PM_{2. 5} Aerosol particles that are smaller than or equal to 2. 5 micrometers or have an aerodynamic diameter of 2. 5 micrometers. In the Clean Air Act Revision of 1997 modified particulate standard to include PM_{2. 5}. Considered fine particle matter, PM_{2. 5} has a greater effect of visibility and health than do larger aerosol particles, such as decreased lung function and premature death.