The ISO 13571 standard document “Life-threatening components of fire – Guidelines for the estimation of time to compromised tenability in fires” has been updated, in particular removing use of “fractional effective concentration” (or FEC) from fire gas emissions, and replacing by “fractional effective dose” (FED). FEC considered the effects of irritant gases to be instantaneous, rather than basing assessment on a fraction of the lethal dose. This led to unfavourable results for heteroatoms, in particular halogens. The change is based on bioassay research showing that toxicity of irritants is dependent on dose (not just concentration) and fire analysis research showing that fires in cables (which often contain PVC) did not generate the high numbers of fatalities which would be expected from FEC calculations.
ISO 13571 standard “Life-threatening components of fire – Guidelines for the estimation of time to compromised tenability in fires” https://www.iso.org/standard/73217.html