Skip directly to search Skip directly to A to Z list Skip directly to navigation Skip directly to page options Skip directly to site content


May 1994
Immediately Dangerous to Life or Health Concentrations (IDLH)

CAS number: 56–38–2

NIOSH REL: 0.05 mg/m3 TWA [skin]

Current OSHA PEL: 0.1 mg/m3 TWA [skin]

1989 OSHA PEL: Same as current PEL

1993-1994 ACGIH TLV: 0.1 mg/m3 TWA [skin]

Description of substance: Pale-yellow to dark-brown liquid with a garlic-like odor.

LEL: . . Unknown

Original (SCP) IDLH: 20 mg/m3

Basis for original (SCP) IDLH: No useful data on acute inhalation toxicity are available on which to base the IDLH for parathion. If the IDLH were estimated from the statement by AIHA [1971] that “the minimum lethal oral dose for humans has been estimated as ranging from less than 10 mg to 120 mg [Bidstrup 1950; Grob 1950; Hayes 1963],” then an IDLH of 5 mg/m3 would be chosen. This appears to be far too conservative, however, because ACGIH [1971] noted that workers regularly exposed to 2 to 15 mg/m3, with an average concentration of 8 mg/m3, exhibited only a 25% decrease in cholinesterase levels [Kay et al. 1952]. The chosen IDLH, therefore, has been estimated from the female rat oral LD50 of 3 mg/kg cited by ACGIH [1971].

Short-term exposure guidelines: None developed


Lethal concentration data:

Species Reference LC50 LCLo Time Adjusted 0.5-hrLC (CF) Derived value
RabbitG. pig



Deichmann et al. 1952Deichmann et al. 1952

Izmerov et al. 1982

USAF 1977



84 mg/m3

50 mg/m314 mg/m3

15 mg/m3


2 hr2 hr


4 hr

80 mg/m322 mg/m3


168 mg/m3

8.0 mg/m32.2 mg/m3


17 mg/m3

Lethal dose data:

Species Reference Route LD50(mg/kg) LDLo(mg/kg) Adjusted LD Derived value




G. pig


Eto et al. 1966Kenaga & Morgan 1978

Kenaga & Morgan 1978

Nishizawa et al. 1961

Perkow 1971/1976

von Dozent et al. 1955

Weiss and Orzel 1967



















35 mg/m370 mg/m3

21 mg/m3

6.5 mg/m3

35 mg/m3

56 mg/m3

14 mg/m3

3.5 mg/m37.0 mg/m3

2.1 mg/m3

0.7 mg/m3

3.5 mg/m3

5.6 mg/m3

1.4 mg/m3

Human data: Workers regularly exposed to 2 to 15 mg/m3 (average of 8 mg/m3) exhibited only a 25% decrease in cholinesterase levels [CDC 1956]. The minimum lethal oral dose has been reported to range from 0.17 to 1.471 mg/kg [Arena 1970; CDC 1956; Hartley and Kidd 1986]. [Note: An oral dose ranging from 0.17 to 1.471 mg/kg is equivalent to a worker being exposed to about 8 to 69 mg/m3 for 30 minutes, assuming a breathing rate of 50 liters per minute and 100% absorption.]

Revised IDLH: 10 mg/m3Basis for revised IDLH: The revised IDLH for parathion is 10 mg/m3 based on chronic inhalation toxicity data in humans [CDC 1956]. This may be a conservative value due to the lack of relevant acute toxicity data in humans exposed to concentrations above 10 mg/m3.


1. ACGIH [1971]. Parathion. In: Documentation of the threshold limit values for substances in workroom air. 3rd ed. Cincinnati, OH: American Conference of Governmental Industrial Hygienists, pp. 195-196.

2. AIHA [1969]. Parathion. In: Hygienic Guide Series. Am Ind Hyg Assoc J 30:308-312.

3. Arena JM [1970]. Poisoning, toxicology, symptoms, treatments. 2nd ed. Springfield, IL: C.C. Thomas 2:73.

4. Bidstrup PL [1950]. Poisoning by organic phosphorus insecticides. Br Med J 2:548-551.

5. CDC [1956]. Clinical memoranda on economic poisons. Atlanta, GA: Communicable Disease Center, Bureau of State Services, Public Health Service, U.S. Department of Health, Education, and Welfare, Public Health Service Publication No. 476, pp. 19-21.

6. Deichmann WB, Pugliese W, Cassidy J [1952]. Effects of dimethyl and diethyl paranitrophenyl thiophosphate on experimental animals. AMA Arch Ind Health Occup Med 5:44-51.

7. Eto M, Kishimoto K, Matsumura K, Ohshita N, Oshima Y [1966]. Studies on saligenin cyclic phosphorus esters with insecticidal activity. Agri Biol Chem 30(2):181-185.

8. Grob D [1950]. Uses and hazards of the organic phosphate anticholinesterase compounds. Ann Intern Med 32:1229-1234.

9. Hartley D, Kidd H, eds. [1986]. Agrochemicals handbook. Nottingham, England: Royal Society of Chemistry. 1983-1986, p. A311.

10. Hayes WJ Jr [1963]. Clinical handbook on economic poisons. Emergency information for treating poisons. Washington, DC: U.S. Government Printing Office, Public Health Service Publication No. 476 (Revised 1963), pp. 35-37.

11. Izmerov NF, Sanotsky IV, Sidorov KK [1982]. Toxicometric parameters of industrial toxic chemicals under single exposure. Moscow, Russia: Centre of International Projects, GKNT, p. 52.

12. Kay K, Monkman L, Windish JP, Doherty T, Pare J, Racicot C [1952]. Parathion exposure and cholinesterase response of Quebec apple growers. AMA Arch Ind Hyg Occup Med 6:252-262.

13. Kenaga EE, Morgan RW [1978]. Commercial and experimental organic insecticides (1978 revision). Entomological Society of America Special Publication 78-1:16.

14. Nishizawa Y, Fujii K, Kadota T, Miyamoto J, Sakamoto H [1961]. Studies on the organophosphorus insecticides. VII. Chemical and biological properties of new low toxic organophosphorus insecticide. O,O-dimethyl-O-(3-methyl-4-nitrophenyl) phosphorothioate. Agri Biol Chem 25(8):605-610.

15. Perkow W [1971/1976]. Wirksubstanzen der pflanzenschutz und schadlingsbekampfungsmittel. Berlin, Germany: Verlag Paul Parey, 1971-1976 (in German).

16. USAF [1977]. Proceedings of the 7th annual conference on environmental toxicology. Wright-Patterson Air Force Base, OH: Air Force Systems Command, Aerospace Medical Division, Aerospace Medical Research Technical Report, AMRL-TR-76-125.

17. von Dozent J, Klimmer OR, Pfaff W [1955]. Vergleichende untersuchungen über die toxicität organischer thiophosphorsäureester. Arzneimittel-Forschung (Drug Research) 5:626-630 (in German).

18. Weiss LR, Orzel RA [1967]. Some comparative toxicologic and pharmacologic effects of dimethyl sulfoxide as a pesticide solvent. Toxicol Appl Pharmacol 11:546-557.