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

Overview

• The Government of Canada conducted a science-based evaluation of hydrogen sulfide, sodium bisulfide, and sodium sulfide, called a screening assessment, to address the potential for harm to Canadians and to the environment.
• Under the Canadian Environmental Protection Act, 1999 (CEPA 1999), the risk posed by a substance is determined by considering both its hazardous properties (its potential to cause adverse human health or ecological effects) and the amount of exposure there is to people or to the environment. A substance may have hazardous properties; however, the risk to human health or to the environment may be low depending on the level of exposure.
  • More information on assessing risk can be found in the Overview of Risk Assessment and related fact sheets, particularly on Types of Risk Assessment Documents and the Risk Assessment Toolbox.
• Hydrogen sulfide has effects of concern for human health and the environment; however, current exposure levels are low. Therefore, it is proposed under CEPA 1999 that hydrogen sulfide, sodium bisulfide, and sodium sulfide are not harmful to human health or to the environment.

About these substances

• This screening assessment focuses on hydrogen sulfide (H₂S), and two salts called sodium sulfide [(Na(SH)), also called 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, and hot springs. In Canada, a number of industrial operations release hydrogen sulfide.
• In Canada, sodium bisulfide is used in the manufacture of other substances or products. It has commercial uses in dyes that are used in textiles, as well as uses in paints and coating products, non-pesticidal agricultural products, and building and construction materials.
• Sodium sulfide is used in pulp and paper processes, wastewater treatment, mining and smelting, and in food packaging that does not have 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 react to form hydrogen sulfide, if released to water. Therefore, the environmental and human health risk characterization of this assessment focused on exposure to hydrogen sulfide.

Exposure of Canadians and the environment

• Natural sources can release hydrogen sulfide mainly to air and to water, resulting in potential exposure of Canadians.
• Hydrogen sulfide is released to the environment as a result of several industrial activities. In Canada, it is released from oil and gas facilities, kraft pulp and paper mills, wastewater treatment systems, mining sites, and intensive livestock operations. It is most likely to be released into air and the aquatic environment.
• In Canada, hydrogen sulfide levels have been measured and reported in wastewater effluents, air, surface water, and near industrial operations associated with release of this substance.
• Canadians would primarily be exposed to hydrogen sulfide through inhalation (breathing it in). Measurements of hydrogen sulfide in outdoor air were therefore used in this assessment.

Key health and ecological effects (hazard)

• In conducting this assessment for hydrogen sulfide, there were no classifications identified for mutagenicity (damage to genetic material) or carcinogenicity (cancer-causing ability) by other national or international regulatory agencies. Also there were no reproductive or developmental classifications identified for this substance.
• Exposure to hydrogen sulfide has been associated with effects on the eye (irritation) and on the respiratory system (increases in airway resistance in people with asthma). It has also been associated with loss of the sense of smell.
• Effects on the nervous system have been observed in humans as a result of accidental exposure to hydrogen sulfide through inhalation in workplace settings. Effects include headache, memory impairment, and at elevated levels, loss of consciousness and death.
• Effects on the eye, the respiratory system and the nervous system were considered to be the important or "critical" effects used for characterizing the risk to human health in this screening assessment.
• Hydrogen sulfide has the potential to harm (including potential death) both aquatic organisms and terrestrial plants at low concentrations. In the case of plants, however, low concentrations can also have stimulatory effects (for example, increased growth).

Risk assessment outcomes

• Using a comparison of levels to which Canadians can be exposed to hydrogen sulfide (excluding in workplace settings) and the levels associated with health effects, it was determined that the risk to human health is low.
• Also, considering all information presented, there is low risk of harm to organisms or the environment from hydrogen sulfide.
• The Government published the Draft Screening Assessment for Hydrogen Sulfide (H₂S), Sodium Sulfide (Na(SH)) and Sodium Sulfide (Na₂S) on September 9, 2017. This publication has a 60-day public comment period ending on November 8, 2017.

Proposed screening assessment conclusions

• The Government is therefore proposing that hydrogen sulfide, sodium bisulfide, and sodium sulfide are not harmful to human health at current levels of exposure.
• The Government is also proposing that hydrogen sulfide, sodium bisulfide, and sodium sulfide are not entering the environment at levels that are harmful to the environment.

Preventive actions and reducing risk

• In Canada, a number of federal, provincial/territorial, and municipal risk management measures are currently in place to minimize releases of hydrogen sulfide to the environment (mainly to air and water). These include regulations targeting releases to the aquatic environment related to pulp and paper effluents, municipal wastewater effluent, oil and gas seepage, environmental emergencies, and transportation of substances.
• There are a number of provincial and territorial regulations, standards, objectives and/or guidelines in place for air quality and pollution control. Some of these measures are sector-specific, for example, some restrict the release of hydrogen sulfide to air from the oil and gas sector.
• Some industrial releases of hydrogen sulfide are reported to and monitored through the federal National Pollutant Release Inventory.
• While exposure of the general population to hydrogen sulfide is not a concern at current levels, hydrogen sulfide is associated with health effects of concern (for example, severe neurological effects) at higher concentrations. Therefore, there may be a concern for human health if exposures of Canadians were to increase.
• Follow-up activities to track changes in exposure to hydrogen sulfide are being considered. These will include current monitoring initiatives at the federal and provincial/territorial levels.
• Stakeholders are encouraged to provide any information relevant to this substance that may help inform the choice of tracking activity, during the 60-day public comment period on the assessment.

Important to know

• In occupational 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 settings, are a great deal higher than levels encountered in a community setting. The high levels are therefore not considered relevant for characterizing the risk of the general population of Canada, which is the focus of this screening assessment.
• Occupational requirements are in place for the protection of workers from inhalation exposure to hydrogen sulfide. Canadians who may be exposed to hydrogen sulfide, sodium bisulfide, or sodium sulfide in the workplace should consult with their employer and an occupational health and safety (OHS) representative about safe handling practices, applicable laws, and requirements under OHS legislation and the Workplace Hazardous Materials Information System (WHMIS).
• The Canadian Centre for Occupational Health and Safety (CCOHS) provides a fact sheet on hydrogen sulfide for information relevant to a workplace setting.