Methyl mercaptan (CH3SH), also known as methanethiol, is a colourless, highly flammable, toxic gas with a distinct, strong unpleasant odour. Microbial degradation of organic matter releases methyl mercaptan. Prolonged exposure of the CH3SH can quickly deaden a person’s sense of smell, making the odour of methyl mercaptan an unreliable indicator of its presence. Hence, other means such as the use of methyl mercaptan monitors is a viable solution to provide adequate warning of hazardous exposure. This article covers information on methyl mercaptan, its sources in the ambient air, permissible levels, health and environmental impact, possible corrective measures, need for methyl mercaptan monitors as well as different methods of CH3SH monitoring.
What is Methyl Mercaptan?
Methyl mercaptan (CH3SH), also known as methanethiol, is a colorless, highly flammable, toxic gas with a distinct, strong unpleasant odour. Its odour can be described as that of rotten cabbage or garlic. According to Central Pollution Control Board’s (CPCB) report, the odour of methyl mercaptan can be detected at a concentration of 0.0005 ppm. Most humans can detect methyl mercaptan odor at concentrations 500 times lower than the accepted safe exposure limit.
Table : Odour Intensity of Methyl Mercaptan
|3||Median, easily noticeable||7.9|
Methyl mercaptan is an organosulfur volatile organic compound (VOC). It can be easily ignited and decomposed, emitting highly toxic fumes and flammable vapours. It is heavier than air and may collect in low-lying areas.
Methyl mercaptan is naturally released in the atmosphere from microbial degradation of organic matter. It volatilizes naturally from soil and water. Once released in the atmosphere, it undergoes photooxidation. During daytime, methyl mercaptan is oxidized by OH radicals while during nighttime, it reacts with NO3 molecules in the atmosphere. As a result of these reactions, the effective lifetime of CH3SH in the atmosphere is about 1 to 26 hours.
Methyl mercaptan is naturally produced from microbial degradation of organic matter such as vegetation and animal wastes. It is present in the marshes, dump sites, underground gas pockets, etc.
Methyl mercaptan is present in natural gas, coal tar and some crude oil. Industrials sources of methyl mercaptan include manufacturing and use of methionine (i.e. poultry feed), pesticides, jet fuels, oil shale, plastics etc. Other sources include sewage treatment plants, petroleum refining, starch manufacturing, wood-pulp mill, rendering plants, pharmaceutical industry, etc.
Permissible exposure limits for Methyl Mercaptan
The permissible exposure limits of CH3SH in terms of continuous occupational exposure are given below:
|OSHA (PEL)||For general industry: 10 ppm Ceiling |
For the construction industry: 0.5 ppm
TWAFor maritime : 0.5 ppm TWA
|ACGIH (TLV)||0.5 ppm TWA|
|NIOSH (REL)||0.5 ppm Ceiling|
|NIOSH (IDHL)||150 ppm|
Permissible Exposure Limit (PEL) given by OSHA (Occupational Safety and Health Administration) defines the maximum concentration of CH3SH to which an unprotected worker may be exposed to. PEL may reference an eight-hour time-weighted average (TWA), a 15-minute short term exposure limit (STEL) or an instantaneous ceiling limit (CL) concentration that cannot be exceeded for any period of time. Similarly, Recommended Exposure Limit (REL) and Immediately Dangerous to Life of Health (IDLH) are the occupational exposure limit recommended by NIOSH (National Institute for Occupational Safety and Health) and the TLVs (i.e. threshold limit values) are the exposure guidelines given by ACGIH (American Conference of Governmental Industrial Hygienists)
The AEGL (Acute Exposure Guideline Levels) values by EPA (Environmental Protection Agency) of methyl mercaptan are given below. They represent threshold exposure limits (exposure levels below which adverse health effects are not likely to occur) for the general public and are applicable to emergency exposures ranging from 10 minutes (min) to 8 h.
Table: AEGL Values for methyl mercaptan (ppm)
|Classification||10 min||30 min||1 h||4 h||8 h|
Health & Environmental Impact of Methyl Mercaptan
Methyl mercaptan exposure causes irritation to moist tissues such as eyes, skin and respiratory tract. High-level exposure can cause headaches, nausea, vomiting, dizziness, tremors, seizures, and lack of coordination. It gets rapidly absorbed by lungs leading to respiratory distress. It depresses the central nervous system. Exposure to high levels of methyl mercaptan can also damage liver and kidneys. It affects the red blood cells causing anaemia. Its prolonged high exposure can lead to more serious health complications such as respiratory paralysis leading to coma and death.
Possible corrective measures
The primary action is methyl mercaptan monitoring i.e. to measure how much of CH3SH concentrations you are exposed to. In addition to this following corrective measures can be taken:
- Avoid going to or staying in low lying areas where methyl mercaptan is produced such as marshes, dump sites, etc.
- Avoid staying upwind of wastewater treatment plants, sewage treatment plants, landfills, and industrial areas of starch manufacturing, wood-pulp mill, etc.
- Always use appropriate personal protective equipment (PPE) while using CH3SH containing products or while going to places where presence of high levels of CH3SH is suspected.
- Also, if the presence of high levels of CH3SH is detected, immediately vacant the area and provide proper ventilation to remove the gas.
Measurement methods of Methyl Mercaptan monitoring
Different working principles for methyl mercaptan monitoring in the ambient environment are chemiluminescence, semiconductor, and electrochemistry.
This method of methyl mercaptan monitoring is based on the chemiluminescent reaction between methyl mercaptan (CH3SH) and ozone (O3). In the methyl mercaptan monitor based on this principle, the CH3SH molecules present in the air sample react with ozone (produced using an ozone generator) to generate activated SO2. During the conversion, light energy at a specific wavelength is produced which is proportional to the amount of methyl mercaptan present in the air sample. The light intensity is measured photometrically and converted to the CH3SH concentration.
When a metal oxide semiconductor-based methyl mercaptan monitor is exposed to air sample, the CH3SH molecules react on the metal oxide surface of the sensor and dissociate into charged ions which alter the resistance of the film. Further, this interaction is measured as a signal and is converted to the gas concentration. However, the energy consumption of such methyl mercaptan monitors is higher compared to others.
CH3SH monitors working on the electrochemical principle are operated based on diffusion of methyl mercaptan gas into the sensor which results in the production of electrical signals proportional to the CH3 SH concentration. It allows accurate measurement of even low concentrations of CH3SH, which is essential in methyl mercaptan monitoring in the ambient air.
Among all the above principles of methyl mercaptan monitoring, applications like ambient air monitoring prefer CH3SH monitors based on electrochemistry are as they yield more accurate CH3SH concentrations and are inexpensive in comparison with the others.
Oizom’s working principle for CH3SH monitoring
Oizom’s ODOSENSE is the real-time odour emission tracking solution. It continuously detects, measures, and monitors the odourful gaseous contaminants including hydrogen sulfide, ammonia, sulfur dioxide, methyl mercaptan, TVOC, formaldehyde, methane, and weather parameters like temperature, humidity, wind speed, and wind direction. The sensor that measures CH3SH works on the principle of electrochemical sensing. With the help of meteorological data, Odosense can trace the odourant dispersion plume incited by conditions like wind speed and wind direction. Odosense is a proactive approach to measure real-time odour emissions. This makes it an ideal choice for landfill sites, wastewater treatment facilities, fertilizers, paper-pulp industries, and soil-treatment sites, etc.
Reasons why Methyl Mercaptan monitoring is important
- Methyl mercaptan is a colorless, highly flammable, toxic gas with a distinct, strong unpleasant odour released from microbial degradation of organic matter.
- It is an organosulfur volatile organic compound (VOC) that can be easily ignited and decomposed, emitting highly toxic fumes and flammable vapours.
- Exposure to methyl mercaptan causes irritation to eyes, skin as well as respiratory tract, respiratory distress, damages liver and kidneys, respiratory paralysis, leading to coma and death.
- Prolonged exposure of the CH3SH can quickly deaden a person’s sense of smell, making the odor of methyl mercaptan an unreliable indicator of its presence. Hence, other means such as use of methyl mercaptan monitors is a viable solution to provide adequate warning of hazardous exposure.
- Real-time monitoring of CH3SH levels helps in determining their source as well as formulating an action plan to control methyl mercaptan emissions.