My cheap Chinese PM2.5 sensors (PMS7003) installed both outside and inside provide accurate readings all year round. They track official government data very closely (monitored by expensive and certified equipment that goes through calibration every year).
My problem with this advice is not that it's difficult to measure pollution levels (it really isn't), but that there's no "fresh air" outside for many of us. In many parts of the world, the air is significantly worse outside than inside even without running an air purifier (and with a purifier the difference in particulate levels can run into 100× or more during winter).
Some years ago I looked at the few papers that measured the difference in gaseous pollutants (like NO2 and SO2) inside and outside with windows open and windows closed, and for some reason closed windows do provide limited protection against them. Nobody really understands why AFAIK, it shouldn't work that way since they're mixed with air in a gaseous mixture from which they can't be filtered out without a specialized chemical filter, but it does help.
> and with a purifier the difference in particulate levels can run into 100× or more during winter
No surprise. A lot of homes are still being heated by combustion. Gas heating is relatively clean if the burners are properly maintained, oil burners are a hit and miss, but everything else... wood pellets are often declared as the "green" alternative, but that's only valid for CO2 (and even there, the actual benefit for the climate is massively debated) - these things spew an awful lot of particulate emissions, brick-wood ovens are even worse, especially when people illegally throw in too humid firewood or, worse, garbage that has no business being incinerated outside of an industrial high temperature facility with proper exhaust stack treatments.
Unfortunately, as you say, in many parts of the world people are too poor to afford proper heating fuel sources, and it's costing them and their countries so much productivity and money in the long run... and in developed countries, there still are a sizable amount of people who have no idea how to properly heat their ovens, how to prepare their firewood or that just outright skirt garbage disposal fees. Jörg Kachelmann, founder of the weather service Meteomedia, has extensively written about that topic in the past [1] and keeps ranting on social media about it under #holzofengate.
We should clarify what type of sensors we're talking about. It seems to be easier to find accurate inexpensive sensors for particulate matter (PM) and CO2 than it is for VOCs.
Exactly this. Is my Awair Element perfectly accurate? No, but VOC at 200 or less when the windows have been open (on a good air day) looks very different than 3,000 when we finish cooking something on the stove.
Maybe my numbers should be 100 and 10,000, but it doesn't matter, I know when I need to turn the air exchanger on turbo and when we are back to "normal".
