Documentation for Immediately Dangerous to Life or Health Concentrations (IDLHs)
The "immediately dangerous to life or health air concentration
values (IDLHs)" used by the National Institute for Occupational
Safety and Health (NIOSH) as respirator selection criteria
were first developed in the mid-1970's. The Documentation for
Immediately Dangerous to Life or Health Concentrations (IDLHs)
is a compilation of the rationale and sources of information
used by NIOSH during the original determination of 387 IDLHs
and their subsequent review and revision in 1994.
Immediately Dangerous to Life or Health Conditions and Respirator Selection 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. The following is from a U.S. Department of Labor bulletin:
The Occupational Safety and Health Administration (OSHA) defines an immediately dangerous to life or health concentration in their hazardous waste operations and emergency response regulation as follows:
In the OSHA regulation on permit-required confined spaces, an immediately dangerous to life or health condition is defined as follows:
As part of their current respiratory protection standard
[29 CFR 1910.134(e)], OSHA requires that a standby person be
present with suitable rescue equipment when self-contained
breathing apparatus or hose masks with blowers are used in
atmospheres immediately dangerous to life or health. Furthermore,
persons using air-line respirators in atmospheres immediately
hazardous to life or health must be equipped with safety harnesses
and safety lines for lifting or removing workers from hazardous
atmospheres. Go to top of page.
In 1974, NIOSH and OSHA jointly initiated the development of occupational health standards consistent with Section 6(b) of the Occupational Safety and Health Act of 1970 for substances with then-existing OSHA permissible exposure limits (PELs). This joint effort was called the Standards Completion Program (SCP) and involved the cooperative efforts of personnel from various divisions within NIOSH and OSHA, and several contractors. The SCP developed 387 substance-specific draft standards with supporting documentation that contained technical information and recommendations needed for the promulgation of new occupational health regulations. Although new standards were not promulgated at that time, these data became the original basis for the NIOSH/OSHA Occupational Health Guidelines for Chemical Hazards [NIOSH/OSHA 1981].
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!
IDLHs were determined for each substance during the 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.
Since chronic exposure data may have little relevance to acute
effects, these types of data were used in determining IDLHs
only when no acute toxicity data were available and only in
conjunction with competent scientific judgment. In a number
of instances when no relevant human or animal toxicity data
were available, IDLHs were based on analogies with other substances
with similar toxic effects. Go to top of page.
Discussion of Original IDLHs
The basis for each of the 387 IDLHs determined during the SCP were reviewed and paraphrased from the individual draft technical standards for this publication. Also included is a complete listing of references cited in the SCP; in many cases where only secondary references were cited, the original sources have also been added. Whenever available, the references (secondary and primary) were obtained to verify the information cited in the SCP. However, a few of the original references such as personal communications and foreign reports could not be located.
Although 387 substances were originally included in the SCP, IDLHs were not specifically determined for all of them. The published data at that time for 40 of these substances (e.g., DDT and triphenyl phosphate) showed no evidence that an acute exposure to high concentrations would impede escape or cause any irreversible health effects following a 30-minute exposure and the designation "NO EVIDENCE" was used in the listing of IDLHs. For all of these substances, respirators were selected on the basis of assigned protection factors. For some (e.g., copper fume and tetryl), an assigned protection factor of 2,000 times the PEL was arbitrarily used to determine the concentration above which only the "most protective" respirators were permitted. However, for most particulate substances for which evidence for establishing an IDLH did not exist (e.g., ferbam and oil mist), the use of an assigned protection factor of 2,000 would have resulted in the assignment of respirators at concentrations that were not likely to be encountered in the occupational environment. In addition, exposure concentrations greater than 500 times the PEL for many airborne particulates could result in exposures that would hamper vision. Therefore, it was decided as part of the SCP (and during the review and revision of the IDLHs) that for such particulate substances, only the "most protective" respirators would be permitted for use in concentrations exceeding 500 times the PEL.
IDLHs could not be determined during the SCP for 22 substances (e.g., bromoform and calcium oxide) because of a lack of relevant toxicity data and therefore the designation "UNKNOWN" was used in the IDLH listing. For most of these substances, the concentrations above which only the "most protective" respirators were allowed were based arbitrarily on assigned protection factors that ranged from 10 to 2,000 times the PEL, depending on the substance. There were also 10 substances (e.g., n-pentane and ethyl ether) for which it was determined only that the IDLHs were in excess of the lower explosive limits (LELs). Therefore, the LEL was selected as the IDLH with the designation "LEL" added in the IDLH listing. For these substances, only the "most protective" respirators were permitted above the LEL in the SCP draft technical standards.
For 14 substances (e.g., beryllium and endrin), the IDLHs
determined during the SCP were greater than the concentrations
permitted based on assigned respiratory protection factors.
In most instances the IDLHs for these substances were set at
concentrations 2,000 times the PEL. Go to top
The current NIOSH definition for an immediately dangerous to life or health condition, as given in the NIOSH Respirator Decision Logic [NIOSH 1987], 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." The NIOSH respirator decision logic uses an IDLH as one of several respirator selection criteria. Under the NIOSH respirator decision logic, "highly reliable" respirators (i.e., the most protective respirators) would be selected for emergency situations, fire fighting, exposure to carcinogens, entry into oxygen-deficient atmospheres, entry into atmospheres that contain a substance at a concentration greater than 2,000 times the NIOSH REL or OSHA PEL, and for entry into immediately dangerous to life or health conditions. These "highly reliable" respirators include either a self-contained breathing apparatus (SCBA) that has a full facepiece and is operated in a pressure-demand or other positive-pressure mode, or a supplied-air respirator that has a full facepiece and is operated in a pressure-demand or other positive-pressure mode in combination with an auxiliary SCBA operated in a pressure-demand or other positive-pressure mode.
When the IDLHs were developed in the mid-1970's, only limited toxicological data were available for many of the substances. NIOSH has recently requested information on the current uses of IDLHs in the workplace and on the scientific adequacy of the criteria and procedures originally used for establishing them [Federal Register, Volume 58, Number 229, p. 63379, Wednesday, December 1, 1993]. The information received in response to the Federal Register announcement is being evaluated and will be used to establish future actions concerning IDLHs. In the interim, however, NIOSH decided to review the existing IDLHs, and revise them as appropriate.
This document includes IDLHs for 85 substances (e.g., benzene
and methylene chloride) determined by NIOSH to meet the OSHA definition
of "potential occupational carcinogen" as given in 29
CFR 1990.103. For all of these substances, except ethylene oxide
and crystalline silica, NIOSH recommends that the "most protective"
respirators be worn by workers exposed at concentrations above
the NIOSH REL, or at any detectable concentration when there is
no REL. For ethylene oxide and crystalline silica, NIOSH recommends
that the "most protective" respirators be worn in concentrations
exceeding 5 ppm and 25 mg/m3, respectively [NIOSH 1989, 1994].
Go to top of page.
The criteria utilized to determine the adequacy of existing IDLHs were a combination of those used during the SCP and a newer methodology developed by NIOSH. These criteria form a tiered approach with acute human toxicity data being used preferentially, followed next by acute animal inhalation toxicity data, and then finally by acute animal oral toxicity data to determine an updated IDLH. When relevant acute toxicity data were insufficient or unavailable, then the use of chronic toxicity data or an analogy to a chemical with similar toxic effects was considered. In order to facilitate the revision process, secondary toxicological data were primarily used. Once a preliminary IDLH was developed, it was compared to the existing IDLH and to several other factors (e.g., existing short-term exposure guidelines and lower explosive limits).
The following "hierarchy" was followed to develop a "preliminary" value for the revised IDLH:
t(hours) correction factor O.5 1.0 1 1.25 2 1.6 3 1.8 4 2.0 5 2.15 6 2.3 7 2.4 8 2.5
Anyone who is aware of additional published data that may affect
the IDLHs determined for particular substances is encouraged to
make this information available to NIOSH. All data will be reviewed
and consideration will be made regarding subsequent revision of
the IDLHs. Go to top of page.
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Buckley LA, Jiang XZ, James RA, Morgan KT, Barrow CS . Respiratory tract lesions induced by sensory irritants at the RD50 concentration. Toxicol Appl Pharacol 74(3):417-429.
NIOSH/OSHA . Occupational health guidelines for chemical hazards. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 81-123 (NTIS Publication No. PB-83-154609).
NIOSH . NIOSH respirator decision logic. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 87-108 (NTIS Publication No. PB-88-149612).
NIOSH . Current Intelligence Bulletin 52. Ethylene oxide sterilizers in health care facilities. Engineering controls and work practices. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 89-115 (NTIS Publication No. PB-90-142571).
NIOSH [in preparation]. Criteria for a recommended standard: occupational exposure to respirable coal mine dust. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health.
ten Berge WF, Zwart A, Appleman LM . Concentration-time mortality response relationship of irritant and systematically acting vapours and gases. J Haz Mat 13:301-309.
Yant WP . Protecting workers against temporary and emergency exposures. In: Protecting plant manpower through the control of air contaminants. Special Bulletin No. 14. Washington, DC: U.S. Department of Labor, Division of Labor Standards. Go to top of page.
*Code of Federal Regulations