demystifying air quality numbers

There are lots of numbers used to describe air quality these days in China. This post is intended as a glossary to demystify the different numbers currently used by various sources.

Concentration: The most direct way to report air pollution would be the concentration of a given pollutant in the air. This is straight-forward science and measurement. Example: PM2.5 concentration is 950 micrograms per cubic meter (ug/m^3).

Although concentration is straightforward scientifically, there are a couple of problems with it from a public outreach perspective. First, not everyone understands what concentration is, or has context for what the numbers mean. Second, it’s difficult to compare different pollutants to each other, since each pollutant affects people differently depending on concentration. Therefore, concentrations are commonly normalized to some scale, on which multiple pollutants can be compared together and a single “score” covering multiple pollutants given to a city.

Confusingly, there are currently three such scales used to describe Chinese air quality: the US Air Quality Index (AQI), the current/outgoing Chinese Air Pollution Index (空气污染指数, API), and the current/future Chinese Air Quality Index (空气质量指数, AQI). Here are the technical details of each one:

US AQI: In the United States, concentrations are converted to the “Air Quality Index” using breakpoints defined in 40 CFR 58 Subpart G. (Edit 1/13/13: The EPA has just revised these breakpoints, though it hasn’t posted to the CFR yet. New breakpoints shown here and in the graph below.)

Note: the US does not define an AQI above 500. On, if you use the AQI->concentration calculator and input an AQI above 500, it gives an error message.

This means that AQIs reported by the Embassy above 500 are not in accordance with US regulations. My guess is that they linearly projected using the same slope as the 400 –> 500 line.

China API: China’s current but outgoing system of API uses breakpoints described in an old post of mine here. This is officially the current system in place nationwide, though it does not include PM2.5 or ozone.

China AQI: In February 2012 (regulation HJ 633—2012), China defined a new air quality index that includes PM2.5 and ozone. It doesn’t take effect nationwide until 2016, but Beijing and many other cities have already adopted it. The China AQI breakpoints are here:

Note that the Chinese system also does not define an index above 500. This is why the data reported by the BMEMC rail at 500 for AQI. I don’t think they project as the Embassy does.

Let’s do a quick comparison of the PM2.5 cutpoints in the US and in China:

1) The US is more strict at low concentrations.
2) The systems are identical above a concentration of 150 (AQI of 200).
3) Neither system is linear, which is annoying and non-intuitive.
4) It is also very annoying that the numbers are so close (as opposed to a 1-10 index, for example). This means it is very easy to confuse AQI and concentration.

– Concentration is the most accurate way of describing air pollution, but isn’t good for public awareness and comparing multiple pollutants.
– Both the US and China use AQI systems. Both systems go from 0-500, and are not technically defined above 500.
– The US and Chinese systems are identical above an index value of 200 (PM2.5 concentration of 150), but slightly different below this level.
– Because the systems aren’t identical and have different slopes, you have to be very careful when saying something like “PM2.5 is 150.” The meaning of this statement is different depending on if you mean concentration OR US AQI OR Chinese AQI.

Confused yet?

7 Responses to “demystifying air quality numbers”

  1. Moy says:

    Interesting. So generally, does that difference have an effect for people not in the industry outside of very rural areas? Seems like for me, 500 is 500 is bad.

  2. Steven Andrews says:

    In Dec. 2012 the US air quality laws were strengthened and the revised annual average PM2.5 standard is now 12ug/m3. The Air Quality Index has also been revised so that “good” air quality needs to have a 24 hr average <= 12.0 ug.m3.
    So in Beijing the cutoff for what would be classified as "good" is now nearly three times higher!

  3. Vance says:

    Thanks Steve. I heard they revised the standard but didn’t realize the changes hadn’t been updated in the CFR yet. I’ve updated the info above to reflect this.

  4. Edward Wong says:

    Good post. Do you know what pollutants the U.S. and China each put into their AQI measurements, and the proportion of each? I could be wrong, but I think Alex Wang mentioned there was some difference between the formulas. The EPA breakpoint chart you posted doesn’t mention the pollutant or pollutants that are being measured to arrive at the AQI. PM 2.5 is a major component, but what about others?

  5. Vance says:

    Ed: both the US AQI and Chinese new AQI systems operate on the same principle. Separate AQIs are calculated for each of 5 pollutants (ozone, PM, carbon monoxide, sulfur dioxide, and nitrogen dioxide) based on different breakpoints. The reported AQI for the area is then the highest of the calculated individual AQIs. There are some differences in exactly how data is processed I think (and how data from multiple monitors is compared), but the basics are the same. If I have time I’ll prepare a separate post on this. All the technical info is in the US CFR and this Chinese standard You know, light bedside reading.

    FYI, the “limiting” AQI in the US is usually either PM2.5 or ozone. On, a single AQI is given, but the individual AQIs for PM2.5 and ozone are also provided. I would expect PM2.5 and Ozone to be the limiting pollutants in China as well.

  6. jian yao says:

    There is a web page where you can convert between the concentration and AQI in US:

    is there a similar website for China’ s API?

  7. Vance says:

    Jian Yao: Sorry, I’m not aware of one.

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