What is the difference between PM2.5 and AQI measurements? There are some good answers here Richard Muller and Sandeep Chowdary. I’ll add a bit more explanation about PM 2.5 AQIs and a funny quirk about how they are calculated using stepwise linear functions.
How Is PM 2.5 AQI Calculated?
(Note: this is for the US AQI scale. We’ll be discussing different countries’ scales in the following section.)
To calculate PM 2.5 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.
Even the definition of the number is different. For example, an AQI of 151 is labelled “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 PM 2.5 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 PM 2.5 from 2008-2015 stacks up against those limits:
(The US standard here is 12 micrograms.)
Where Do These Limits Come From?
The WHO bases their limits on studies of the health effects of pollution. From what I understand, there is compelling evidence that PM 2.5 has harmful effects even at levels under 20 micrograms.
Bottom Line: PM 2.5 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.
Breathe safe!
View the original article on Quora here.
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