What has changed in the definition of an IDLH atmosphere? NIOSH has removed the mention of the 30 minute time period, based on intraspecies variation factors. There is no mention of the 30-minutes in the new definition. A little history about the IDLH designation…
The concept of using respirators to protect workers in situations that are immediately dangerous to life or health was discussed at least as early as the 1940’s. IDLH values used by NIOSH as respirator selection criteria were first developed in the mid-1970’s. IDLHs were determined for each substance during the Standards Completion Program (SCP) on a case-by-case basis, taking into account the toxicity data available at the time. Whenever possible, IDLHs were determined using health effects data from studies of humans exposed for short durations. However, in most instances, a lack of human data necessitated the use of animal toxicity data. When inhalation studies of animals exposed for short durations (i.e., 0.5 to 4 hours) were the only health effects data available, IDLHs were based on the lowest exposure causing death or irreversible health effects in any species. When lethal dose (LD) data from animals were used, IDLHs were estimated on the basis of an equivalent exposure to a 70-kg worker breathing 10 cubic meters of air. As part of the respirator selection process for each draft technical standard, an IDLH was determined. The definition for an IDLH that was derived during the SCP was based on the definition stipulated in 30 CFR 11.3(t). The purpose for establishing this IDLH was to determine a concentration from which a worker could escape without injury or without irreversible health effects in the event of respiratory protection equipment failure (e.g., contaminant breakthrough in a cartridge respirator or stoppage of air flow in a supplied-air respirator) and a concentration above which only “highly reliable” respirators would be required. In determining IDLHs, the ability of a worker to escape without loss of life or irreversible health effects was considered along with severe eye or respiratory irritation and other deleterious effects (e.g., disorientation or incoordination) that could prevent escape. Although in most cases, egress from a particular worksite could occur in much less than 30 minutes, as a safety margin, IDLHs were based on the effects that might occur as a consequence of a 30-minute exposure. However, the 30-minute period was NOT meant to imply that workers should stay in the work environment any longer than necessary following the failure of respiratory protection equipment; in fact, EVERY EFFORT SHOULD BE MADE TO EXIT IMMEDIATELY! The current NIOSH definition for an immediately dangerous to life or health condition, is a situation “that poses a threat of exposure to airborne contaminants when that exposure is likely to cause death or immediate or delayed permanent adverse health effects or prevent escape from such an environment.” It is also stated that the purpose of establishing an IDLH is to “ensure that the worker can escape from a given contaminated environment in the event of failure of the respiratory protection equipment.