Why are particulate matter toxic to humans?
Particulates can accumulate in the aveolae in the lungs. This leads to inflammation and tissue damage and scarring. This reduces the available surface area for gas transfer. Low level exposure over long periods is harmful. Asbestos workers are well known to get sick, but miners can get black lung or silicosis from inhaling mining dust, and farmers can get lung disease from inhaling dirt that gets blown up as dust. This damage usually isn't seen until they are in their 50s or 60s.