How dangerous is mercaptan?
by Anne Zammit
Enemalta’s furtive disposal of unwanted mercaptan gas by burning it in a field in 2009 has caused outrage. The predictable blame game that ensued has fuelled much debate in the media over who was politically responsible for this high-profile environmental foul.
Yet the people who may have been most directly affectedby the incident seem to have been overlooked.
The direct health and safety of the workers who were contracted to carry out the environmental misdemeanour hardly seem to have come up for mention as the story continued to unfold in the press.
Normal exposure to many everyday substances goes without risk but when exposed to the same substances for a longer time or at high doses, an incident can sometimes be fatal.
Mercaptan is a naturally occurring substance found in some nuts and cheeses and during digestive and metabolic processes. The general population will most likely have already been exposed to trace amounts of this compound.
Also used in pharmaceuticals, pesticides and livestock-feed additives, the gas is mainly known as an odouriser.
It is normally added in minute quantities to butane or propane gas, used for cooking and heating. Even when present in very small amounts, mercaptan attracts attention with a permeating smell when gas in the home escapes unlit.
A gas company in the US encourages its customers who do not know what mercaptan smells like to ring up and order a ‘scratch ’n’ sniff’ card so they can become familiar with the odour.
Air, water or rust in a gas cylinder can cause the odour to fade, making it less effective as a harmless warning device.
The powerful aroma of mercaptan gas is not unlike garlic. More often, the gas is described as having a highly noticeable smell not unlike rotten cabbage or smelly socks.
While it burns easily, mercaptan’s most popular function as an additive is to provide an early warning system for gas leaks. But when present in large amounts the overpowering odour can deaden the olfactory senses so that sense of smell no longer acts as a warning signal.
Other uses for mercaptan in industry include pharmaceuticals and livestock feed additives. Made up of carbon, hydrogen and sulphur (which is responsible for the strong odour), it is the world’s smelliest substance and almost unbearable in concentrated form.
Notably, mercaptan is generally less corrosive and less toxic than similar sulphur compounds found naturally in rotten eggs, onions and garlic. Yet prolonged or acute exposure can cause headaches and nausea.
A number of different factors determine whether harmful health effects occur and what the type and severity of those health effects are when people are exposed to chemicals. Individual characteristics such as age, gender, family traits and state of health also come into it.
Effects depend on dose, duration, exposure route (breathing, eating, drinking, or skin contact) and whether any other chemicals are present.
A US government agency toxic profile records the death of a 53-year-old black man who fell into a coma and died after spending a week emptying tanks of methyl mercaptan.
This extremely high-level event of occupational exposure happened over 50 years ago and the unfortunate worker suffered a fatal pulmonary embolism after 28 days in hospital.
Workers in sewage treatment plants, pulp mills, chemical plants, and industrial or agricultural settings where chemical or microbiological formation of mercaptan is significant would potentially have a higher-than- normal exposure to this compound. People living close to hazardous waste sites may also have higher exposure than the general population.
However, mercaptan’s penetrating smell and low odour threshold (the point at which it can be detected by smell) of 1.6 parts per billion makes it unlikely that most people would willingly tolerate exposure to concentrations much above the threshold for any substantial amount of time.
Over-exposure can bring on fever, cough, shortness of breath, double vision, a feeling of tightness and burning in the chest, dizziness, memory loss, tremors, difficulty swallowing and abdominal pain.
Exposure limits for workers over a normal eight-hour work day within a 40-hour week are a fraction of the limit for the presence of mercaptan in air, laid down by US guidelines.
Industrial hygienists put the threshold at lower than 0.5 parts per million.
Workers should be provided with and required to use chemical protective clothing, gloves and face shields. Where thereis any possibility for a worker’s body being exposed to mercaptan, facilities for quick drenching of the body should be provided within the immediate work area.
Emergency showers and eyewash should be close at handas contact can cause skin oreye irritation.
Planned or emergency entry into environments containing unknown concentrations require self-contained breathing apparatus with a full face piece.
Before working with mercaptan, all workers likely to have contact with it should be properly trained as to its storage and handling.
The US agency for toxic substances and disease registry claims to know “very little” about what happens to mercaptan after it is released into the environment. Because it is a gas, most of it probably goes into the air where sunlight breaks it down into other substances.
Food chain bio-accumulation of mercaptan is unlikely. It is not held in animal tissues as it is highly volatile and water soluble.
It is the transport of mercaptan to a field for burning that may have been the most dangerous part of the whole misguided exercise.
Highly flammable in confined spaces, the gas is also used in jet fuel and self-ignites at a very high temperature with vapour which can cause a flash fire. On combustion, sulphur dioxide fumes are produced, which can irritate the eyes and lungs.
Contact with water or steam creates an instant reactionwhich produces toxic, flammable vapours. Hopefully the Enemalta firemen were made aware of all the hazards although information on this aspect is lacking.
The 1960s were the dark ages for chemical safety given all the new products being developed and freely distributed then, with next to nothing known about their health effects.
Today we know better, and those responsible for safety when there is any risk of chemicals being released into the environment should be made fully accountable.