AQI, API, PM2.5 – Which One Should I Check?
What are the benefits of using PM2.5 and not AQI to measure air quality, and what difference does it really make? PureLiving explains:
Part-I The Different Measures
AQI stands for "Air Quality Index” and rates air quality on a 0 - 500 scale, with 0 being the best, and 500 representing the worst. This method was introduced to China by the US Embassy in Beijing and is used most often because it is the easiest to remember and doesn't cause confusion by listing the exact concentrations of individual pollutants.
API or “Air Pollution Index” is used by the Chinese government. Although it is a similar index to AQI, the two cannot be compared as they correspond to different levels of various air pollutants.
Mass concentration, more commonly known as PM or PM2.5, refers to the amount of micrograms under a certain size of particulate in a cubic meter (µg/m³).
AQI vs. API vs. Mass Concentration
AQI (Air Quality Index)
● Unitless scale between 0-500 that US EPA uses to describe the pollution level
● Normally composed of multiple pollutants: PM10, PM2.5, NOx, SO2, ozone
● Reflects the highest pollutant, so AQI can be high even if PM2.5 is low
● Not linear – AQI of 200 is 4x worse than AQI of 100
API (Air Pollution Index)
● Unitless scale between 0-500 that China MEH uses to describe the pollution level
● Behaves differently than AQI when under 200, e.g. API 50 = AQI 100
● Same as AQI when over 200
Mass Concentration/PM (micrograms per cubic meter)
● Amount of dust that is in specified volume (m³)
● Can compare across regions and equipment types
● All health standards use mass concentration
○ Maximum daily average µg/m³: US EPA – 35, WHO – 25, China – 75
● Recommended option for policy formulation and setting targets for indoor air quality
PM2.5, AQI China, and AQI US
Part-II Why PM is a better measure than AQI
1. Technically, AQI doesn’t measure just PM2.5.
When AirNOW, a US company, created the AQI system for the US Environmental Protection Agency, it was intended to be used mainly as a policy tool. Instead of placing emphasis on PM2.5, the AQI system measures a "breadbasket" of various pollutants, such as NO2, SO2, ozone, and carbon monoxide, and generates the AQI figure based on the highest value. This means if one pollutant is high, it will skew the overall figure. For example, if NO2 is particularly high on a given day, it will push the AQI higher too - even if PM2.5, which is what really matters health-wise, is low. NO2 is helpful for policymakers because it is a precursor for PM2.5, but its impact on your health is not as significant as PM2.5. So, a high NO2 rating doesn’t necessarily mean the air outside is particularly harmful.
2. There are multiple AQI systems, each of which reports PM2.5 differently
Mass concentration is the same, no matter where you are in the world or what system you are using. However, using versions of other systems causes confusion. China has AQI and also API, while the US has both the AirNOW AQI system and a modified AQI, which the US Embassy team in China uses to measure just PM2.5. This means there are at least four different air quality indexes being used in China, the most common reason for confusion over pollution levels.
This screenshot shows two reporting points at virtually the same mass concentration: one is 37 µg/m³, the other is 38 µg/m³. However, the reported AQI for the Chinese AQI is 53, while the US Embassy AQI reading is 105.
The different AQI systems can lead to fairly large differences in the measurements because you aren’t comparing like with like. A more efficient way of accurately monitoring air quality is to look for a monitoring station close to where you are based, and focus on the PM2.5 levels reported at that station. Choose an app that offers a breakdown between the different pollutants to really measure outdoor air quality accurately. We usually use the app from http://air-matters.com/ .
3. Health standards and scientific guidelines are based on mass concentration, not on AQI
To avoid confusion, organisations such as WHO and environmental ratings such as LEED all use PM2.5 data rather than AQI. It is simply a more accurate measure. It is also logical - if, for example, 35 µg/m³ is the EPA's 24-hr standard, then 70 µg/m³ is twice as bad. However, 35 µg/m3 is equivalent to an AQI of 100, but 70 µg/m³ is equivalent to an AQI of 158, rather than 200. AQI does not have a linear relationship with mass concentration, so it is not straightforward to correlate the mass concentration increase to the AQI. For instance, while at 500 µg/m³, the AQI is the same, with a mass concentration rating also of 500. But at the lower levels, the relationship between the two is not close, with PM2.5 of 10 µg/m³ equaling 42 AQI.
This graph shows how different reporting schemes are related to mass concentration and that the numbers are not all proportional.
However, despite the differences, all reporting schemes are derived from mass concentration in the first place.
4. The AQI reporting system cannot handle China-levels of pollution
At PM2.5 levels of 500 µg/m³, the AQI level might be exactly the same, but above that, AQI maxes out. So, when you read in the news that air pollution is above 500, then the measure being used can only be PM2.5. AQI can only report "Beyond Index” after 500, or as the US Embassy tweeted accidentally in 2012, “Crazy Bad”. In China, sadly, the air all-too-often exceeds 500 µg/m³, so using AQI means there is no way to measure whether the air quality is 501 or 2000.
Put simply, AQI was not designed for China.
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