Final answer:
Particulate matter smaller than 2.5 micrometers (PM2.5) is a measurable environmental factor used to understand air pollution dose. PM2.5 consists of tiny solid or liquid particles suspended in the air that can cause respiratory diseases.
Step-by-step explanation:
Particulate matter smaller than 2.5 micrometers (PM2.5) is a measurable environmental factor that is quantified to understand the air pollution dose that an individual receives. PM2.5 refers to tiny solid or liquid particles suspended in the air, such as dust, dirt, viral particles, and bacteria. These particles are small enough to penetrate deeply into the lungs and can cause respiratory diseases.