AQI vs. PM2.5 ug/m3: why the numbers differ

One of the more confusing aspects of understanding air quality involves the different numbers and metrics used to determine how good/bad the air is. What exacerbates the problem is that foreigners in Korea naturally gravitate toward air quality smartphone apps created in their native tongue. These International apps are usually in English, and rely on AQI (Air Quality Index) measurement numbers, while local Korean apps, signs at bus stops, and local news reports, use a number in ug/m3 (Micrograms per Cubic Meter of Air). The large variance in numbers confuses users who are unaware of the differences, and it creates some odd looks when you quote a high AQI number to Koreans, who see the PM2.5 ug/m3 number on their app is much lower.

If you have ever looked at your app, and become confused by the wide gap in numbers compared to Korean apps, then this blog entry is for you. Please note that we discuss this a lot on the Clean Air Ideas in Korea Facebook Group, as well. Join us!

Before I continue, here is a simple graphic for those who just want a straight numbers comparison between Base PM2.5 in ug/m3 used by Koreans and those handy personal Air Quality Monitors, and AQI on most English apps. Some color zones are divided to show color differences between Korea (left) and US AQI (right) for that same measurement zone:

Direct comparison of PM2.5 in ug/m3 with Air Quality Index (AQI) metric.

I must add a quick note that the zones for PM10 dust are completely different, and the numbers do not equal those of the PM2.5 measurement. You can type those numbers in here, if you choose PM10 as the pollutant, to learn what they are. Or, just refer to your favorite app.

What is AQI?

Apps such as AQICN, AirVisual, and Plume use AQI (Air Quality Index) as their barometer for air quality in a given area. A large "pro" is that apps are available in English, and are popular for their world-wide coverage. A "con" is that these apps rely on 3rd party data, and some may include sensor data from individual home sensors, possibly of random and/or inferior placement. In my experience, updates from Korean sources to most English apps are sometimes delayed. If you insist on an English app, please see MiseGo, which is an English copy of MiseMise -- a trusted Korean app which uses excellent sensors (MiseGo sometimes requires a manual hit of the refresh arrow, to update, but results are correct). Or, you can view the original raw data in Korean or English at the AirKorea website.  They also have an app, but at this time, it is not a good one.

Let's talk more about AQI.  According to AirNow, a website by the US Environmental Protection Agency: "The AQI is an index for reporting daily air quality. It tells you how clean or polluted your air is, and what associated health effects might be a concern for you. The AQI focuses on health effects you may experience within a few hours or days after breathing polluted air. EPA calculates the AQI for five major air pollutants regulated by the Clean Air Act: ground-level ozone, particle pollution (also known as particulate matter), carbon monoxide, sulfur dioxide, and nitrogen dioxide. For each of these pollutants, EPA has established national air quality standards to protect public health .Ground-level ozone and airborne particles are the two pollutants that pose the greatest threat to human health in this country."

In my personal opinion, AQI is a superior measurement, and I wish Korea would use it. The AQI number encompasses a range of potential pollutants, and the numbers are driven by what particular pollutants are the worst. There are days when PM10 might be greater than PM2.5, such as in the spring, when dust traditionally blows in from deserts in Western Asia. The simple truth, however, is that Koreans are used to seeing the PM2.5 and PM10 measurements split-up and monitored separately. 

Here are all of the AQI breakpoints for all pollutants, as defined by the US Environmental Protection Agency (EPA). 

What are ug/m3, PM2.5, and PM10?

The definition of ug/m3: Micrograms per cubic meter of air. It is a unit of measurement over time (usually an hour). 

The Korean apps, bus stop signs, and television reports generally give a ug/m3 result denoting what the ug/m3 was over the past hour. It's important to emphasize that all of the apps (AQI included) give results as an average from the previous hour -- they are NOT in real-time.

So why is the ug/m3 sometimes given in PM2.5 or PM10? First, lets understand what the difference is. Here is a nice definition from Airveda:

"PM10: The coarse particles are between 2.5 and 10 micrometers (from about 25 to 100 times thinner than a human hair). These particles are called PM10 (we say "P M ten", which stands for Particulate Matter up to 10 micrometers in size). These particles cause less severe health effects mostly in the upper respiratory tract. These consist of smoke, dirt and dust from factories, farming and roads as well as mold, spores and pollen. They are made from crushing and grinding rocks and soil then blown by wind.

PM2.5. The small particles are smaller than 2.5 micrometers (more than 100 times thinner than a human hair). These particles are called PM2.5 (we say "P M two point five", as in Particulate Matter up to 2.5 micrometers in size). These consist of toxic organic compounds and heavy metals. They are made from automobile exhaust, burning garbage and landfill, smelting and processing of metals."

MiseMise Korea Air Quality App

There are several great Korean apps for air quality, and although I feel AQI is a better overall metric, I have gravitated toward using Korean apps, because they are far faster at updating, and more reliable than the English apps. This was not always the case, but Korean interest in air quality has resulted in some excellent tools for smartphones.

If you are someone who craves simplicity, does not read Korean, and just wants an accurately fast app to show you Korean air quality, then for Android, MiseMise is hard to beat. Apple users look here. Its simple interface is cute, fun, and obvious. You can also read the base PM10 (미세 먼지) and PM2.5 (초 미세 먼지) numbers in very small text, at the bottom.

Another great Korean app is CAI by Softworx. Apple users find CAI here. I love CAI because it's very detailed, and even provides a link to AQICN for those days when I want to look at the AQI map for South Korea. A problem with it, however, is that it defaults to a poor scale for quality, and you have to change it to WHO 6 in the menus, which requires navigating Korean. MiseMise and Misego are just easier. CAI and MiseMise apps have custom smartphone widgets, and update quickly.

Confusing colors and differing color zones

Are you prepared for more confusion? Different countries have different opinions as to what number ranges and colors are used to measure air quality. To add more gasoline to the fire, the goalposts seem to change every few years.

China, for example, has its own AQI measurement zones, and they are far more lax than US AQI. If you buy an air purifier such as a Xiaomi from China, it will think a PM2.5 ug/m3 of 75 is still within the green zone, because China's measurements say so. According to international measurements, 75 is considered quite high, and is already well into the red zone. For a fun read, see this article discussing the diplomatic history of how the US Embassy air quality monitor in Beijing inadvertently angered the Chinese government. 

Korea also has a different standard than other countries. The good news is that public pressure (I assume) has caused the government to modify its zones to more closely match international numbers. Unfortunately, Koreans culturally use different colors to describe air quality. To Koreans, "blue air" is considered better than "green air," so you'll see that air purifiers will light-up blue within their range of 0-15 ug/m3, which is the current Korean "blue air" zone. The good news is that this part of their metric is easy to follow, and if you keep your air purifier light blue, you are in perfectly decent air. Korea's "green zone" also differs, a bit.

Whatever air quality measurement you choose to use, be vigilant. Know the air quality and be prepared!