHI tries to express short-term health risk directly: each extra unit is meant to represent a similar step up in risk. The AQI tries to classify pollution levels so they can be compared across pollutants and regions, and the value published is the highest of its pollutant sub-indices.
| AQHI (Canada) | AQI (US EPA) | |
|---|---|---|
| Scale | 1 to 10, with 10+ for very high risk | 0 to 500 |
| Pollutants used | PM2.5, ozone (O3), nitrogen dioxide (NO2) | Six criteria pollutants, including PM2.5, PM10, O3, CO, SO2 and NO2 |
| Formula shape | Exponential: each extra unit of concentration adds more risk | Piecewise linear interpolation inside fixed breakpoint ranges |
| How the number is chosen | One combined calculation from all three pollutants | A sub-index per pollutant; the highest is reported |
| Averaging period | A three-hour rolling average, updated hourly | Depends on the pollutant; PM2.5 uses a 24-hour average |
| Categories | Low, moderate, high, very high | Good, moderate, unhealthy for sensitive groups, unhealthy, very unhealthy, hazardous |
The consequence: AQHI 7 and AQI 7 are unrelated numbers, and a city reporting an AQI of 160 on a given afternoon might be reporting an AQHI of 7 or an AQHI of 10, depending on how much of the pollution is particulate matter.
How the AQHI is calculated
The AQHI puts three pollutant concentrations into a single exponential formula. PM2.5 is measured in micrograms per cubic metre, while ozone and nitrogen dioxide are measured in parts per billion.
Each pollutant contributes an excess-risk term. Health Canada's published multipliers are 0.000487 for PM2.5, 0.000537 for O3 and 0.000871 for NO2 (Stieb et al., 2008); the three terms are summed and then scaled by 1000 divided by 10.4. These are the same values the converter uses.
| Pollutant | Unit | Multiplier | Term |
|---|---|---|---|
| PM2.5 | micrograms per cubic metre | 0.000487 | e to the power of (0.000487 times PM2.5), minus 1 |
| O3 | ppb | 0.000537 | e to the power of (0.000537 times O3), minus 1 |
| NO2 | ppb | 0.000871 | e to the power of (0.000871 times NO2), minus 1 |
| Total | none | times 1000, divided by 10.4 | AQHI equals (1000 divided by 10.4) times the sum of the three terms |
Because those terms are exponential, the AQHI is not linear in concentration: at higher levels, doubling a concentration more than doubles its contribution. That is deliberate, since the index is meant to track health risk rather than mass.
- 1 to 3, low risk: air quality is ideal and normal outdoor activity is fine.
- 4 to 6, moderate risk: sensitive people may want to shorten intense outdoor exertion.
- 7 to 10, high risk: sensitive groups should cut back on outdoor activity.
- Above 10, very high risk: everyone should reduce outdoor activity.
Exactly 10.0 counts as high risk; the very-high category starts above 10, where the official scale is open-ended (reported as “10+”). Values above 10 appear when you convert from an AQI, and the AQHI input accepts them up to 15 so an extreme reading can be entered directly.
How the AQI is calculated
The AQI is not a formula over concentrations; it is a set of tables. Each pollutant has a breakpoint table mapping a concentration range onto an index range, and the index is interpolated linearly inside that band.
The converter uses the PM2.5 table, the 24-hour fine particulate scale. These are the exact bands it applies.
| PM2.5 from | PM2.5 to | AQI from | AQI to | Category |
|---|---|---|---|---|
| 0.0 | 9.0 | 0 | 50 | Good |
| 9.1 | 35.4 | 51 | 100 | Moderate |
| 35.5 | 55.4 | 101 | 150 | Unhealthy for sensitive groups |
| 55.5 | 125.4 | 151 | 200 | Unhealthy |
| 125.5 | 225.4 | 201 | 300 | Very unhealthy |
| 225.5 | 325.4 | 301 | 500 | Hazardous |
The US EPA revised the PM2.5 breakpoints with effect from 6 May 2024, which is why the bands above differ from the older tables still circulating. Concentrations that fall into the small gaps between two published bands, such as 9.05, are pulled into the neighbouring band before interpolating, so nothing is lost. Anything above 325.4 µg/m³ is beyond the AQI and is reported as 500.
Published AQI values take the maximum across every measured pollutant, so a high-ozone afternoon can show a worse AQI than PM2.5 alone would suggest. The converter computes the PM2.5 sub-index only.
The equivalent ranges, worked out
There is no official conversion between the AQHI and the AQI. The converter bridges them with PM2.5: it turns one index into a PM2.5 concentration, then turns that concentration into the other index. Leaving ozone and nitrogen dioxide at zero attributes the whole AQHI to PM2.5, which keeps the comparison unambiguous.
| AQHI | AQHI category | PM2.5 (micrograms per cubic metre) | AQI | AQI category |
|---|---|---|---|---|
| 1 | Low | 11.9 | 49 | Good |
| 3 | Low | 35.2 | 100 | Moderate |
| 4 | Moderate | 46.7 | 129 | Unhealthy for sensitive groups |
| 6 | Moderate | 69.3 | 158 | Unhealthy |
| 7 | High | 80.5 | 164 | Unhealthy |
| 10 | Very high | 113.3 | 181 | Unhealthy |
Two patterns stand out. First, the AQHI compresses a wide range of PM2.5 into a narrow scale: everything from roughly 150 micrograms per cubic metre upward lands at AQHI 10, while the AQI keeps climbing towards 500. Second, the moderate AQHI band already reaches AQI categories that sound alarming, which is why the two numbers look inconsistent side by side.
Read the category rather than the number. AQHI 4 and AQI 40 do not describe the same air, and there is no sense in which one is four times the other.
Using the converter
The tool converts in both directions, and the result updates as you type.
- Choose the direction: AQHI to AQI, or AQI to AQHI.
- Enter the value you have. An AQHI input is capped at 10 and an AQI input at 500.
- Read the PM2.5 concentration next to both indices, because that is the value the two scales actually share.
- Open the advanced options to enter ozone and nitrogen dioxide for a more realistic baseline.
The advanced options matter more than they appear to. Setting O3 and NO2 to zero makes the AQHI depend on PM2.5 alone. Raising them adds a fixed baseline risk: with roughly 30 ppb of ozone and 20 ppb of nitrogen dioxide, the background alone accounts for about AQHI 4.3, and any AQHI below that cannot be attributed to particulate matter at all, so the converter returns a PM2.5 value of zero.
That zero baseline is a deliberately simple starting point, not a claim about real air. Urban air always carries some ozone and nitrogen dioxide, so treat the default result as one end of a range rather than a measurement.
Results are rounded to two decimals for PM2.5 and one decimal for the AQHI, using the category thresholds described above.
Which number should you look at?
Use whichever index your local authority publishes, and read its category rather than comparing raw numbers across systems.
- In Canada the AQHI is the official public scale, written around short-term risk and aimed especially at people with heart or lung conditions.
- In the United States, and in many countries that adopted the same scheme, the AQI is the standard, and it also drives the colour codes in weather apps.
- When you compare cities across borders, convert first. Putting a raw AQHI next to a raw AQI will mislead you.
- Neither index tells you what is in the air. A wildfire-smoke afternoon and a stagnant traffic-congestion afternoon can produce similar numbers for very different reasons.
Air quality changes quickly. Any conversion here describes an instantaneous relationship between two scales, not a forecast.
AQHI and AQI are two translations of the same air, and the honest way to compare them is through the pollutant they share. The AQHI to AQI converter on this site performs that translation in both directions, shows the PM2.5 concentration in between, and lets you add ozone and nitrogen dioxide when you want a more realistic background.
Frequently asked questions
Why is AQHI 4 not equal to AQI 4 or AQI 40?
They are separate scales with separate formulas. The AQHI is exponential and runs from 1 to 10, while the AQI is piecewise linear and runs to 500. The conversion goes through a PM2.5 concentration, so an AQHI of 4 corresponds to roughly AQI 129 when all of the pollution is particulate.
Can I convert an official AQHI reading into an official AQI reading?
Only approximately. Official values come from real-time measurements of several pollutants, while the converter uses PM2.5 plus whatever ozone and nitrogen dioxide values you enter.
Why does raising ozone and NO2 push the PM2.5 result down to zero?
Because the AQHI adds their risk into one shared total. Once the background gases alone account for the AQHI value you entered, nothing is left over for particulate matter.
Which PM2.5 breakpoints does the converter use?
The 24-hour PM2.5 bands, from 0 to 12.0 micrograms per cubic metre for AQI 0 to 50 up to 350.5 to 500.4 for AQI 401 to 500, with linear interpolation inside each band and anything higher reported as 500.
Is this good enough for health decisions?
No. Use it to understand how the two scales relate. For actual readings and advice, check the monitoring service that covers where you live.
Tools used in this guide
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