Air Quality Index: what it doesn't tell you!
Do you know what the Air Quality Index (AQI) is and how it is calculated?
Some governments (e.g. China, EU, India, UK, USA) measure the air quality and publish the current Air Quality Index (AQI), which indicates how clean or polluted the air is. Although the calculation of the AQI varies from country to country (depending on their national Air Quality Standard), they are quite similar in meaning. What all Air Quality Indexes have in common:
Measuring the dose (air pollutant averaged over a specified time).
Different pollutants are considered:
Particulate Matter (PM) / dust / fine particles (e.g. PM2.5, PM10).
Gases (e.g. ground level ozone O3, carbon monoxide CO, nitrogen dioxide NO2, sulfur dioxide SO2).
There is a AQI scale from good, to moderate, to bad air quality.
There is a real-time global AQI map (http://aqicn.org/map/world/). Here a snapshot, taken on 2017-July-21 at 06:26 AM UTC(0).
What the AQI tells you
The AQI 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 .
What the AQI doesn't tell you
As you know now, the AQI is a value that is based on averaged concentrations of gases and particle pollution over a specific time. This is where the problem starts: the averaging time is sometimes up to 24 hours. Therefore, if you want to know how polluted the air is right now, the AQI is the wrong value to look at, because it may tell you the 24h-average, not the current state of pollution.
For example: when the AQI is 170 (unhealthy), one need to read it as "if I stay out for 24 hours, and the AQI is 170 during those 24 hours, then the health effect is unhealthy". This is quite different from saying that "if the AQI reported now is 170, then the health effect is unhealthy".
It may be possible that winds clean or pollute the air within less than an hour (e.g. the Indonesian wildfire causing the Singapore Smog when the winds are heading to the north).
To overcome this issue, initiatives were started like the NowCast AQI, which is based on values over the last 12h (PM) or 8h (O3) and weights the more recent hours higher than the hours before.
Note: The reason behind this 24h-averaging is, that health effect studies on humans are based on this 24h-averaging in epidemiological studies. And these studies were not just done in a few weeks, it took decades.
Another issue you may face is, that the next outdoor air quality measurement station is tens or even hundreds of kilometers away. In this case, the relevance of the measured AQI on you is highly questionable.
AQI of your country
If you want to know more about the AQI in your country and the national pollution limits, you can find here a list (please forgive me if your country is not there and let me know where I can find out more about your countries AQI):
If you do not find your country in this list, go to this link, which is a good place to start finding your countries air quality measurement data.
For you to remember: whenever you look at the AQI of your country or city: check the averaging-time.
So, that's all for today's blog post. Go out and enjoy the summer (or winter, wherever you live) with a walk or a run in hopefully fresh air! I am going to do the same today :-)
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