How is PM2.5 AQI Calculated?

PM2.5 AQI is the most common measurement to track air pollution and air quality. In this article, we will discuss how PM2.5 AQI is calculated and its funny quirk.

How Is PM2.5 AQI Calculated?

To calculate US PM2.5 AQI, governments use a machine called a beta attenuation monitor that estimates the number of micrograms (the mass or weight) per cubic meter of air. (Here’s how they work.)

So the raw number is the number of micrograms. Then they take that number of micrograms and convert it to an AQI. (Here’s a handy converter calculator.) So what’s the formula, 1 microgram = 2 AQI points? 1 microgram = 10 AQI points?

The actual answer is much weirder. Here’s what it looks like:

Those first 10 micrograms count for 42 AQI points! But around the 100 microgram point, adding 10 micrograms contributes just 5 AQI points. Toward the end, it becomes 1 to 1.

AQI Readings for Other Countries

This calculation is for the US AQI scale, which many other countries use. However, different countries follow different scales. For instance, China uses its own AQI scale and India uses a scale it calls National Air Quality Index (NAQI).

Both of these are more “forgiving” scales. For example, A PM2.5 concentration of 45 micrograms would be an AQI of 124 in the US but just 75 in India.

PM2.5 AQI vs NAQI

Even the definition of the number is different. For example, an AQI of 151 is labeled “unhealthy” in the US but “moderate” in India.

Why I’ve Stopped Using AQI

That can make AQI numbers really confusing. Oftentimes apps report AQIs without making it clear what scale they’re using.

For these reasons, the more I’ve gotten into the nerdery of air quality (such as analyzing what time of day is PM2.5 the lowest), the more I start to ignore AQI and just pay attention to the direct measure -micrograms. Micrograms don’t have ever-changing conversion formulas, and they don’t depend on your government’s scale.

How to Use Micrograms

But wait, AQI is great because 100 is roughly “bad,” so it’s easy to understand. If we use micrograms, how do we know what’s good and what’s bad? I use the WHO guidelines:

Annual limit = 10 micrograms

24-hour limit = 25 micrograms

For a reference point, here’s how Beijing’s average PM2.5 from 2008-2015 stacks up against those limits:

(The US standard here is 12 micrograms.)

Where Do These Limits Come From?

The WHO bases its limits on studies of the health effects of pollution. From what I understand, there is compelling evidence that PM2.5 has harmful effects even at levels under 20 micrograms.

Bottom Line: PM2.5 v.s. AQI v.s. Micrograms

The number of micrograms per cubic meter of air is estimated through a beta attenuation monitor and then converted to AQI based on formulas, which vary among countries.  Micrograms are more direct and independent.

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View the original article on Quora here.


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