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About H2S (Hydrogen Sulphide) Awareness

What is H2S?

Hydrogen Sulphide is the chemical compound with the formula H2S. It is a colorless, very poisonous, flammable gas with the characteristic foul odor of rotten eggs. It often results from the bacterial breakdown of organic matter in the absence of oxygen, such as in swamps and sewers; this process is commonly known as anaerobic digestion. It also occurs in volcanic gases, natural gas, and some well waters.

Hydrogen sulphide is a highly toxic and flammable gas. Being heavier than air, it tends to accumulate at the bottom of poorly ventilated spaces. Although very pungent at first, it quickly deadens the sense of smell, so potential victims may be unaware of its presence until it is too late. For safe handling procedures, a hydrogen sulphide material safety data sheet (MSDS) should be consulted.

What is the difference between Hydrogen Sulphide and Hydrogen Sulfide?

Nothing! Some countries, like Canada and the UK, spell it ‘Sulphide’, others, like the US, spell it ‘Sulfide’.

I think I may have been exposed to Hydrogen Sulphide. How can I tell if this is the case?

Hydrogen sulfide can be measured in your breath, if the sample is taken within two (2) hours after exposure. After then a test can be conducted to measure the level of thiosulphate levels in your urine. This test has to be performed within 12 hours of exposure. Both tests require equipment not routinely available in a doctor’s office. Samples must be sent to a special laboratory for the tests. These tests check if you have been exposed to hydrogen sulphide, but not whether harmful effects will occur.

I have heard that special materials must be used in pipes that might come into contact with H2S. Is this true?

Yes. Hydrogen sulphide is corrosive and can make some steels brittle, leading to sulphide stress cracking. This is of particular concern for handling “sour gas” and “sour crude oil” in the oil industry.

I know H2S is harmful, but does it have any useful uses in industry?

Yes, it can be converted to elemental sulphur using partial combustion via the Claus process. Suphur is used primarily in fertilizers, black gunpowder, matches, insecticides and fungicides.

What are the exposure limits for H2S in Canada?

The levels of allowable exposure by unprotected workers were set in accordance with the American Conference of Governmental Industrial Hygienists (ACGIH).

These are published in the form of Threshold Limit Values (TLV’s) expressed as a Time Weighted Average (TWA) of H2S for an 8 hour day/ 40 hour week and a Short Term Exposure Limit (STEL) of 15 minute of exposure no more that 4 times per day.

Until 2010 the limits for unprotected workers was a TWA of 10 Parts per million (PPM) and a CEV of 15PPM. The working practices and provision of protection and detection equipment for the oil & gas industries across Canada are all based on these levels.

Some Provinces (Alberta, Manitoba, Ontario, New Brunswick, PEI, Nova Scotia, Newfoundland and Labrador) and the Federal Government have clauses in their Health & Safety Regulations to automatically incorporate any new ACGIH TLV’s.

In 2010 the ACGIH issued new TLV’s for H2S of a TWA of 1 (compared to 10 previously) and a STEL of 5 (15 previously). ACGIH TLV’s are health-based and no consideration is paid to economic feasibility. This could be a huge problem for the oil & gas industry. Many of today’s measuring devices are not capable of measuring such low concentrations. Compliance would require major expenditures in personal protective equipment and a significant loss of productivity in affected workplaces.

Newfoundland has formally responded to the revised ACGIH H2S exposure levels (see http://www.enviromed.ca/Documents/Letter2.pdf). This acknowledges the practical problems and states that their OCCUPATIONAL HEALTH & SAFETY Officers will not issue orders under the new TLV’s “until further notice”.

An enquiry to the Ministry of Labour in Ontario elicited this response:

“In regards to the limit for Hydrogen Sulphide the current limit in Ontario Regulation 833 Control of Exposure to Biological or Chemical Agents is 10ppm TWA and 15ppm STEL. The Regulation and Table of Occupational Exposure Limits for Ontario Workplaces can be accessed at: http://www.labour.gov.on.ca/english/hs/topics/oels.php.

“In the summer of 2010 the Ministry of Labour consulted on proposed new and revised occupational exposure limits including a proposed updated limit for Hydrogen Sulphide of 1ppm TWA and 5ppm STEL. The Ministry is continuing to review the results of this consultation.”

Every employer whose workforce might come into contact with Hydrogen Sulphide must comply with any Regulations that apply. They must also maintain a watching brief on these issues concerning exposure values permitted for their workers.