This post talks about airborne particles from .01 to 10 microns in size.
Particles can be organic like mold, bacteria, and viruses; or inorganic like ozone, smoke, and air pollution.
Particles are separated into repairable and non repairable groupes. Non-repairable means it’s so small it gets stuck in your lungs. The chemical composition of particles also determines the effect they have on your health.
There are a lot of people flooding the market with particle sensing devices that only test for “course” (over 2.5 µg/m3) particles. Those sensors are significantly cheaper than the “fine” (under 2.5 µg/m3) particle sensors. We have tested a lot of different, low cost sensors and most don’t work. We recommend Purple Air or EPA Smoke Sense Study mobile app for outside air. There is an AI “prediction” of indoor air here: Breezmeter.
There are a wide variety of methods to test for airborne particles. If your primary concern is urban smog, then look for a sensor/particle meter/IAQ device that can be measure fine particles.
Regardless of how accurately the sensor is that you are using; the main objective is to lower the particulates you are breathing. The best way to do that is with a HEPA only air purifier. You may be able to add a HEPA filter to your heating or HVAC system. Portable HEPAs work very well, also.