Hi everyone.
I am in the Po valley (closer to its borders in the Bologna area) and as every user of this forum surely knows there are always significant PM levels around here in the months of January and February with the arrival of cold and fog for then improved significantly over the course of the year without any particular problems.
In any case, we are now in February.
When I read the WHO data with recommended thresholds of 10 Micrograms/daily average of PM2,5 it makes me laugh ( British humour:)) …a dream.
For this reason I wanted to test an air purifier with a CADR of 70m³/hour with a HEPA13 (H13) filter and my Nova SDS011 which I usually use mainly indoors (knowing the effect of high humidity (over 70%) which distorts sensor values too much by having over 80% humidity outdoors frequently this season).
Generally, from the internal values of the same study room I can obtain a true range of the average external value of PM10 released by the control unit of the air quality agency of my municipality the following day. I can always guess whether the values were slightly below or above the released average threshold.
It’s a DIY system but it almost always works 
Since mine is a common study room, based on the CADR 70m³/hour specifications of the purifier I calculated that in about 40 minutes there is at least complete filtered air circulation.
The sensor was very useful to me, SDS011 responds very well to state changes…
From the graph it is clear how from the first minute the PM2,5 values drop significantly.
It’s fascinating how everything happens in just a few seconds.
I learned a lot from these tests.
I also learned that as soon as I turn off the purifier, the recovery begins from the first minute, very slowly but constantly. and this in a house with 14cm insulation in the walls, closed door and double glazing on the window.
The values never fall below a certain threshold (5-7micrograms) probably for the reasons mentioned above and constant PM infiltration from the walls…
It is very clear how the constant variations in outdoor particulate matter influence indoor aerosols, although obviously very slowly. all taking as a reference my study room which is usually frequented only by me and closed for the rest of the day.
So for the moment the tests continue and if anyone has questions, advice or criticisms they are welcome
Thank you!