Just google brands and find that they may be not so accurate or may be calibrated for some specific region of China https://smartairfilters.com/en/blog/how-acc

Just google brands and find that they may be not so accurate or may be calibrated for some specific region of China https://smartairfilters.com/en/blog/how-accurate-are-common-particle-counters-comparison-test/ https://smartairfilters.com/en/blog/how-accurate-are-common-... So may be better solution are local laboratories with calibrated equipment Masks are tricky. Most "light" masks are for huge particles and can't defend against most of dangerous chemicals Military and science masks are working good against some chemicals and do not protect from another. Except solutions with O2 tanks on your back