Immediately Dangerous To Life or Health (IDLH) Values

Criteria for Determining IDLH Values

Since the development of the original IDLH values in the 1970s and their subsequent revision in 1994, NIOSH has continued to review relevant scientific data and conduct research on methods for develop­ing IDLH values. This has led to the development of Current Intelligence Bulletin (CIB) 66: Derivation of Immediately Dangerous to Life or Health (IDLH) [NIOSH 2014-100].

1994 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).

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.

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: SCP-derived 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.

Page last reviewed: March 2, 2017 (archived document)