Methyl mercaptan is a flammable, toxic, reactive, irritating gas with a strong, disagreeable odor (similar to that of garlic or rotten cabbage) that is detectable at extremely low concentrations. Common uses of methyl mercaptan include in the manufacturing of additives for jet fuels, catalysts, dyes, fungicides, pesticides, pharmaceuticals, plastics, and other chemicals. The name methyl mercaptan is slowly being replaced by its proper chemical name methanethiol, although the U.S. Department of Transportation (DOT) and the International Maritime Organization (IMO) still are using the shipping names methyl mercaptan and methylmercaptan, respectively. “Thiol” is the newer designation for all chemicals previously called mercaptans.


Methyl mercaptan is a flammable gas with a relatively wide explosive (flammable) range of 3.9 to 21.8 percent in air and an ignition temperature that has not been determined. It has the following properties: 42.4°F boiling point, -18°F freezing point, 0.87 specific gravity (liquid), 48 molecular weight, and 1.66 vapor density. It may react with warm or hot water. Its chemical formula is CH3SH.


Methyl mercaptan`s main hazard is that it is a flammable gas. Its relatively wide explosive (flammable) range means that once the gas has reached a concentration of 3.9 percent (its lower explosive limit), it will be almost impossible to find the gas in a “too rich” condition–that is, the upper explosive limit is so high (21.8 percent) that it may be impossible to get a concentration above that and therefore it may be impossible to ignite. Its vapor density of 1.66 means that the vapors will settle to the ground in any available low spots and, if undisturbed by breeze or wind, will “hang together” and flow along the ground, seeking the lowest terrain. It will disperse on its own slowly, but there is always the danger that it will flow until it finds an ignition source. When this occurs, the vapors (if within the explosive range) will ignite and flash back to the source of origin, igniting the vapors from a leaking container or a pool of liquid.

Although the literature lists no officially recognized ignition temperature, it certainly will be within the temperature range reachable by all common ignition sources.

Another hazard of methyl mercaptan is that it reacts vigorously with many chemicals including azo compounds, copper, diazo compounds, halogenated hydrocarbons, hydrazines, inorganic acids, isocyanates, ketones, lead, mercury, the metallic elements of Groups IA and IIA (the alkali and alkaline earth metals), nitrides, all strong oxidizers, all strong reducing agents, unsaturated hydrocarbons and their derivatives, and zinc.

Methyl mercaptan is toxic even at low concentrations. Its TLV-TWA (threshold limit value-time weighted average) is 0.5 ppm (parts per million of air). The TLV-TWA is the amount of material to which a person may be exposed for eight hours a day, five days a week (the normal American workweek) with no harmful effects. Time weighted average means that the concentration of the material in question may be higher than the TLV for certain periods of time but that concentration will be calculated into the total exposure so that the average concentration over the eight hours does not exceed the TLV. This does not mean that a worker cannot work in an atmosphere where the concentration of methyl mercaptan is higher than 0.5 ppm. What it means is that the worker must be provided protection if exposed to concentrations higher than the TLV-TWA. The ceiling limit set by the Occupational Safety and Health Administration (OSHA) is 10 ppm, meaning that for any 15-minute period, a worker in an unprotected state may not be exposed to more than 10 ppm of the product. This 10 ppm for 15 minutes must be averaged into the total exposure for the eight-hour period.

Inhaling methyl mercaptan vapors will cause respiratory problems, usually in the form of respiratory paralysis or pulmonary edema. The symptoms of inhalation are breathing difficulties, cyanosis, depression of the central nervous system, increased respiration, muscular weakness, narcosis, convulsions, and death. Nonfatal levels of methyl mercaptan can cause liver and kidney damage.

The odor of methyl mercaptan is so strong that some people can can detect it at levels as low as 0.01 ppb (parts per billion of air). Since methyl mercaptan contains sulfur within its molecules, the final combustion products evolved by its burning will include carbon, carbon monoxide, carbon dioxide, water, and sulfur dioxide.

Methyl mercaptan usually is shipped and stored in its liquefied form. The liquid is highly volatile because it is a liquefied gas. (Volatility means that the gas in its liquefied state has a high vapor pressure and therefore the liquid will evaporate rapidly and produce high amounts of vapor in a very short time.) The implication of volatility is that the liquid, once accidentally released, will flow more rapidly than water and will float on top of it and evaporate considerably faster as it is warmed by the water.

Methyl mercaptan is considered stable in normal transportation. Shippers protect it so that water or moisture in the air does not reach it. The substance reacts vigorously and dangerously with epoxides, copper, lead, mercury, and zinc, which, of course, should not be found in containers used to transport and store the chemical.


A release of any appreciable quantity of methyl mercaptan is dangerous because of the various hazards the material presents and the different manners in which these hazards will threaten the community and its inhabitants and visitors. A release should trigger the community`s emergency response plan, which will provide the incident commander with all the resources needed to mitigate the emergency.

Methyl mercaptan leaking from a container should be contained and recovered or covered as soon as possible. Because of its high mobility, the liquid will move rapidly and also rapidly percolate into the soil. Its high volatility means a tremendous amount of vapors that may react with the moisture in the air will be generated. Pools of methyl mercaptan, therefore, will generate large amounts of flammable and toxic gas. For this reason, consider downwind evacuation from the immediate spill area at the beginning of the incident.

Since methyl mercaptan is such a volatile material and the flammable vapors may explode if an ignition source is reached, approach all releases from upwind and uphill. The material`s toxicity is another reason for being cautious when approaching the incident site.

Released product may be contained in a pond constructed by pushing up earth or sand around the released material. If the proper equipment is available, you can dig a containment pit, but removing contaminated soil may be more difficult when a pit is used instead of a pond. Since methyl mercaptan is really a gas in liquid form, the boiling and rapid evaporation of the liquid may make it difficult to carry out these procedures. Needless to say, only sparkproof tools and equipment should be used, and those tools and equipment must not contain any metal that will react with methyl mercaptan.

If it is determined that the methyl mercaptan contained can be covered, use a plastic sheet or sheets made from some other impervious material. The covering will slow evaporation and protect the methyl mercaptan from moisture. The product may be siphoned from the containment pond or pit into secure containers, again using tools and equipment compatible with methyl mercaptan. Only professional members of a salvage firm should perform salvage activity on a hazardous material; they have been properly educated, equipped, and trained in such activities.

If the product cannot be covered, use a water spray or high-pressure fog to sweep the methyl mercaptan gas. Be careful not to allow the water to contact the released liquid, since it will speed up the evaporation procedure. The water fog or spray will help somewhat in controlling the movement of the gas. However, it will be difficult to control the runoff if it begins to rain.

Prevent methyl mercaptan from entering the sewer system or waterways. Catch basins and entries to waterways may be dammed to divert liquid approaching them. Any of the liquefied gas flowing into a sewer system will boil away, releasing such large amounts of gas that a great amount of the sewer system will be filled with it. Therefore, an explosion will be possible at any point in the system where the gas may be forced out into the atmosphere. Since methyl mercaptan`s upper explosive limit is so high, the explosion may involve all of the gas spread throughout the system.

Accumulated liquid pools may be recovered with appropriate compatible hoses, pumps, and storage containers or vacuum trucks. Any remaining spilled liquid may be controlled by absorbing it with cement powder, clay, earth, fly ash, sand, commercial sorbents, or other compatible substances. Once used, sorbent materials pose the same hazards as the spilled product and must be handled in the same safe manner as the pure product. Their use adds to the overall volume of contaminated material. All contaminated soil and sorbents must be removed and disposed of in accordance with all federal, state, and local regulations.

If methyl mercaptan enters a pond or lake, it could be fatal to aquatic life near the point of entry. Some product will dissolve in the water and slowly spread away from the point of entry; the rest of it will boil and produce great quantities of flammable and toxic gas. If the entry is into a river or stream, the contamination will be carried away and mixed throughout the water at a rate proportional to the speed of the moving water. If the volume of moving water is great, the material will dissolve faster and be diluted at a faster rate. Notify immediately all possible downstream users of the water. The environmental authorities will have to determine the extent of contamination and when the water is safe for use. Caution is needed all along the waterway, since methyl mercaptan is heavier than air and may be contained within the waterway`s banks, particularly if the banks are steep.


If any type of methyl mercaptan container is leaking and the gas is burning, the flame must not be extinguished unless the flow of gas can be stopped immediately. The proper way to fight any flammable gas fire is to stop the flow of gas. If the leaking container is isolated from people and property, no system is threatened, and the environment is not being harmed, the best tactic may be to allow the fire to continue to burn until the product is used up or the flow can be safely stopped. Extinguishing the flame of burning methyl mercaptan as it escapes from a broken pipeline can cause flammable gas to build up and possibly result in a devastating explosion.

If the container releasing liquefied methyl mercaptan has ignited, the same rule applies: Stop the flow of fuel before the flame is extinguished and allow the remaining released gas to burn off if it poses no danger to anyone and does not threaten to cause the leaking container to BLEVE (boiling-liquid, expanding-vapor explosion).

Since all containers of liquefied gas contain liquids stored above their boiling points, tremendous amounts of fuel will be ready to enter the atmosphere and explode with devastating results on ignition. Therefore, apply water to the container holding the liquefied gas to keep the contents cool and the pressure below the design strength of the tank, although the pressure-relief valve might be operating. Be aware that water applied near the leak will freeze almost instantly, and the leak may be plugged with ice. This may or may not be desirable, depending on the amount of pressure rise of the gas within the container. Water applied to any pool of liquefied methyl mercaptan will warm up the water rapidly (as the water freezes), causing rapid evolution of explosive gas.

Whenever methyl mercaptan burns, sulfur dioxide gas will be formed as a final decomposition product. This gas is toxic and has a powerful, choking odor. It is also somewhat corrosive since it forms sulfurous acid when it dissolves in water.

The pressure-relief device on a container of methyl mercaptan will be operating from the first time the container is heated, and vapors will be rushing into the atmosphere. Water from firefighters` hoses may react with that gas. Experts who have responded to the incident will have to advise the incident commander with regard to the safest mitigation techniques.

Protect containers of methyl mercaptan from the heat of a fire by applying water from unmanned monitors positioned as far away as possible. Never approach a container of liquid being heated by a fire. Methyl mercaptan is especially dangerous because it is so volatile–as the product is heated, more vapors are produced than would be the case with “ordinary” liquids. The internal pressure of a container of methyl mercaptan may rise faster than its pressure-relief device can relieve, and the container will fail catastrophically.


Choose protective clothing and equipment that will prevent any possibility of skin or eye contact with methyl mercaptan or its vapors. Rubber boots, gloves, face shields, splashproof safety goggles, and other impervious and chemical-resistant clothing may be needed. Total encapsulating suits with self-contained breathing apparatus are necessary to prevent contact with the vapors. An SCBA with full face piece (or the equivalent) must be worn when the incident involves a release of methyl mercaptan.

As always, consult with the manufacturers of total encapsulating suits and methyl mercaptan for their recommendations of material. Remember, protection is a relative term. A material in the protective suit may be used because it resists a particular chemical in the laboratory, but that same material used in a total encapsulating suit may fail because it was thinner than that used in the lab tests. Also, seam integrity, zipper coverings, the concentration of the chemical involved, and the exposure time will determine the degree of protection needed.


Inhalation. Move the victim to fresh air and keep him warm and at rest. If breathing stops or becomes difficult, administer artificial respiration. (Avoid mouth-to-mouth contact, since it could expose the first-aid giver to the chemical in the victim`s lungs or vomit.) Get medical attention immediately.

Eye contact. Immediately flush the contaminated eye(s) with lukewarm, gently flowing water for 20 minutes, holding the eyelid(s) open. Take care not to rinse contaminated water in the nonaffected eye. If irritation persists, repeat flushing. Obtain immediate medical attention. Frostbite might have occurred because of the temperature of the gas or liquid.

Skin contact. Carefully remove all contaminated clothing at once. Wash all affected body areas with large amounts of water. Get medical attention immediately.

Ingestion. If the victim is conscious, administer large quantities of water or milk immediately while attempting to induce vomiting. Never attempt to force an unconscious person to drink anything or to vomit. Get immediate medical attention. n




methyl sulfhydrate


thiomethyl alcohol



(Chemical Abstract Services)



(Standard Transportation Commodity Code)



(Registry of Toxic Effects of Chemical Substances)



(United Nations/North America)



(Chemical Hazard Response Information System)



(U.S. Department of Transportation)

flammable gas

NFPA 704 Rating

(National Fire Protection Association)



(International Maritime Organization)

2.1, flammable gas

FRANK L. FIRE is the vice president of marketing for Americhem Inc. in Cuyahoga Falls, Ohio. He`s an instructor of hazardous-materials chemistry at the University of Akron as well as an adjunct instructor of haz mats at the National Fire Academy. Fire is the author of The Common Sense Approach to Hazardous Materials and an accompanying study guide, Combustibility of Plastics, and Chemical Data Notebook: A User`s Manual, published by Fire Engineering Books. He is an editorial advisory board member of Fire Engineering.

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