Immediately Dangerous To Life or Health (IDLH)

May 1994

Immediately Dangerous to Life or Health Concentrations (IDLH)

Introduction

The immediately dangerous to life or health (IDLH) values 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 (IDLH) 

Concentrations is a compilation of the rationale and sources of information used by NIOSH 

during the original determination of 387 IDLH values. In addition, NIOSH continues to 

review, document, and revise the science and methodology behind the existing IDLH values 

when appropriate, and derive new IDLH values.

Background

The concept of using respirators to protect workers in situations that are IDLH was discussed 

at least as early as the 1940's. The following is from a U.S. Department of Labor bulletin:

The situations for which respiratory protection is required may be designated as, (1) 

nonemergency and (2) emergency. Nonemergency situations are the more or less normal ones 

that involve exposure to atmospheres that are not immediately dangerous to health and life, 

but will produce marked discomfort, sickness, permanent harm, or death after a prolonged 

exposure or with repeated exposure. Emergency situations are those that involve actual or 

potential exposure to atmospheres that are immediately harmful and dangerous to health or 

life after comparatively short exposures. [Yant 1944]

The Occupational Safety and Health Administration (OSHA) defines an IDLH value in their 

hazardous waste operations and emergency response regulation as follows:

An atmospheric concentration of any toxic, corrosive or asphyxiant substance that poses an 

immediate threat to life or would cause irreversible or delayed adverse health effects or would 

interfere with an individual's ability to escape from a dangerous atmosphere. [29 CFR* 

1910.120]

In the OSHA regulation on permit-required confined spaces, an IDLH condition is defined as 

follows:

Any condition that poses an immediate or delayed threat to life or that would cause irreversible 

adverse health effects or that would interfere with an individual's ability to escape unaided 

from a permit space. Note: Some materials--hydrogen fluoride gas and cadmium vapor, for 

example--may produce immediate transient effects that, even if severe, may pass without 

medical attention, but are followed by sudden, possibly fatal collapse 12-72 hours after 

exposure. The victim "feels normal" from recovery from transient effects until collapse. Such 

materials in hazardous quantities are considered to be "immediately dangerous to life or 

health." [29 CFR 1910.146]

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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.

The Standards Completion Program

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 value was 

determined. The definition for an IDLH value that was derived during the SCP was based on 

the definition stipulated in 30 CFR 11.3(t). The purpose for establishing this IDLH value 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 IDLH values, 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, IDLH values 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!

IDLH values 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, IDLH values 

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, IDLH values were based on the lowest exposure causing 

death or irreversible health effects in any species. When lethal dose (LD) data from animals 

were used, IDLH values 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 IDLH values 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, IDLH values were based on analogies with other 

substances with similar toxic effects.

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Discussion of the Original IDLH Values

The basis for each of the 387 IDLH values 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, IDLH values 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 value 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 IDLH values) that for such 

particulate substances, only the "most protective" respirators would be permitted for use in 

concentrations exceeding 500 times the PEL.

IDLH values 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 value 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 IDLH values were in excess of the lower explosive 

limits (LELs). Therefore, the LEL was selected as the IDLH value 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 IDLH values 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.

Current NIOSH Use of IDLH Values

The current NIOSH definition for an IDLH condition, as given in the NIOSH Respirator 

Selection Logic, is one 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 [NIOSH 2004]. The purpose of establishing an 

IDLH value is (1) to ensure that the worker can escape from a given contaminated environment 

in the event of failure of the respiratory protection equipment and (2) is considered a 

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maximum level above which only a highly reliable breathing apparatus providing maximum 

worker protection is permitted [NIOSH 2004]. In establishing the IDLH value, the following 

conditions must be assured:

A. The ability to escape without loss of life or immediate or delayed irreversible health effects. 

(Thirty minutes is considered the maximum time for escape so as to provide some margin of 

safety in calculating an IDLH value.)

B. The prevention of severe eye or respiratory irritation or other reactions that would hinder 

escape.

The NIOSH respirator decision logic uses an IDLH value 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 positivepressure mode in combination with an auxiliary SCBA operated in a pressure-demand or other 

positive-pressure mode.

When the IDLH values were developed in the mid-1970's, only limited toxicological data were 

available for many of the substances. In 1993, NIOSH requested information on the uses of 

IDLH values 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 was evaluated and used to establish future actions concerning IDLH values.

This document includes IDLH values 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, 2005].

Revised Criteria for Determining IDLH Values

The criteria utilized to determine the adequacy of existing IDLH values 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 value. 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 value was developed, it was compared to the 

existing IDLH value and to several other factors (e.g., existing short-term exposure guidelines 

and lower explosive limits).

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The following "hierarchy" was followed to develop a "preliminary" value for the revised IDLH 

value:

A.Human acute toxicity data were used if sufficient to determine a concentration that for up to 

30 minutes does not cause death, serious or irreversible health effects, or does not impair or 

impede the ability to escape.

B.Animal acute lethal concentration (LC) data were considered next. The only animal lethal 

concentration data used involved mammals; the vast majority of the data was from studies of 

rats, mice, guinea pigs, and hamsters. It was decided to generally use the lowest reliable LC 

data, with LC50 data preferred. If acute LC data determined during a 30-minute period were 

not available, then the data, based on a study by ten Berge et al. [1986], were "adjusted" to an 

equivalent 30-minute value using the following relationship:

Adjusted LC50 (30 minutes) = LC50(t) * (t/0.5)** (1/n)

where: LC50(t) = LC50 determined over t hours

n = constant* 

*Note: ten Berge et al. [1986] determined the relationship shown above based on experimental 

data. The constant "n" was determined by ten Berge et al. to be less than 3.0 for 18 of the 20 

substances studied. Although the individual "n" values determined by ten Berge et al. [1986] 

were utilized when applicable during the review and revision of the original IDLHs, as a 

conservative estimate, an "n" = 3.0 was assumed when "adjusting" the LC data to 30 minutes 

for all other substances.

This equation with an "n" = 3.0 results in the following correction factors:

 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 

The LC values (after "adjusting" if necessary to 30 minutes) were divided by a safety factor of 

10 to determine a "preliminary" IDLH for comparison purposes.

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C.Animal lethal dose (LD) data were considered next. As was the case with the lethal 

concentration data, the only animal lethal dose data used involved mammals; the vast majority 

of the data were from studies of rats, mice, guinea pigs, and hamsters. It was decided to 

generally use the lowest LD data with oral LD50 data preferred. The LD data was used to 

determine the equivalent total dose to a 70-kg worker and, as was done during the SCP, the air 

concentration containing this dose was determined by dividing by 10 cubic meters. [Note: A 

worker breathing at a rate of 50 liters per minute for 30 minutes would inhale 1.5 cubic meters 

of air.] A "preliminary" IDLH for comparison purposes was determined by dividing these air 

concentrations by a safety factor of 10. D. Chronic toxicity data were considered if no relevant 

acute toxicity data existed. However, the fact that chronic exposures may have limited 

relevance to acute effects was taken into consideration.

E. When relevant toxicity data applying specifically to the chemicals in question were lacking, 

and if it was determined to be justified, then analogies to substances with similar acute toxic 

effects were considered. F. All "preliminary" IDLHs derived during this update were checked 

against the following factors prior to establishing the final "revised" IDLH:

1. Lower explosive limit (LEL): It was decided to restrict the "routine" entry into a possible 

explosive atmosphere to concentrations no greater than 10% of the LEL. [Note: SCPderived IDLHs were set at 100% of the LELs if there were no known serious health 

hazards below these values. However, OSHA considers concentrations in excess of 10% of 

the LEL to be a hazardous atmosphere in confined spaces [29 CFR 1910.146(b)].]

2. RD50 data: An RD50 is defined as the 10-minute exposure concentration producing a 

50% respiratory rate decrease in mice or rats and can be used to estimate severe 

respiratory irritation. Prolonged exposure to an RD50 concentration has been shown to 

produce respiratory tract lesions consistent with irritation [Alarie 1981; Buckley et al. 

1984].

3. Other short-term exposure guidelines such as the American Industrial Hygiene 

Association's emergency response planning guidelines (ERPGs) and the National 

Research Council's emergency exposure guidance levels (EEGLs) and short-term public 

emergency guidance levels (SPEGLs), and occupational exposure standards or 

recommendations such as OSHA PELs, NIOSH RELs, or the American Conference of 

Governmental Industrial Hygienists (ACGIH) TLVs.

4. Based on the NIOSH respirator decision logic, the revised IDLH values could not be 

greater than 2,000 times the NIOSH REL (or OSHA PEL).

5. The revised IDLH values would not be greater than the original IDLH values derived 

during the SCP.

Anyone who is aware of additional published data that may affect the IDLH values 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 IDLH 

values.

References

Alarie Y [1981]. Dose-response analysis in animal studies: prediction of human responses. 

Environ Health Persp 42:9-13.

Buckley LA, Jiang XZ, James RA, Morgan KT, Barrow CS [1984]. Respiratory tract lesions 

induced by sensory irritants at the RD50 concentration. Toxicol Appl Pharacol 74(3):417-429.

NIOSH/OSHA [1981]. Occupational health guidelines for chemical hazards. Cincinnati, OH: 

U.S. Department of Health and Human Services, Public Health Service, Centers for Disease 

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Page last reviewed: December 4, 2014

Page last updated: December 4, 2014

Content source: National Institute for Occupational Safety and Health Education and Information Division

Centers for Disease Control and Prevention 1600 Clifton Rd. Atlanta, GA 

30329-4027, USA

800-CDC-INFO (800-232-4636) TTY: (888) 232-6348 - Contact CDC-INFO

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No. 81-123 (NTIS Publication No. PB-83-154609).

NIOSH [1989]. Current Intelligence Bulletin 52. Ethylene oxide sterilizers in health care 

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Occupational Safety and Health, DHHS (NIOSH) Publication No. 89-115 (NTIS Publication 

No. PB-90-142571).

NIOSH [2004]. NIOSH respirator selection 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. 2005-100.

NIOSH [2005]. NIOSH Pocket Guide to 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. 2005-149.

ten Berge WF, Zwart A, Appleman LM [1986]. Concentration-time mortality response 

relationship of irritant and systematically acting vapours and gases. J Haz Mat 13:301-309.

Yant WP [1944]. 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.

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