## Immediately Dangerous To Life or Health (IDLH)

May 1994

Immediately Dangerous to Life or Health Concentrations (IDLH)



### Introduction
The immediately dangerous to lifeor health (IDLH) values usedby the National Institute for Occupational Safety and Health (N IOSH) as respirator selection cr iteria were first developed in the mid-1970's. The Documentation for Immediat ely Dangerous to Life or Health (IDLH) Concentrations is a compilation of the ration ale and sources of information used by NIOSH during the original determination of 387 ID LH values. In addition , NIOSH continues to review, document, and revise th e science and methodol ogy behind the exis ting IDLH values when appropriate, and derive new IDLH values. 

### Background
The concept of using respirators to protect workers in situation s that are IDLH was discussed at least as early as the 1940's. The following is from a U.S. Departme nt of Labor bulletin:
The situations for which respir atory protection is requiredmay 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 discom fort, sickness, permanent harm, or death after a prolonged exposure or with repeated exposure. Emergency situations are those th at involve actual or potential exposure to atmospheres that are i mmediately harmful and dangerous to health or life after comparatively sh ort exposures. [Yant 1944]
The Occupational Safety and He alth Administration (OSHA) de fines an IDLH value in their hazardous waste operations and emerge ncy response regulation as follows:

An atmos p heric concentration of any toxic, corrosive or as p hy xiant substance that p oses an immediate threat to life or woul d cause irreversible or delayed adverse health effects or would interfere with an individual's ability to es cape from a dangerous atmosphere. [29 CFR* 1910.120]
In the OSHA regulation on permit-required confi ned spaces, an IDLH co ndition is defined as follows:

Any condition that p oses an immediate or delay ed threat to life or thatwould cause irreversible adverse health effects or that would interfere wi th an individual's abil ity to escape unaided from a permit space. Note: So me 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 follo wed by sudden, possibly fata l collapse 12-7 2 hours after exposure. The victim "feels normal" from recover y from transient effects until collapse. Such materials in hazardous quantiti es are considered to be "immedi ately dangerous to life or health." [29 CFR 1910.146] Page 1 of 7 CDC - Immediately Dangerous to Life or He alth Concentrations (IDLH): Introduction - N...

8 / 7 / 201 5 http://www.cdc.gov/nios h/idlh/idlhintr.htm lAs p art of their current res p irator y p rotection standard [29 CFR 1910.134(e)], OSHA re quires that a standby person be present with suit able rescue equipment when self-contained breathing apparatus or hose masks with bl owers are used in atmospheres immediately dangerous to life or health. Furthermore, pers ons 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 wo rkers 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 Heal th Act of 1970 for substances with then-existing OSHA permissible exposure limits (PEL s). This joint effo rt was called the Standards Completion Program (SCP) and involved the cooperative effort s of personnel from various divisions within NIOS H and OSHA, and several contractors. The SCP developed 387 substance-specific draft standards with suppor ting documentation that contained technical information and recommendations needed for th e promulgation of new occupational health regulations. Although new standa rds were not promulgated at thattime, these data became the original basis for the NIOSH/OSHA Occupational Health Guidelines for Chemical Hazards [NIOSH/OSHA 1981].

As p art of the res p irator selection p rocess 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 stipulatedin 30 CFR 11.3(t). The purpose for es tablishing 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 prot ection equipment failure (e.g., contaminant breakthrough in a cartridge respiratoror stoppage of air fl ow in a supplied-air respirator) and a concentration above which on ly "highly reliable" respirators would be required. In determining IDLH values, the ability of a worker to escape wi thout 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. A

lthou g h in most cases, e g ress from a p articular worksite could occu r in much less than 30 minutes, as a safety ma rgin, IDLH values were based on the effects that might occur as a consequence of a 30-minute ex posure. However, th e 30-minute period was NOT meant to imply that workers should stay in the work en vironment any longer th an necessary following the failure of respiratory prot ection 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 availabl e at the time. Whenever possible, IDLH values were determined using health e ffects data from studies of huma ns exposed for short durations. However, in most instances, a la ck of human data necessitated th e 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 he alth effects in any species. When le thal dose (LD) data from animals were used, IDLH values were es timated on the basis of an equivalent exposure to a 70-kg worker breathing 10 cubic meters of air.
Since chronic exposure data may ha ve little relevance to acute effe cts, 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 we re available, IDLH values were based on analogies with other substances with similar toxic effects. Page 2 of 7 CDC - Immediately Dangerous to Life or He alth Concentrations (IDLH): Introduction - N...

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### Discussion of the Original IDLH Values
The basis for each of the 387 IDLH values de termined during the SCP were reviewed and paraphrased from the individual dr aft technical standards for this publication. Also included is a complete listing of referenc es 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 primar y) were obtained to verify th e information cited in the SCP. However, a few of the original references, such as person al communications and foreign reports, could not be located.

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lthou g h 387 substances were ori g inally included in the SCP, IDLH values were not specifically determined for all of them. The published data atthat time for 40 of these substances (e.g., DDT and tripheny l phosphate) showed no evidence that an acute exposure to high concentrations would impede escape or caus e any irreversible heal th 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 se lected 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 th e concentration above which only the "most protective" respirators were permitted. Howeve r, 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 en countered in the occupati onal environment. In addition, exposure concentrati ons greater than 500 times the PEL for many airborne particulates could result in exposures that woul d 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 dete rmined during the SCP for 22 su bstances (e.g., bromoform and calcium oxide) because of a lack of relevant toxicity data and therefore the designation "UNKNOWN" was used in the ID LH value listing. For mostof these substances, the concentrations above which only the "most prot ective" respirators were allowed were based arbitrarily on assigned protection factors th at 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 LE L 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 dete rmined during the SCP were greater than the concentrations permitted based on assi gned respiratory protection factors. In most instances th e IDLHs for these substances we re set at concentrations 2,000 times the PEL.



### Current NIOSH Use of IDLH Values

The current NIOSH definition for an IDLH conditi on, as given in th e 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 adve rse health effects or prevent escape from such an environment [NIO SH 2004]. The purpose of establishing an IDLH value is (1) to ensure that the worker ca n escape from a given contaminated environment in the event of failure of the respiratory protection equipment and (2) is considered a Page 3 of 7 CDC - Immediately Dangerous to Life or He alth Concentrations (IDLH): Introduction - N...

8 / 7 / 201 5 http://www.cdc.gov/nios h/idlh/idlhintr.htm lmaximum level above which only a hi g hly reliable breathin g a pp aratus p rovidin g maximum worker protection is pe rmitted [NIOSH 2004]. In establishi ng the IDLH value, the following conditions must be assured:

A . The ability to esca p e without loss of life or immediate or delay ed irreversible health effects. (Thirty minutes is conside red the maximum time for escape so as to provide some margin of safety in calculatin g an IDLH value.)
B. The prevention of severe eye or respiratory irritation or ot her reactions that would hinder
escape. The NIOSH respirator deci sion logic uses an IDLH value as one of several respirator selection criteria. Under the NIOSH respirator decision logi c, "highly reliable" respirators (i.e., the most protective respirators) would be selected for em ergency situations, fire fighting, exposure to carcinogens, entry into oxygen-deficient atmosp heres, entry into atmo spheres that contain a substance at a concentration greater than 2,000 times the NIOSH REL orOSHA PEL, and for entry into immediately dangerous to life orhealth conditions. Th ese "highly reliable" respirators include either a self-contained brea thing apparatus (SCBA) th at has a full facepiece and is operated in a pressure-demand or ot her positive-pressure mode, or a supplied-air respirator that has a full facepi ece and is operated in a pressure-demand or other positivepressure mode in combination wi th an auxiliary SCBA operated in a pressure-demand or other positive-pressure mode.

When the IDLH values we re developed in the mid- 1970's, only limited toxicological data were available for many of the subs tances. In 1993, NIOSH requeste d information on the uses of IDLH values in the workplace and on the scient ific adequacy of the criteria and procedures originally used for establishing them [Federal Register, Vo lume 58, Number 229, p. 63379, Wednesday, December 1, 1993]. The information received in response to the Federal Register announcement was evaluated and used to es tablish future actions con cerning IDLH values.

This document includes IDLH values for 85 substances (e.g., benzene and methylene chloride) determined by NIOSH to meet th e OSHA definition of "potenti al occupational carcinogen" as given in 29 CFR 1990.103. For all of these substances, except ethylene oxide and crystalline silica, NIOSH recommends that th e "most protective" respirators be worn by workers exposed at concentrations above the NIOSH REL, or at any detectable concentrati on when there is no REL. For ethylene oxide and crystalline silica , NIOSH recommends that the "most protective" respirators be worn in concentrations exceedin g 5 ppm and 25 mg/m3, respectively [NIOSH 1989, 2005]. 

### Revised Criteria for Determining IDLH Values

The criteria utilized to determin e the adequacy of existing IDLH values were a combination of those used during the SCP and a newer methodol ogy 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 valu e. When relevant acute toxici ty data were insufficient or unavailable, then the use of chro nic toxicity data or an analogy to a chemical wi th similar toxic effects was considered. In order to facilitate the revision proce ss, secondary toxi cological 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-te rm exposure guidelines and lower explosive limits). Page 4 of 7 CDC - Immediately Dangerous to Life or He alth Concentrations (IDLH): Introduction - N...

8 / 7 / 201 5 http://www.cdc.gov/nios h/idlh/idlhintr.htm lThe followin g "hierarch y " was followed to develo p a " p reliminar y " value for the revised IDLH value:

A .Human acute toxicity data were used if s ufficient to determine a con centration that for u p to 30 minutes does not cause death, serious or irreve rsible health effects, or does not impair or impede the ability to escape.

B.Animal acute lethal concentrat ion (LC) data were considered next. The only animal lethal concentration data used involved mammals; the va st majority of the data was from studies of rats, mice, guinea pigs, and hams ters. It was decided to generally use the lowest reliable LC data, with LC50 data preferred. If acute LC da ta determined during a 30-minute period were not available, then the da ta, based on a study by ten Berge et al. [1986], were "adjusted" to an equivalent 30-minute value usin g the following relationship:

A d justed L C 50 (30 minutes ) = LC50 ( t) * ( t /0.5)** (1/n)

where: LC50(t) = LC50 de termined over t hours

n = constant* *Note: ten Berge et al. [1986] determined the relati onship shown above based on experimental data. The constant "n" was determinedby ten Berge et al. to be le ss than 3.0 for 18 of the 20 substances studied. Although th e individual "n" values determin ed by ten Berge et al. [1986] were utilized when applicable du ring the review and revision of the orig inal 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 resu lts in the followin g 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 "adjus ting" if necessary to 30 minutes) we re divided by a safety factor of 10 to determine a "preliminary" IDLH for comparison purposes. Page 5 of 7 CDC - Immediately Dangerous to Life or He alth Concentrations (IDLH): Introduction - N...

8 / 7 / 201 5 http://www.cdc.gov/nios h/idlh/idlhintr.htm lC.Animal lethal dose (LD) data were considered next. As wa s the case with the lethal concentration data, the only anim al lethal dose data used invo lved 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 datawith oral LD50 data preferre d. 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 cubi c meters. [Note: A worker breathing at a rate of 50 liters per minute for 30 minute s would inhale 1.5 cubic meters of air.] A "preliminary" IDLH fo r comparison purposes was dete rmined by dividing these air concentrations by a safety factorof 10. D. Chronic toxicity datawere considered if no relevant acute toxicity data existed. However, the fact that chroni c exposures may have limited relevance to acute effects wa s taken into consideration.
E. When relevant toxicity data applying specifically to the chem icals in question were lacking, and if it was determined to be justified, then analogies to subs tances with simi lar acute toxic effects were considered. F. All "preliminary" IDLHs de rived during this update were checked against the following factor s prior to establishing the final "revised" IDLH:

1. Lower explosive limit (LEL): Itwas decided to restrict the "r outine" entry into a possible explosive atmosphere to conce ntrations no greater than 10 % of the LEL. [Note: SCPderived IDLHs were set at 10 0% of the LELs if there we re no known serious health hazards below these values. However, OSHA con siders 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 lesion s 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 guidan ce levels (EEGLs) an d short-term public emergency guidance levels (SPEGLs), and occupational exposure standards or recommendations such as OSHA PELs, NIOS H RELs, or the American Conference of Governmental Indu strial Hygienists (ACGIH) TLVs. 4. Based on the NIOSH respirator decision lo gic, the revised IDLH values could not be greater than 2,000 times th e NIOSH REL (or OSHA PEL). 5. The revised IDLH values woul d not be greater than the or iginal IDLH values derived during the SCP.

Any one who is aware of additional p ublished data that may affect the IDLH values determined for particular substances is en couraged to make this informatio n available to NI OSH. All data will be reviewed and cons ideration will be made regarding subsequent revision of the IDLH values.



### References

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larie Y [1981]. Dose-res p onse analy sis in animal studies: p rediction of human res p onses. 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 chemic al hazards. Cincinnati, OH: U.S. Department of Health and Human Services , Public Health Servic e, Centers for Disease Page 6 of 7 CDC - Immediately Dangerous to Life or He alth Concentrations (IDLH): Introduction - N...

8 / 7 / 201 5 http://www.cdc.gov/nios h/idlh/idlhintr.htm lPage 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 Preventi on 1600 Clifton Rd. Atlanta, GA 30329-4027, USA 800-CDC-INFO (800-232-4636) TTY: (888) 232-6348 - Contact CDC-INFO Control, National Institute for Occu p ational Safety and Health, DHHS (NIOSH) Publication No. 81-123 (NTIS Public ation No. PB-83-154609).
NIOSH [1989]. Current Intelligence Bulletin 52. Ethylene oxide sterilizers in health care facilities. Engineering controls an d work practices. Cincinnati, OH : U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Co ntrol, National Institute for Occupational Safety and Heal th, 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 Co ntrol, National Institute for Occupational Safety and Health, DHHS (NIOSH). Publication No. 2005-100.

NIOSH [2005]. NIOSH Pocket Guid e to Chemical Hazards. Cinc innati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Sa fety and Health, DHHS (NIOSH ). Publication No. 2005-149.
ten Berge WF, Zwart A, Applem an LM [1986]. Concentratio n-time mortality response relationship of irritant and systematically acting vapours and gases. J Haz Mat 13 :301-309.

Yant WP [1944]. Protectin g workers a g ainst tem p orar y

and emer g enc y

ex p osures. In: Protecting plant manpower thro ugh the control of air contamina nts. Special Bulletin No. 14. Washington, DC: U.S. Department of Labor, Division of Labor Standards. Page 7 of 7 CDC - Immediately Dangerous to Life or He alth Concentrations (IDLH): Introduction - N...

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