PM2.5 AQI (air quality index) is the most common measurement used to track air pollution and air quality. In this article, we will discuss how PM2.5 AQI is calculated, how you can use it, and we’ll uncover its funny quirk.
How Is PM2.5 AQI Calculated?
To calculate the PM2.5 AQI value for a given area, governments use a machine called a beta attenuation monitor (BAM) that estimates the total mass of PM2.5 particles per cubic meter of air. (Here’s how they work.)
So the raw number these BAM machines give is concentration, given in micrograms per meter cubed. Then they take that number of micrograms and convert it to an AQI. How does this formula work? Unfortunately it’s not as simple as 1 microgram = 2 AQI points ,or 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. Sound confused? Here’s a handy PM2.5 & AQI calculator for you to use.
AQI Readings for Other Countries
This calculation is used for the US AQI scale. The US AQI scale has become the common scale in many other countries as well. 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
The reason governments came up with the AQI scale was to make reporting easier – PM2.5 concentrations can then fall into easy to understand ‘bands’ like ‘0-50′ and ’50-100’. However, because there are so many different scales AQI numbers actually make things more confusing. Oftentimes apps report AQIs without making it clear what scale they’re using (is it the US AQI, or the India AQI, or the China one?). What’s more, some governments decided not to use AQI at all. Singapore, for example, uses PSI or “Pollutant Standards Index”. Even more confusing!
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. They are the same across the world.
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 = 5 micrograms
24-hour limit = 25 micrograms
For a reference point, here’s how Beijing’s average PM2.5 from 2014-2020 stacks up against those limits:
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.
View the original article on Quora here.
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