Summary of public comments received on the first draft assessment of hydrogen sulfide (H2S), sodium sulfide (Na(SH)) and sodium sulfide (Na2SH)

Comments on the first draft Assessment of Hydrogen Sulfide (H2S), sodium sulfide (Na(SH)) and sodium sulfide (Na2SH), assessed under the chemicals management plan (CMP), were submitted by Albemarle Corporation, Arcelor Mittal Dofasco, Canadian Association of Petroleum Producers, Free Range Worker Cooperative, Southern Alberta Group for the Environment (SAGE), and private citizens.

Summarized public comments and responses are provided below, organized by topic:

Human health assessment

Comment summary 1: There is concern that the government has failed to adopt a science-based, precautionary approach to the assessment of hydrogen sulfide health risks in Canada. Hydrogen sulfide should be classified as toxic pursuant to subsection 64(c) of the Canadian Environmental Protection Act (CEPA), 1999.The draft assessment must examine the risks hydrogen sulfide releases pose to the health of many Canadians, especially those living or working near oil and gas facilities and infrastructure, or impacted by geological fracturing.

Hydrogen sulfide has been raised as an ongoing and persistent problem in rural oil and gas producing communities. The problems are not only related to industrial settings, as the draft assessment concludes. Hydrogen sulfide exposure of the general population in southeast Saskatchewan is a concern at current levels and the air shed monitors do not capture the spikes in hydrogen sulfide concentrations at point sources during short periods, yet these acute levels are poisoning people in their communities. Hydrogen sulfide is entering the environment in a quantity or concentration and under conditions that constitute a danger in Canada to human life and health.

Response 1: The assessment has been revised to take into consideration new information that has become available regarding incidental releases of hydrogen sulfide. This new information includes reports of events during which members of the general population are potentially exposed to spikes of high concentrations of hydrogen sulfide caused by releases from inactive wells. Although it is recognized that there are limitations associated with the nature of the data in these incidental exposure reports, it is considered appropriate to propose that there is a potential concern for human health associated with incidental exposure to hydrogen sulfide. The basis for the concern includes the levels at which acute health effects can occur, the magnitude of incidental concentrations reports, and the number of incidences that were reported, combined with the number of inactive wells in Canada. Accordingly, it is proposed to conclude that there is a potential concern for human health associated with incidental exposure to hydrogen sulfide. It is proposed that hydrogen sulfide meets the criteria under paragraph 64(c) of CEPA. There is also a potential concern for the environment associated with incidental exposure to hydrogen sulfide. It is also proposed that hydrogen sulfide meets the criteria under paragraph 64 (a) of CEPA.

Comment summary 2: Looking at the human health effects as well as taking into consideration that there are also occupational health hazards for individuals working at some types of facilities where varying levels of hydrogen sulfide occur for example, agricultural operations, it is important that hydrogen sulfide be listed as a “toxic substance”.

Response 2: In the draft assessment, it is acknowledged that levels specific to occupational settings can be several orders of magnitude higher than concentrations encountered in a community setting.

The Government of Canada is working with the provinces and territories to explore ways to enhance the protection of workers from exposure to chemicals by integrating and leveraging the information, tools, and/or technical expertise of the CMP and Health Canada’s Workplace Hazardous Products Program. For information concerning occupational exposure, Canadians should consult with their employer or an occupational health and safety (OHS) representative in their jurisdiction for information on what steps to take in the workplace.

Comment summary 3: It is recommended that the Ministers recognize and accept the growing body of regulatory and common law findings that human health is or may be detrimentally affected, hence endangered, by odours and chronic inhalational exposures.

Response 3: Chronic inhalation exposure and neurotoxic effects of hydrogen sulfide were addressed in the assessment and rely on human and animal data. These effects, along with other health effects, are considered in the risk characterization. While the unpleasant odour of hydrogen sulfide is well documented and can produce symptoms of stress and annoyance, whether it instigates an adverse toxicological effect as a result of olfactory nuisance is not clear as other factors are likely implicated (for example, psychosomatic and stress-related factors). The perception of odours is subjective, varies among the population, and reported effects are usually transient or reversible. These variables, compounded by the complexity of the sense of smell, introduce significant uncertainty in determining a direct link between the unpleasant odour of hydrogen sulfide and any reported health effects. On this basis, olfactory nuisance was not considered as a critical health effect of hydrogen sulfide on which to base the conclusions of the assessment. Additional information was added to the revised draft assessment to support this approach further.

Comment summary 4: It is unclear why the current consultation would include sulfide salts, but not other 3-atom reduced sulphur compounds that are toxicologically similar (albeit slower acting) and may pose risks to larger numbers of workers and residents of rural and industrialized areas. Reduced sulphur compounds were not addressed in any of the petroleum streams assessments to date, either. This is a large gap.

Response 4: One of the primary objectives of the Chemical Management Plan (CMP) has been to address the approximately 4300 existing substances that have been identified as priorities for assessment through the Government of Canada’s categorization of the substances on the Domestic Substances List (DSL).

As indicated in the introduction of the assessment, hydrogen sulfide, sodium bisulfide (CAS RN 16721-80-5) and sodium sulfide (CAS RN 1313-82-2), were included in the assessment because they met categorization criteria as described in ECCC, HC (modified 2017). These substances were identified as they may present the greatest potential for exposure to Canadians, or are persistent or bioaccumulative and inherently toxic to human beings or to non-human organisms. No other sulphur compounds on the DSL were identified as meeting these categorization criteria.

Comment summary 5: It is recommended that the neurological acute no observable adverse effect concentration (NOAEC) be lowered to reflect the neurological adverse health effects of odour. The California Office of Environmental Health Hazard Assessment (“OEHHA”) uses an acute reference exposure level (“REL”) of 42 μg/m3 (0.03 ppm) based on neurological effects.

Response 5: A NOAEC of 30 ppm from an inhalation study was identified in the draft assessment for characterization of risk of neurological effects from exposure to hydrogen sulfide. This is based on the observation of significant reductions in spontaneous motor activity (ambulations and total movements) in rats following a 5- day inhalation exposure to the next tested concentration of 80-ppm hydrogen sulfide in a study by Struve et al. (2001). This is considered a conservative indication of possible neurological effects. Although a decline in auditory verbal learning from baseline was observed at all concentrations tested in human volunteer subjects to 2-hour exposures no dose-dependent response was observed. As well, and as indicated by the author, the decrease in auditory verbal recall could be attributed to subject fatigue (having spent more than 2 hours being tested and undergone several cognitive and physical tests before the second AV recall test).

The (REL) value determined by OEHHA relies on studies that use olfactory nuisance (and physical effects that possibly arise from it) as an endpoint. As indicated in the assessment, olfactory nuisance is not considered an adverse effect for the purpose of this assessment. Additional details on olfactory nuisance are provided in a response above.

Comment summary 6: It is recommended that uncertainty factors be applied to the chronic inhalation no observable adverse effect concentration (NOAEC) to set a chronic inhalation reference exposure level, as was done by the California OEHHA.

Response 6: The purpose of the human health risk characterization in the assessment is to determine whether hydrogen sulfide represents a concern to human health for the general population at current levels of exposure and is not to set a reference exposure level.

The quantitative risk characterization in the assessment compares the potential exposure level of the general population with levels that are observed to cause health effects in human or animal studies. Risk is determined to be of concern or not depending on the magnitude of this margin between exposure and effect levels; adequacy of the margin of exposure accounts for uncertainties in the data used to inform the assessment.

Of note, the effects level used for determination of risk from chronic exposure in the draft assessment is 10 ppm, which is more conservative than the 30 ppm used as the basis for the California OEHHA.

Comment summary 7: The margins of exposures determined in the draft assessment are within the range that requires further analysis, according to Health Canada’s policy.

Response 7: The adequacy of margins of exposure depends on a number of factors, including intra and interspecies variability, the conservative nature of the exposure assessment and selection of critical health effects level. For example, the acute respiratory critical effect level of 2 ppm is based on a human study and a margin of exposure based on this health effect level would not be confounded by interspecies variability. A rationale for determining the adequacy of each margin of exposure is stated in the quantitative component of the human health risk characterization section of the assessment.

Comment summary 8: The 2-ppm acute respiratory effect level identified in the draft assessment lacks experimental validation.

Response 8: The acute respiratory effect level of 2 ppm was based on the Jappinen et al study (1990), in which the respiratory pattern of 10 asthmatic volunteers was measured before and after exposure to 2-ppm hydrogen sulfide. The authors indicate that there are no statistically significant changes in the tested subjects; however, as a group, large changes in airway resistance and airway conductance after exposure to hydrogen sulfide were observed, which are considered of biological significance.

Therefore, given the limitations of the study (only 10 volunteers) and the biological significance of the effects, it was considered appropriate to select 2 ppm as a conservative effect level on which to base the short-term risk characterization.

While acknowledging the limitations of the study, other agencies (ATSDR and IPCS) relied on it to derive reference values for hydrogen sulfide exposure.

Comment summary 9: The community air-quality studies provided in the assessment related to intensive livestock operations are limited, and the examples provided in the assessment may not be indicative of the industry as a whole.

Additional information on hydrogen sulfide near livestock operations was submitted for consideration, which provide confirmation of probable harm to the health of health-vulnerable neighbours from barn and lagoon emissions. There should be further investigation of the nighttime air quality at these residences, because the scientific literature from researchers indicates we should be alarmed.

Response 9: The available information with respect to intensive livestock operations in Canada is not indicative of a concern for human health based on a comparison of hydrogen sulfide concentration levels with critical health effect levels.

The additional information submitted was considered. Generally, the references provided were studies that were not specific to hydrogen sulfide or did not provide sufficient detail to associate the levels of hydrogen sulfide with a specific location in Canada.

Comment summary 10: There is a need to acknowledge and assess epidemiological and community-based health studies not cited in this assessment and to recognize the impacts of chronic and low-level concentrations of intermittent hydrogen sulfide over a long-term.

Response 10: The assessment is based on the best available scientific data to characterize the risk from exposure to hydrogen sulfide.

The references that were submitted were carefully examined. Some studies were associated with significant limitations and were not included in the draft assessment. However, other studies were considered appropriate for use as supplementary evidence for the health effects of hydrogen sulfide on the respiratory and nervous systems in humans and were added to the draft assessment (for example, Campagna et al. (2004), Kilburn et al. (2012)).

Environmental releases

Comment summary 11: We disagree with the statement, “if proper manure management practices are followed at intensive livestock operations, most of this hydrogen sulfide will be incorporated into soil using techniques to avoid evaporative losses”. As long as there is manure present there will always be hydrogen sulfide (and other gases, vapors, particulate matter and bioaerosols) emitted continuously into the atmosphere.

Response 11: It is acknowledged that hydrogen sulfide releases can occur at various locations at intensive livestock operations. The statement has been revised to make it specific to manure application to land:

“If proper manure management practices are followed at intensive livestock operations, hydrogen sulfide from manure application to land will be incorporated into soil using techniques to minimize evaporative losses.”

Overarching comments

Comment summary 12: We support the draft assessment and find it to be balanced and representative of relevant public hazard and exposure pathway information.

Response 12: Noted.

Comment summary 13: We agree that occupational exposures are a great deal higher than levels encountered in a community setting, and that those high levels are therefore not reflective of risks to the general population of Canada.

Response 13: Noted.

Comment summary 14: We are subject to provincial air management systems. Hydrogen sulfide will not cause acute health effects at the levels measured outside facilities meeting the ambient air quality standards or objectives.

We agree with the proposed conclusion that the substances are not entering the environment in a quantity or concentration or under conditions that constitute or may constitute a danger in Canada to human life or health.

Hydrogen sulfide presents a significant risk in the workplace when present at high concentrations. Highly effective administrative and engineered controls have been developed to mitigate the risk. Occupational Health and Safety codes in all Canadian provinces address hydrogen sulfide hazard and risk mitigation in the occupational context.

Response 14: Noted.

Comment summary 15: There is uncertainty in the quantity, source and life cycle of hydrogen sulfide in Canada’s environment.

Response 15: Assessments are based on the best available data. Uncertainties in the available data are acknowledged.

Follow-up activities

Comment summary 16: Information was provided by a stakeholder regarding their use activities for hydrogen sulfide, to help inform the choice of follow-up activity.

Response 16: Noted.

Comment summary 17: We recommend that ECCC and HC continue to use the existing data sources that formed the basis of the draft assessment to monitor concentrations in the vicinity of oil and gas operations.

Response 17: Available current air monitoring data near oil and gas operations were considered in the assessment. In the revised draft assessment, new data sources such as reports of incidental exposure are also being considered.

Comment summary 18: We support ongoing monitoring of health risks to the Canadian public from increased exposures to hydrogen sulfide. Such efforts should be focused on those situations where there is the highest probability of significant releases or unexpected exposure. In addition, any follow up activities to track changes in exposure should be harmonized to the extent possible between Environment and Climate Change Canada, Health Canada, and Transport Canada.

Response 18: The draft assessment has been updated to incorporate additional information on potential incidental exposure. Hydrogen sulfide is proposed to meet paragraph 64, therefore risk management measures will be investigated.


Campagna D, Kathman SJ, Pierson R, Inserra SG, Phifer BL, Middleton DC, Zarus GM, White MC. 2004. Ambient hydrogen sulfide, total reduced sulphur, and hospital visits for respiratory diseases in northeast Nebraska, 1998–2000. J Expo Anal Environ Epidemiol. 14:180–187.

[ECCC, HC] Environment and Climate Change Canada, Health Canada. [modified 2017 Mar 12]. Categorization of chemical substances. Ottawa (ON): Government of Canada. [accessed 2022 Mar 31].

Jappinen P, Vilkka V, Marttila O, Haahtela T. 1990. Exposure to hydrogen sulfide and respiratory Function. British Journal of Industrial Medicine. 47:824-828.

Kilburn KH. 2012. Human impairment from living near confined animal (hog) feeding operations. Journal of Environmental and Public Health 2012.

Struve MF, Brisbois JN, James RA, Marshall MW, Dorman DC. 2001. Neurotoxicological effects associated with short-term exposure of Sprague-Dawley rats to hydrogen sulfide. Neurotoxicology 22(3):375–385.

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