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

CAS number: 2104–64–5

NIOSH REL: 0.5 mg/m3 TWA [skin]

Current OSHA PEL: 0.5 mg/m3 TWA [skin]

1989 OSHA PEL: Same as current PEL

1993-1994 ACGIH TLV: 0.5 mg/m3 TWA [skin]

Description of Substance: Yellow solid with an aromatic odor.

LEL:. . Noncombustible Solid

Original (SCP) IDLH: 50 mg/m3

Basis for original (SCP) IDLH: No data on acute inhalation toxicity are available on which to base the IDLH for EPN. The chosen IDLH, therefore, has been estimated from the female rat oral LD50 of 8 mg/kg [Gaines 1969 cited by NIOSH 1976].

Short-term exposure guidelines: None developed


Lethal dose data:







Adjusted LD

Derived value





AAPCO 1966

Gaines 1969

Gaines 1969

Hodge et al. 1954

Nishizawa et al. 1962
















140 mg/m3

56 mg/m3

252 mg/m3

49 mg/m3

85 mg/m3

14 mg/m3

5.6 mg/m3

25 mg/m3

4.9 mg/m3

8.5 mg/m3

Human data: Ingestion of 3 mg EPN per day for 32 days did not depress plasma or red blood cell (RBC) cholinesterase; 6 mg EPN per day for 88 days did not cause a significant depression of RBC or plasma cholinesterase [Rider et al. 1959]. It has been reported that the threshold of incipient toxicity appears to be 9 mg [Moeller and Rider 1962].

Revised IDLH: 5 mg/m3

Basis for revised IDLH: No inhalation toxicity data are available on which to base an IDLH for EPN. Therefore, the revised IDLH for EPN is 5 mg/m3 based on acute oral toxicity data in humans [Rider et al. 1959] and animals [Gaines 1969; Hodge et al. 1954]. This may be a conservative value due to the lack of relevant acute inhalation toxicity data for workers.



1. AAPCO [1966]. Pesticide chemicals official compendium. Topeka, KS: Association of the American Pesticide Control Officials, Inc., pp. 478-479.

2. Gaines TB [1969]. Acute toxicity of pesticides. Toxicol Appl Pharmacol 14:515-534.

3. Hodge HC, Maynard EA, Hurwitz L, DiStefano V, Downs WL, Jones CK, Blanchet HJ Jr [1954]. Studies of the toxicity and of the enzyme kinetics of ethyl p-nitrophenyl thionobenzene phosphonate (EPN). J Pharmacol Exp Ther 112:29-39.

4. Moeller HC, Rider JA [1962]. Plasma and red blood cell cholinesterase activity as indications of the threshold of incipient toxicity of ethyl-p-nitrophenyl thionobenzenephosphonate (EPN) and malathion in human beings. Toxicol Appl Pharmacol 4:123-130.

5. NIOSH [1976]. TB19250. Phosphonothioic acid, phenyl-, O-ethyl O-(P-nitrophenyl)ester. In: Registry of toxic effects of chemical substances, 1976 ed. Cincinnati, OH: U.S. Department of Health, Education, and Welfare, Public Health Service, Center for Disease Control, National Institute for Occupational Safety and Health, DHEW (NIOSH) Publication No. 76-191, p. 877.

6. Nishizawa Y, Kuramoto S, Kadota T, Miyamoto J, Fujimoto K, Sakamoto H [1962]. Studies on organophosphorus insecticides. Part X. Chemical and biological properties of O,O-dimethyl-O-(4-cyanophenyl) phosphorothioate and O-ethyl-O-(4-cyanophenyl) phenyl phosphonothioate. Agri Biol Chem 26(4):257-264.

7. Rider JA, Moeller HC, Swader J, Devereaux RC [1959]. A study of the anticholinesterase properties of EPN and malathion in human volunteers. Clin Res 7:81-82.

Page last reviewed: December 4, 2014