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Hydrogen sulfide (H2S), sodium sulfide (Na(SH)) and sodium sulfide (Na2S) – information sheet

Hydrogen sulfide (H2S)
CAS Registry Number 7783-06-4

Sodium sulfide [Na(SH); sodium bisulfide]
CAS RN 16721-80-5

Sodium sulfide (Na2S)
CAS RN 1313-82-2

Publications summarized:

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Overview

  • The Government of Canada conducts risk assessments of substances under the Canadian Environmental Protection Act, 1999 (CEPA) to determine whether they present or may present a risk to human health or to the environment.
    • The risks posed by a substance are determined by both its hazardous properties (potential to cause adverse human health or ecological effects) and the amount or extent of exposure to people and the environment.
    • When needed, the Government implements risk management measures under CEPA and other federal acts to help prevent or reduce potential harm.
  • In September 2017, the Government published a first draft assessment for hydrogen sulfide (H2S), sodium sulfide (Na(SH); referred to as sodium bisulfide), and sodium sulfide (Na2S). Based on the data at that time, all 3 substances were proposed to be not harmful to human health or to the environment. Following the public comment period of the first draft assessment, new information was received reporting incidental releases of hydrogen sulfide from inactive oil and gas wells at levels that are associated with concerns for human health and the environment. As such, a risk to human health and the environment was identified. Since the new information led to a change in conclusion for 1 of the 3 substances (hydrogen sulfide) an updated draft assessment and a risk management scope are published for public consultation.
  • As a result of the updated assessment, the Government is proposing that hydrogen sulfide may be harmful to human health and to the environment due to incidents of release of high levels of hydrogen sulfide to air from inactive oil and gas wells.
    • Potential acute inhalation exposure of Canadians to these high levels of hydrogen sulfide is the concern for human health. Hydrogen sulfide is associated with acute effects on the human respiratory system.
    • Potential incidents of release of high levels of hydrogen sulfide to air may cause harm to the environment.
  • To manage potential risks to human health and the environment, the Government is considering risk management options that would contribute to reducing exposures to incidental releases of hydrogen sulfide from inactive oil and gas wells. Existing provincial and territorial regulatory regimes and programs for oil and gas well management, as well as federal initiatives, will be reviewed to inform the risk management options.
  • The Government is also proposing that the other 2 substances (sodium bisulfide and sodium sulfide) are not harmful to human health or to the environment.

About these substances

  • The assessment focuses on hydrogen sulfide (H2S) and two salts, called sodium sulfide (Na(SH); referred to as sodium bisulfide) and sodium sulfide (Na2S). These substances were assessed as part of the Chemicals Management Plan (CMP).
  • Hydrogen sulfide is a naturally occurring gas known for its rotten egg odour. It is produced from the breakdown of organic matter in the absence of oxygen, and therefore is widely present in sediments and water as well as in biological wastes. It is found naturally in crude oil, natural gas, volcanic gases, and hot springs and is released from these natural sources mainly to air and water under specific environmental conditions.
  • Hydrogen sulfide can also be released from human activities. In Canada, industrial operations that release hydrogen sulfide include oil and gas facilities, kraft pulp and paper mills, wastewater treatment systems, mining production, and intensive livestock operations. Hydrogen sulfide can also be released from inactive oil and gas wells. These oil and gas wells provide conduits for methane and bacteria, which can then metabolize the methane, generating by-products such as hydrogen sulfide.
  • According to information gathered by the Government, sodium bisulfide is used commercially in non-pesticidal agricultural products, or as a dye or in the manufacture of paints and coatings and building and construction materials (wood and engineered wood). Sodium sulfide is used in pulp and paper processing, wastewater treatment, and mining and smelting. It may also be used in the manufacture of food packaging materials that have no contact with food.
  • Sodium bisulfide and sodium sulfide rapidly and completely break down in the body to produce hydrogen sulfide. Also, sodium bisulfide and sodium sulfide break apart to form bisulfide, sulfide anions, and hydrogen sulfide, if released to water. Therefore, the human health risk characterization as well as the environmental assessment are focused on exposure to hydrogen sulfide.

Human and ecological exposures

  • People in Canada may be exposed to hydrogen sulfide mainly through inhalation (breathing it in). Hydrogen sulfide can be present in ambient (outdoor) air from both natural sources and industrial sources.
  • As described above, hydrogen sulfide is released into the environment as a result of several industrial activities, mainly to air and water. In Canada, concentrations of hydrogen sulfide have been measured in air, surface water, groundwater, and wastewater effluents near pulp and paper operations, oil and gas facilities, wastewater treatment systems, and livestock operations.
  • Incidental releases of hydrogen sulfide have also been measured near inactive oil and gas wells in Ontario, Canada. An inactive well is a well that has not been in production over consecutive weeks, months or years (depending on the jurisdiction, regulations or guidelines). There are inactive oil and gas wells in provinces and territories across Canada with the majority of them identified in British Columbia, Alberta, Saskatchewan, and Ontario. Incidents of release of high levels of hydrogen sulfide to air from inactive oil and gas wells may result in acute exposure of people and the environment.

Key health and ecological effects (hazard)

  • To help inform the health and ecological effects characterization in the assessment, international and national data reports were considered, among other sources of information when available.
  • No classifications by other agencies were identified for hydrogen sulfide for genotoxicity (damage to genetic material) or carcinogenicity (ability to cause cancer).
  • Critical health effects associated with hydrogen sulfide include respiratory effects (increases in airway resistance in people with asthma) and neurological effects (nausea, headaches, poor memory, as examples). Exposure to hydrogen sulfide is also associated with effects on the eye (irritation).
  • Hydrogen sulfide has the potential to cause adverse effects on terrestrial organisms at low concentrations. In the case of plants, however, low concentrations can also have stimulatory effects (for example, increased growth).

Consideration of subpopulations who may be have greater susceptibility or greater exposure

  • There are groups of individuals within the Canadian population who, due to greater susceptibility or greater exposure, may be more vulnerable to experiencing adverse health effects from exposure to substances.

  • Certain subpopulations are routinely considered throughout the assessment process, such as infants, children, and people of reproductive age.

    For example, developmental and reproductive toxicity studies are evaluated for potential adverse health effects. For hydrogen sulfide, these subpopulations were taken into account.

  • In addition, people living near inactive oil and gas wells who are more likely to be exposed to high levels of hydrogen sulfide from release events were considered. In the assessment, data was available to account for people who have asthma and who may be more susceptible to respiratory effects from exposure to hydrogen sulfide.

Risk assessment outcomes

  • Assessments focus on information critical to determining whether substances are harmful to human health or the environment under CEPA. This is done by considering scientific information, including information, if available, on subpopulations who may have greater susceptibility or greater exposure, vulnerable environments and cumulative effects and by incorporating a weight of evidence approach and precaution.
  • As a result of the updated draft assessment, a potential risk to human health was identified from incidental releases of hydrogen sulfide to air from inactive oil and gas wells.
    • This potentially results in acute exposure of Canadians to levels of hydrogen sulfide that are near to or above the levels that may cause acute respiratory effects.
  • For the ecological assessment, it was also determined that there may be a risk of harm to the environment posed from incidental releases of hydrogen sulfide to air from inactive oil and gas wells.
  • Other sources of exposure, such as releases from other industrial activities to air or water, were not a concern at current levels.
  • It was also determined that sodium bisulfide and sodium sulfide were not associated with hydrogen sulfide exposures of concern for human health or the environment.

Proposed assessment conclusions

  • The Government is proposing that hydrogen sulfide may be harmful to human health at levels of exposure considered in the assessment.
  • The Government is also proposing that hydrogen sulfide may enter the environment at levels that are harmful.
  • It is also proposed that sodium bisulfide and sodium sulfide are not harmful to human health or to the environment at levels of exposure considered in the assessment.
  • Also, it is proposed that hydrogen sulfide meets the persistence but not the bioaccumulation criteria as set out in the Persistence and Bioaccumulation Regulations of CEPA.

Preventive actions and risk reduction

  • If the proposed conclusion is confirmed in the final assessment, the Government will consider proposing to add hydrogen sulfide to Part 2 of Schedule 1 to CEPA. Adding a substance to Schedule 1 enables the Government to take enforceable risk management actions under CEPA, using a 2-track approach to manage risks.
    • Toxic substances that pose the highest risk (that is, meet certain criteria) are added to Part 1 of Schedule 1. These are prioritized for total, partial or conditional prohibition.
    • Other toxic substances are added to Part 2 of Schedule 1 and are prioritized for pollution prevention.
    • Regulations specifying criteria for the classification of substances that pose the highest risk or that are carcinogenic, mutagenic or toxic to reproduction may be developed. When criteria are available, some substances considered for addition to Part 2 of Schedule 1 may instead be considered for addition to Part 1 of Schedule 1.
  • Publication of the risk management scope aims to inform interested parties of proposed risk management options and initiate discussion about their development. To address the potential concerns for human health and the environment, the Government is considering:
    • risk management options that would contribute to reducing exposure of the general population and the natural environment to incidental releases to air of hydrogen sulfide from inactive oil and gas wells, to levels that are protective of human health and the environment.
  • It is recognized that provinces and several territories have requirements in place for oil and gas well management. For example, there are ambient air quality standards or guidelines for the presence of hydrogen sulfide in air. As such, current regulatory regimes and programs will be reviewed, including federal initiatives that may contribute to reducing incidental releases of hydrogen sulfide from inactive oil and gas wells, to inform potential risk management options.
  • Information is being sought by the Government to inform risk management decision-making. Details can be found in the risk management scope, including where to send information during the public comment period, ending April 3, 2024.
  • Risk management actions may evolve through consideration of assessments and risk management options for other substances. This is to ensure effective, coordinated, and consistent risk management decision-making.

Where to find updates on risk management actions

Related resources

  • Visit Healthy Home for information on chemical safety in and around the home, including on hydrogen sulfide (for consumers).
  • In workplace settings, severe health effects (for example, loss of consciousness and death) have been reported due to accidental acute exposure of workers to high levels of hydrogen sulfide. These exposure levels, specific to industrial workplace settings, are a great deal higher than levels encountered in a non-workplace setting. Occupational requirements are in place for the protection of workers from inhalation exposure to hydrogen sulfide.
  • Assessments conducted under CEPA focus on risks of exposure of the general population. Hazards related to chemicals used in the workplace are defined within the Workplace Hazardous Materials Information System (WHMIS). If a substance is harmful to the general population, it could also be of concern for individuals in situations where a higher volume of the substance is used or where the substance is used for a longer duration (for example, the workplace). The Government of Canada recognizes that it is the responsibility of the federal, provincial and territorial occupational health and safety organizations to coordinate legislation for the safe use of chemicals in the workplace. We are working to support this role by integrating the information, tools, and technical expertise of the CMP and Health Canada's Workplace Hazardous Products Program.
  • The Canadian Centre for Occupational Health and Safety (CCOHS) provides a fact sheet on hydrogen sulfide for information relevant to a workplace setting.

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