Methyl mercaptan (CH₃SH), also known as methanethiol, is a colorless, highly flammable, toxic gas with a distinct, strong unpleasant odour. Moreover, microbial degradation of organic matter releases methyl mercaptan. As a result, prolonged exposure to 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 are 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, the 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. Moreover, its odour can be described as that of rotten cabbage or garlic. Above all, 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

Intensity	Description	Concentration (ppm)
0	No odour	0.0030
1	Threshold	0.041
2	Faint	0.57
3	Median, easily noticeable	7.9
4	Strong	110
5	Most Intense	1500

That is to say, methyl mercaptan is an organosulfur volatile organic compound (VOC). Further, it can be easily ignited and decomposed, emitting highly toxic fumes and flammable vapors. It is heavier than air and may collect in low-lying areas. 

However, methyl mercaptan is naturally released in the atmosphere by microbial degradation of organic matter. As a result, it volatilizes naturally from soil and water. Once released in the atmosphere, it undergoes photooxidation. During the 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.  

What are the Sources of Methyl Mercaptan

 

Methyl mercaptan is naturally produced from microbial degradation of organic matter such as vegetation and animal wastes. On the other hand, It is present in marshes, dumpsites, underground gas pockets, etc.

Certainly, methyl mercaptan is present in natural gas, coal tar, and some crude oil. Industrial sources of methyl mercaptan include manufacturing and use of methionine (i.e. poultry feed), pesticides, jet fuels, oil shale, plastics, etc. Subsequently, 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

Firstly, the 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. For instance, 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. Above all, 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
AEGL-1(non-disabling)	–	–	–	–	–
AEGL-2(disabling)	40	29	23	14	7.3
AEGL-3(lethal)	120	86	68	43	22

 

Health & Environmental Impact of Methyl Mercaptan

 

Methyl mercaptan exposure causes irritation to moist tissues such as the eyes, skin, and respiratory tract. Subsequently, high-level exposure can cause headaches, nausea, vomiting, dizziness, tremors, seizures, and lack of coordination. It gets rapidly absorbed by the lungs leading to respiratory distress and it also depresses the central nervous system. Exposure to high levels of methyl mercaptan can also damage the liver and kidneys and affects the red blood cells causing anemia. Its prolonged high exposure can lead to more serious health complications such as respiratory paralysis leading to coma and death. 

How Can I Reduce My Exposure to Methyl Mercaptan?

 

The primary action is methyl mercaptan monitoring i.e. to measure how much 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, dumpsites, etc.
• However, avoid staying upwind of wastewater treatment plants, sewage treatment plants, landfills, wood-pulp mills, and industrial areas of starch manufacturing, etc. 
• Always use appropriate personal protective equipment (PPE) while using CH3SH-containing products or while going to places where the presence of high levels of CH₃SH is suspected. 
• Also, on detection of high levels of CH₃SH, immediately vacate the area and provide proper ventilation to remove the gas.

Measurement methods of Methyl Mercaptan monitoring

 

For example, different working principles used for methyl mercaptan monitoring in the ambient environment are chemiluminescence, semiconductor, and electrochemistry.

Chemiluminescence

 

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. Consequently, the light intensity is measured photometrically and converted to the CH₃SH concentration. 

Semiconductor

 

When a metal oxide semiconductor-based methyl mercaptan monitor is exposed to an 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.

Electrochemical

 

CH₃SH monitors working on the electrochemical principle are operated based on the diffusion of methyl mercaptan gas into the sensor. As a result, it produces electrical signals proportional to the CH₃ SH concentration. Hence, it allows accurate measurement of even low concentrations of CH₃SH, which is essential in methyl mercaptan monitoring in the ambient air. 

So, from all the above principles of methyl mercaptan monitoring, applications like ambient air monitoring prefer CH₃SH monitors based on electrochemistry as they yield more accurate CH₃SH  concentrations and are inexpensive in comparison to others.

Oizom’s working principle for CH₃SH monitoring

 

Oizom’s ODOSENSE is a real-time odour emission tracking solution. Therefore, 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. As a result, The sensor that measures CH₃SH works on the principle of electrochemical sensing. Subsequently, with the help of meteorological data, Odosense can trace the odourant dispersion plume incited by conditions like wind speed and wind direction. So, Odosense is a proactive approach to measuring real-time odour emissions. Meanwhile, this makes it an ideal choice for landfill sites, wastewater treatment facilities, fertilizers, paper-pulp industries, 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 as a result of microbial degradation of organic matter.
• It is an organosulfur volatile organic compound (VOC) hence, easily ignitable and decomposable, emitting highly toxic fumes and flammable vapors.
• Exposure to methyl mercaptan causes irritation to the eyes and skin, as well as the respiratory tract, respiratory distress, damage liver and kidneys, and respiratory paralysis, leading to coma and death. 
• As a result, prolonged exposure to 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 the 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.