What is the main reason cause the PM2.5 air pollution

Decoration materials are the most important factors of indoor air pollution.
Outdoor air pollution is serious, but it can also affect the indoor air quality.
We use air purifier in home but do you know if it is really works?
Particle counter is the best helper to check the air quality around us.
Also the handheld particle counter can demo air purifier capability. and also the indoor temperature.