Respiratory Protection for Firefighters During Overhaul Operations: Operational Field Assessment Report [open pdf - 1MB]
From the Executive Summary: "After extinguishing structural fires, firefighters carry out fire overhaul operations to locate and extinguish smoldering hot spots. Early on in the overhaul operations, firefighters wear self-contained breathing apparatus (SCBA); although, it is common practice to remove the SCBA when the carbon monoxide (CO) concentration drops to a safe level. Studies have shown, however, that after CO dissipates, particulate matter and harmful chemicals are still present during overhaul environments. As a result, firefighters must continue to wear heavy SCBA equipment throughout overhaul operations or risk breathing in the hazardous material present in the overhaul environment. To address this hazard, the Respiratory Protection for Firefighters during Overhaul Operations project developed a filter module that is designed to be used in passive air-purifying respirators (APRs) and powered APRs (PAPRs) to protect firefighters from particulate and chemical hazards while being lighter and more comfortable than the traditional SCBA."
U.S. Department of Homeland Security: https://www.dhs.gov/