Methyl iodide is a federal hazardous air pollutant and was identified as a toxic air contaminant in April 1993 under AB 2728.

CAS Registry Number: 74-88-4

Structure: CH3I

Molecular Formula: CH3I


Methyl iodide is a colorless liquid which turns brown on exposure to light. It has a sweet ethereal odor. It is soluble in alcohol, carbon tetrachloride, and ether, and partially soluble in water. Methyl iodide is nonflammable (HSDB, 1991; Merck, 1983; Sax, 1989).

Physical Properties of Methyl Iodide

Molecular Weight141.94
Boiling Point42.5 oC
Melting Point-66.5 oC
Vapor Density4.9 (air = 1)
Density/Specific Gravity2.28 at 20/4 oC (water = 1)
Vapor Pressure405.9 mm Hg at 25 oC
Log Octanol/Water Partition Coefficient1.51
Water Solubility1,389 mg/L at 25 oC
Conversion Factor1 ppm = 5.81 mg/m3

(Howard, 1990; HSDB, 1991; Merck, 1983; Sax, 1989; U.S. EPA, 1994a)


A. Sources

Methyl iodide is used as a methylating agent, an alkylating agent, in microscopy, as imbedding material for examining diatoms, in testing for pyridine, as a chemical intermediate, and as a fire extinguisher. It has been detected in the exhaust gases of nuclear reactors (HSDB, 1991).

B. Emissions

No emissions of methyl iodide from stationary sources in California were reported, based on data obtained from the Air Toxics "Hot Spots" Program (AB 2588) (ARB, 1997b).

C. Natural Occurrence

Methyl iodide occurs naturally in the ocean as a product of marine algae, with an estimated production of 44 million tons per year (HSDB, 1991).


No Air Resources Board data exist for ambient measurements of methyl iodide. However, the United States Environmental Protection Agency (U.S. EPA) has compiled ambient air data from several urban and suburban locations throughout the United States from 1972-85. From these data, the U.S. EPA has calculated a mean ambient air concentration of 0.12 micrograms per cubic meter or 0.02 parts per billion (U.S. EPA, 1993a).


No information about the indoor sources and concentrations of methyl iodide was found in the readily-available literature.


In the troposphere, methyl iodide will photolyze and react with the hydroxyl (OH) radical. The calculated half-life and lifetime of methyl iodide due to reaction with the OH radical are about 140 days and 200 days, respectively. Methyl iodide absorbs solar radiation out to 335 nanometers, and photolysis should dominate as a tropospheric loss process, with a lifetime of the order of about one day (Atkinson, 1995).


Methyl iodide emissions are not reported from stationary sources in California under the AB 2588 program. It is also not listed in the California Air Pollution Control Officers Association Air Toxics "Hot Spots" Program Revised 1992 Risk Assessment Guidelines as having health values (cancer or non-cancer) for use in risk assessments (CAPCOA, 1993).


Probable routes of human exposure to methyl iodide are inhalation, ingestion, and dermal contact (Howard, 1990).

Non-Cancer: Exposure to methyl iodide in air may cause skin blistering, severe eye and respiratory tract irritation, and pulmonary edema. Methyl iodide is neurotoxic. Symptoms include nausea, vomiting, vertigo, ataxia, slurred speech, drowsiness, convulsions, and coma. Central nervous system symptoms may last for weeks. Methyl iodide is hepatotoxic and a highly reactive alkylating agent (U.S. EPA, 1994a).

The U.S. EPA has not established a Reference Concentration (RfC) or an oral Reference Dose (RfD) for methyl iodide (U.S. EPA, 1994a).

No information is available on adverse developmental or reproductive effects in humans or animals from methyl iodide. (U.S. EPA, 1994a).

Cancer: There are no adequate data on the carcinogenicity of methyl iodide in humans. Rats and mice exposed to methyl iodide developed lung tumors. The U.S. EPA has classified methyl iodide as Group C: Possible human carcinogen (U.S. EPA, 1994a). The International Agency for Research on Cancer has classified methyl iodide as Group 3: Not classifiable (IARC, 1987a). The State of California has determined under Proposition 65 that methyl iodide is a carcinogen (CCR, 1996).