They're not necessarily large, and the worst for humans is small particulate which gets into the bloodstream through lungs, PM2.5.

https://www.health.ny.gov/environmental/indoors/air/pmq_a.ht...

Breathing in unhealthy levels of PM2.5 can increase the risk of health problems like heart disease, asthma, and low birth weight. Unhealthy levels can also reduce visibility and cause the air to appear hazy.

Outdoor sources include vehicle exhaust, burning wood, gas and other fuels, and fires. Particle pollution can also travel long distances from its source; for example from wildfires hundreds of miles away. Outdoor particle pollution levels are more likely to be higher on days with little or no wind or air mixing.