The size of pollution particles, called particulate matter or PM, are directly linked to their potential for causing health problems. PM is measured in micrometers (µm)
When a wildfire smoke event is taking place, there could be many different particles in the air. Larger PM like ash (PM with a diameter greater than 10µm) can be seen with the naked eye and may be concerning, but our bodies have natural defenses to keep these larger particles from entering our lungs, though they can be an irritant to the eyes, nose, and throat.
Other particles from wood, or wildfire smoke, can be much smaller (PM with a diameter of 2.5µm or less, than 2.5µm, about 3% the diameter of a human hair) and are more dangerous to our health. These smaller particles cannot be seen with the naked eye and can be inhaled into the deepest part of the lungs. There these particles may cause greater health concerns, like breathing and heart problems.