It is clear that devices like the AirLink, PurpleAir and organisations like AirNow are busy setting a de facto standard. My advise to the user would be if you are confused or your country does not really has great support for an Air quality standard, go for the US standard. It is clear, well worked out and supported worldwide.
Many sites show Air Quality worldwide. Look e.g. at PurpleAir , IQAir or Davis (although for the last I can't find the map - Anybody?). A special site is aqicn.org which not only shows a lot of info, has great FAQ but also has a great sensor page. It is a good place to start.
Some scientific articles on sensor comparison and AQI can be found here on the forum with this post. Scientists have looked into these sensors to see how they compare to the official measurements. The cheap sensors compare positively. Some articles on this subject are included as well.
The cheap sensors are often referred to as toys. They are not. Many environmental organisations set up a community of users using this type of sensors to get indicative measurements. They can be a bit compared to WOW but are more open. Take a look at Sensor.Cummunity, Luftdaten.info and the Dutch RIVM community page (leading to Sensor.Community). Look in your own country for organisations like this.
For meteorology most people know where to find the information or are educated in that field. For Air Quality that is less so and it is a good idea to read yourself into the matter. Knowing that an AQI is basically a unit related to the statistic of how many people are submitted to hospital with certain types of diseases and the fact that the AQI is apparently not a clear objective figure like temperature. That is kind of worrying in my opinion.
I will modify and extend this post when and if I find additional info or when required. You are invited to add information yourself in the comments. It is more or less independent of the AirQuality module.
[Edit: 29 November 2020]
While finishing my AirQuality module of CumulusUtils I added this info.
The numbers used by the Air Link (and stored each minute by CMX) are:
- Minute values;
- 1 hour running averages;
- 3 hour running averages;
- 24 hour running averages;
- nowcast (see below, also a figure every minute)
You can easily see from the graphs on CumulusUtils that the difference between the 1hr/3hr and the nowcast running averages is not that big. Nevertheless, the nowcast figure is a hard demand in the VS apparently. Just as the absolute concentrations may differ in percentages from the official figures, it is the trend which shows it all. If a cheap sensor shows 100 μg/m3 and somebody else says it is only 20 μg/m3 than you are in for an argument. And that is the real value of the cheap sensors. That is also where the European sensor.community is appreciated by (e.g.) the Dutch National Health Organisation.
BTW: These sensors become dirty by definition because of the constant airflow through the measurement channel where the laser and the lens are. That needs cleaning once in a while (let's say once a year): opening the sensor, carefully cleaning the dirt away.
[Edit: 13 december 2020]
As AQI is very different in different countries I will publish here the documentation and a summary sheet in the following comment.