You can read more on it on the See the Air Blog: https://seetheair.wordpress.com/2021/04/26/thailand-school-s...
If the air quality is decent outside, it is easy to maintain a PM2.5 rating of near <10 like this.
In California fire season I have a box fan on high in the window pushing air in (and sealing the areas around the window) through an MPR 2800 furnace filter (which is better than the 99.97% rated hepa filters, would be marked as a ULPA filter I believe in different contexts) with this I can maintain the ~0 PM2.5 rating inside even when the AQI outside is >300 while keeping CO2 in check.
It's been quite surprising how easily the CO2 spikes with closed windows and doors in the bedroom with just one person... I essentially always have a window in each room open several inches, cracked isn't enough, usually with at least one fan pushing air through a window.
Positive pressure systems can virtually achieve zero AQI inside even on the worst polluted days. You can read more here: [1] https://www.airgradient.com/blog/2020/01/08/positive-pressur... [2] https://seetheair.wordpress.com/2021/04/26/thailand-school-s...
i put my air purifiers on low when the windows are open to deal with the particles i throw up by moving around and doing stuff indoors. with the windows closed (usually only at night), i put the air purifiers on medium/high.
edit: i mentioned the road to note that outdoor pollution is higher during the day, yet opening the windows despite that lowers overall particulate levels indoors. it may be different in indian cities or cities like beijing with much higher pollution levels.
Very much depends on where you live. I have a few DIY air quality stations with data being piped to Grafana. I just looked at the latest data, the average outside PM level for the past 5 months has been around 100 µg/m³, while inside it's around 10 µg/m³.
This spring happens to be pretty windy and this skews the outside levels down, otherwise the ratio would be much worse.