The USEPA has announced the revision of their 24-h National Ambient Air Quality Standards (NAAQS). The 24-h standard for fine particulate matter was reduced from 65 to 35 micrograms per cubic meter (μg/m³).
This is an important development, because it indicates that the USEPA has reviewed many peer-reviewed studies, and they have found that harm can occur from exposures to fine particles that exceed the new, lower level.
At the SimSmoke.Org website, we use the NAAQS to judge the severity of exposure to secondhand smoke particles under a range of conditions. Output generated from the SimSmoke.Org simulations will soon be updated to reflect the new USEPA standard.