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. (.)
So the raw number is the number of micrograms. Then they take that number of micrograms and convert it to an AQI. (.) 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.
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), 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:
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?
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.
View the original article on Quora here.
How I Protect Myself
Smart Air is a certified B Corp committed to combating the myths big companies use to inflate the price of clean air.
Smart Air provides empirically backed, no-nonsense purifiers and masks, that remove the same particles as the big companies for a fraction of the cost. Only corporations benefit when clean air is a luxury.