Hello!
I live in a small town near Barcelona, Spain. After 2-3 months with PM2.5 readings averaging 7, it grew to 30-40-50, right when the cold weather started.
My feeling is that pellets stoves (the last trend in climate friendly heating in the area of wich I am also a victim) in the neighborhood are causing this.
This is the last 24h graph. The spike around 10-12 AM is because I took the unit outside to compare air outside vs inside. I did not turn my heating on for the whole day.
Is smoke coming through my old house’s windows? Time to change windows and invest in an air purifier system? 
Thanks!
Hi,
I think it’s definitely smoke from the stoves. I’ve observed similar behaviour. I’m waiting for an outdoor unit and I’ll perform a long term test during the cold season.