Just to add to what others have said: smog is made up of particles called PM10 and PM2.5. These are classes of particles that fall into size categories of 10 and 2.5 microns in size.

PM10s are small enough to cause irritation in the eyes, throat, and nose. PM2.5s however are so small that they can pass the blood/brain barrier. And those are what are in smog.