Occupational injuries caused by exposure to hydrogen sulfide generated by microbial activity

Abstract

Objectives

The objective of this study was to analyze the risks associated with occupational exposure to hydrogen sulfide and to identify the main factors that lead to its formation and accumulation in work environments. The study also aimed to propose measures that may reduce the frequency and severity of related incidents.

Methods

The study was based on an evaluation of workplace accident records involving hydrogen sulfide and an assessment of available scientific findings on its generation, behavior, and effects on the human body. Particular focus was placed on confined or poorly ventilated spaces, where the gas is most likely to accumulate. The collected data were reviewed to identify recurring patterns and critical risk factors.

Results

The analysis shows that exposure to hydrogen sulfide often leads to severe or fatal outcomes. In most fatal incidents, the presence of hydrogen sulfide was a decisive factor. High concentrations were repeatedly recorded in enclosed spaces, such as sewers, wastewater treatment facilities, and biogas plants. These incidents frequently involved rapid loss of consciousness caused by suppression of the respiratory center and the sudden loss of the ability to detect the gas by smell. Several events affected multiple workers, demonstrating a high potential for large-scale accidents.

Conclusions

The findings highlight the need for improved prevention strategies in industries where hydrogen sulfide may develop. Recommended measures include the application of advanced detection technologies, improved ventilation, and stricter safety procedures. These measures may help increase awareness of the hazard and enhance the protection of workers in high-risk environments.

Introduction

Exposure to hydrogen sulfide (H₂S) poses a serious hazard in occupational environments. H₂S can be generated during microbial decomposition in various processing technologies. Microbial activity, particularly the action of sulfate-reducing bacteria (SRB), plays a key role in the production of H₂S. SRB utilize oxidized sulfur compounds, such as sulfate (SO₄²⁻) or thiosulfate (S₂O₃²⁻), as terminal electron acceptors, reducing them to H₂S¹⁾. SRB are present across a wide range of environments, including wastewater treatment plants, biogas facilities, and sewer systems, where conditions favor their growth and H₂S production²⁾.

H₂S is classified as an extremely flammable gas, highly toxic to aquatic organisms, and potentially fatal upon inhalation. Low-level exposure (0–100 ppm) can cause irritation of the eyes, nose, and respiratory tract, while prolonged exposure or exposure to concentrations above 500 ppm may result in severe neurological damage or death³⁾. Oesterhelweg and Püschel reported that H₂S can cause a temporary loss of olfactory perception at higher concentrations, hindering timely recognition of exposure risk⁴⁾. In the case of H₂S toxicity, concentration is a far more critical factor than the duration of exposure. Even brief exposure to high concentrations can lead to immediate loss of consciousness, respiratory distress, or fatal outcomes⁵⁾. As demonstrated by the cases analyzed in the present study, situations frequently arise where, following the collapse or unconsciousness of a worker, colleagues seeking to provide assistance enter the contaminated area. Such responses often result in additional casualties, increasing the number of affected workers.

Occupational exposure to H₂S poses a significant risk, particularly in confined or poorly ventilated spaces. Due to its higher density compared to air, H₂S tends to accumulate in lower-lying areas of technological facilities, such as pits, shafts, tanks, and sewers⁶⁾. The accumulation of gas in these areas, without sufficient ventilation, substantially elevates the likelihood of fatal occupational accidents.

The occurrence of H₂S in industrial and geothermal processes is well-documented, and established methods exist to minimize exposure risks. For example, geothermal power plants employ technologies such as catalytic oxidation, gas reinjection into geological formations, or the application of specific chemical additives to control H₂S emissions^7,8). These measures aim to maintain H₂S concentrations below 5 ppm to protect the health of workers and nearby populations. However, in certain sectors, the risks associated with H₂S exposure appear to be underestimated. This is particularly evident in operations involving organic waste handling and agricultural facilities, where fatal incidents related to H₂S exposure continue to occur.

The authors’ aim is to contribute to the understanding of this issue. The study identified workplaces in which microbial H₂S generation occurs using records of occupational accidents and incidents involving microbial H₂S exposure. For selected cases, causal event chains were established to determine the conditions leading to H₂S formation and to identify possible preventive measures to mitigate H₂S exposure in occupational settings.

Methods

In the initial phase, records of occupational accidents and injuries caused by microbial H₂S exposure were identified. The selection criteria focused on facilities that demonstrably handle organic matter, such as wastewater treatment plants, biogas facilities, and sewer system maintenance or repair operations — environments where anaerobic conditions promote the growth of sulphate-reducing bacteria and subsequent H₂S production. Occupational accident records were retrieved from scientific databases, occupational injury databases, and industrial accident databases, specifically from the following sources:

• Scientific databases: Web of Science, Scopus, Google Scholar

• Occupational injury and incident databases: Occupational Safety and Health Administration (OSHA), WorkSafeBC

• Industrial accident databases: Zentrale Melde und Auswertestelle für Störfälle und Störungen in verfahrenstechnischen Anlagen (ZEMA), Accidental Release Inventory and Analysis (ARIA)

The search was conducted using the following keywords: “hydrogen sulfide accident”, “industrial accident H₂S”, “hydrogen sulfide” AND “organic waste” AND “injury”, and “hydrogen sulfide” AND “occupational accident”.

For further analysis, only accidents that met the following criteria were included:

1. Origin of H₂S: only cases where H₂S was generated through microbial activity, such as the decomposition of organic waste, were considered.

2. Health impacts: only articles describing injuries or health consequences related to H₂S exposure were included.

3. Data availability: selected cases contained detailed information of H₂S concentration, formation conditions, and accident circumstances.

Temporal trends in fatal occupational accidents were evaluated using simple linear regression, with the coefficient of determination (R²) and the corresponding p-value used to assess the strength and statistical significance of the trend. All statistical analyses were performed using the Statistica software (Dell Inc., Round Rock, TX, USA).

Subsequently, threat and top event identification was conducted using the Bow-Tie method. This approach serves as a visual risk analysis tool that systematically connects the causes (threats) and consequences of a top event (incident). The starting point of the analysis is the hazard, defined as an operation, activity, or material with the potential to cause harm.

On the left side of the Bow-Tie diagram, threats are listed, these are situations, events, or conditions that may lead to the occurrence of the top event. Each threat can be managed through preventive barriers aimed at preventing escalation toward the top event. In the center of the diagram, the top event is depicted, representing the point at which control over the hazardous situation is lost. On the right side of the diagram, the consequences are shown, these are the adverse outcomes resulting from the top event, the severity of which can be reduced through mitigation barriers⁹⁾.

The analysis also includes degradation controls that protect the effectiveness of individual barriers. Degradation factors, phenomena that diminish the performance of preventive or mitigation measures, are detected and eliminated through these controls to ensure the continued functionality of the barriers. The Bow-Tie method thus enables systematic identification of hazards, threats, the top event, consequences, protective measures, degradation factors, and their controls, providing a comprehensive framework for risk management⁹⁾.

The Bow-Tie analysis was conducted as a structured qualitative (non-automated) analysis. Individual Bow-Tie elements (hazard, threats, top event, consequences, preventive and mitigative barriers, including degradation factors) were identified manually based on a systematic review of accident descriptions, followed by discussion and consensus among the co-authors. The analysis did not rely on any automated or algorithmic data processing. The Bow-Tie diagrams were graphically developed using Microsoft Excel (Microsoft Corp, Redmond, WA, USA).

Results and Discussion

Event analysis in databases

Using the aforementioned keywords, a total of 105 accidents were identified and subsequently analyzed (Table 1).

Table 1. Overview of the database and literature search strategy and results.

Data source category	Database	Search keywords (examples)	Records identified (n)	Incidents included in analysis (n)
Scientific databases	Web of Science, Scopus, Google Scholar	hydrogen sulfide accident; industrial accident H2S; hydrogen sulfide AND organic waste AND injury; hydrogen sulfide AND occupational accident	90	36
Occupational injury and incident databases	OSHA	same as above	240	58
Occupational injury and incident databases	WorkSafeBC	same as above	1	1
Industrial accident databases	ARIA	same as above	56	9
Industrial accident databases	ZEMA	same as above	1	1
Total	–	–	388	105

In the ARIA database, 56 records were found based on the search keywords, but only 9 records met the selected criteria. In the OSHA database, out of 240 recorded accidents, 58 cases were used for analysis. From the WorkSafe British Columbia and ZEMA databases, only one record from each met the selection criteria. An additional 36 events were extracted from scientific articles found in Scopus and Web of Science databases.

A substantial proportion of the initially identified records was excluded during the data selection process. The most frequent reason for exclusion was the chemical origin of hydrogen sulfide (eg, industrial production or chemical reactions), whereas the objective of this study was to analyze exclusively accidents associated with microbiologically generated H₂S. Additional exclusion reasons included the absence of information on health impacts or insufficient details regarding accident circumstances, which prevented reliable classification and subsequent analysis. Additional information on the analyzed cases is provided in eTable 1.

The highest number of incident records originated from the OSHA database, maintained by a United States governmental agency, which provided a large number of records, including historical cases (49 fatal accident records).When considering only the sample from the United States, the data appear scattered without a clear trend, which corresponds to a low coefficient of determination (R²=0.004, p=0.012), indicating a non-parametric nature of the data.

Furthermore, the database included records from ten countries: Bangladesh, China, Denmark, France, Italy, Japan, Canada, Germany, Pakistan, and the United Kingdom. The accident records span from 1980 to 2023. However, 18 cases lack a reported accident date. The highest frequency of fatal incidents was recorded in 2016 and 2023. The data exhibited considerable variability, making it difficult to determine a clear trend in the number of fatalities over time. The coefficient of determination was low (R²=0.189), although the trend was statistically significant (p=0.001).

Overall, both datasets described in the preceding paragraphs, namely the dataset of accidents recorded in the United States and the global dataset including accidents from other countries, exhibit considerable variability. However, several factors may further complicate the determination of a clear trend. It is highly likely that the presented data in both cases do not represent the complete dataset, for two main reasons. The first reason is that it is impossible to retrieve all records, particularly historical ones. As a result, the presented dataset is more of a sample rather than a comprehensive collection. The second reason is that the capacity for digitalizing records has increased over time. Therefore, the frequency of fatalities in recent years may not be solely due to a higher mortality rate but may also reflect a general increase in digitalization efforts and record-keeping standards compared to the past.

Figure 1 provides an overview of accident severity based on the number of fatalities. Incidents involving a single fatality are the most frequent (39 cases), suggesting that H₂S exposure often affects individuals entering contaminated environments, such as industrial facilities or confined workspaces. The second most common category comprises incidents with two fatalities (29 cases), representing approximately 33% of all fatal accidents (29 out of 87). In several documented cases^10,11,12), as well as OSHA investigation reports from 1987, 1998, 2003, 2008, 2014, 2017, 2020, 2021, and 2023, the secondary fatality occurred as a result of an attempted rescue of the initially affected worker, leading to secondary fatal exposure.

Fig. 1.

Frequency of deaths due to hydrogen sulfide intoxication by number of victims

Additionally, cases involving multiple fatalities were recorded, specifically incidents with three victims (7 cases). One notable example is an accident at a composting facility in Canada in 2008, where five additional workers risked their lives by entering a contaminated facility in an attempt to save a colleague. Three workers did not survive the accident¹³⁾. Accidents involving four victims (11 cases) and even six victims (1 case) were also recorded. In total, 18 accidents resulted in non-fatal injuries, where the affected workers required hospitalization.

Figure 2 presents the number of accidents categorized by the sector of economic activity. The highest proportion of incidents was recorded in activities related to wastewater management, water supply, and waste handling. This category includes sewers, wastewater treatment plants (WWTPs), WWTP sludge management facilities, pumping stations, composting plants, biogas plants, and waste collection centers. The “sewer systems” category accounted for 28% of all recorded incidents. Accidents in these settings typically occur during the cleaning of blocked pipelines or during inspection or repair of sewer shafts. Almost one-quarter of all recorded cases involved wastewater treatment plants, pumping stations, and facilities specializing in sludge management at WWTPs. The “Agriculture, forestry and fishing” sector represented 12% of the cases, primarily associated with animal farming operations. Intoxications occurred near areas where manure was being mixed, loaded/unloaded, or where connecting pipes were being cleaned. The “Manufacturing” sector included a wide range of processing facilities, such as food production (meat, dairy, sugar), textile manufacturing, fertilizer and feed production, and paper and cardboard manufacturing. This category also encompassed rendering plants, with a total of six recorded incidents. The least represented category involved accidents occurring during transportation and storage activities, specifically related to maritime transport and the movement of materials in tankers.

Fig. 2.

Number of accidents across different sectors of human activity

Accidents occurred in more than 60% of cases within confined spaces such as tanks, shafts, and pits (see Figure 3). In these areas, workers were typically engaged in pump repairs, cleaning activities, or routine inspections. Fatal intoxication occurred in 82% of cases precisely in these workplaces. A total of 23 accidents happened during pipeline cleaning or the maintenance of shut-off valves, with only one of these incidents not resulting in a fatality. A detailed overview of the results is provided in Table 2.

Fig. 3.

Number of accidents in specific parts of facilities/operations

Table 2. Summary of accidents, fatalities, and causes by economic activity.

Economic activity	Number of accidents	Number of fatalities	Fatalities per accident	Accident cause (location and activity)
Manufacturing	17	29	1,71	storage areas, pipelines, tanks, and pits – inspection, cleaning, maintenance, and confined space entry
Transportation and storage	9	15	1,67	tanks and cisterns – inspection, cleaning, unloading, and confined space entry
Water supply, sewerage, waste management and remediation activities	66	105	1,59	sludge line, piping, tank, manhole, pit, fermenter, and pump – cleaning, inspection, and repair
Agriculture, forestry and fishing	13	19	1,46	tanks, piping, and storage areas – cleaning, inspection, repair, and transfer operations

Frequently, exposure led to the loss of olfactory perception, followed by unconsciousness, cardiac arrest, and coma. In some cases, H₂S poisoning was the sole cause of death. In others, death resulted from a combination of factors, such as loss of consciousness due to H₂S exposure followed by a fatal fall. Only 18 accidents did not result in the death of one or more workers. However, in all cases, the affected workers were transported to hospitals for treatment and observation. In cases of intoxication, not only the duration of gas exposure but also the worker’s health condition is critical. The physicochemical properties of the environment — particularly the composition, temperature, and pH of the wastewater or organic material — also significantly influence the form and concentration of the released toxic gas.

Temperature is an important factor influencing microbial hydrogen sulfide generation. The data show that the highest number of deaths occurred in summer, supporting the assumption that work in facilities prone to microbial production may involve a higher intoxication risk during warmer periods. However, this cannot be universally applied. Sewer maintenance is often scheduled outside winter, and temperatures inside sewer systems remain relatively stable year-round.

Identification of threats and top events

The identification of threats and top events in the article was based on the analysis of available records of occupational accidents caused by the microbial generation of H₂S. Records containing detailed information about the course of the accidents were selected, primarily from one study¹⁴⁾ and an accident report available in the ARIA database¹⁵⁾. These cases were deliberately chosen based on the availability of sufficiently detailed event descriptions, as both provided comprehensive information on accident progression, operating conditions, mechanisms of H₂S generation, barrier failures, and exposure consequences. The selection was therefore not random but was based on the suitability of the data for conducting a Bow-Tie analysis. Based on these records, as well as additional information from other sources, threats and causal dependencies leading to consequences — such as fatalities or health damage due to H₂S exposure — were identified. The identification of individual elements of the Bow-Tie model was carried out in accordance with the framework proposed by book A Concept Book for Process Safety⁹⁾. Subsequently, graphical representations of these models were developed and are presented in Figure 4 and Figure 5. Their structured tabular representation is provided in the supplementary materials (eTable 2 and eTable 3).

Fig. 4.

Example of an emergency scenario for the hazard “Storage of material in a sump” and the consequence “Release of H₂S from the sump and worker intoxication”

Fig. 5.

Example of an emergency scenario for the hazard “Maintenance and cleaning of the sewer system” and the consequence of worker fatalities

It is important to note that the models do not precisely depict how the accident events actually unfolded. The authors’ intention was not to reconstruct the events as accurately as possible, as is typically done during investigative processes. Given the uncertainty about the available information, such an approach would not have been feasible. Instead, the authors aimed to capture the events as broadly as possible, with a focus on identifying potential preventive measures. Thus, the studies^14,15) should be understood as representative examples of activities associated with the risk of exposure to fatal concentrations of H₂S.

For H₂S to form, optimal conditions for the growth and proliferation SRB must be present at the site. Key factors influencing this process include temperature, moisture, material composition (ie, an appropriate nutrient balance), redox potential, and pH. The growth of bacteria also follows certain dynamics. Based on these prerequisites, possible preventive measures can be proposed, such as altering conditions to inhibit SRB growth or minimizing the residence time of material in each location. However, regulating these parameters can be particularly challenging in certain facilities, such as sewer systems (see Figure 5).

In facilities where organic material is intentionally stored, changes in environmental parameters can be implemented (see Figure 4). Examples include maintaining temperatures outside the optimal range for SRB (SRB can thrive between 4–85°C, with an optimum around 30–35°C¹⁶⁾), or converting anaerobic conditions into aerobic ones (eg, by introducing air and homogenizing the material at regular intervals). This can be achieved by pneumatic mixing triggered at preset times.

Other approaches include altering the composition of the material (adjusting the nutrient ratio) or limiting the maximum storage time. This can be enforced operationally by regularly inspecting incoming materials and establishing strict storage time requirements in facility operating rules. In the event of prolonged malfunctions, it is essential to halt further waste intake to minimize the risk of dangerous concentration forming.

Mitigating barriers aim to reduce the consequences if areas with dangerous H₂S concentrations do form. One effective measure is to ensure a physical separation between areas where H₂S could accumulate and areas where workers are present. In the emergency scenarios shown in Figure 4, this can be achieved by installing a solid cover separating the storage tank from the work hall. Hedlund and Madsen¹⁴⁾. discuss the failure of such a barrier, reporting a gas release incident that occurred because a cover was not closed immediately after unloading, likely due to worker negligence or oversight. Thus, barriers should be designed to be “resistant” to human error, for example, through the installation of automatically closing lids (triggered by material flow sensors) or by equipping covers with visual and auditory alarms to signal if left open too long. Such measures could also prevent the release of other hazardous gases (eg, ammonia, methane, carbon dioxide), which can also form under optimal microbial activity conditions and pose risks to workers. Another important mitigating barrier is ensuring regular air exchange within the facility using forced or natural ventilation. However, natural ventilation (eg, utilizing the chimney effect) can be highly dependent on meteorological conditions.

The preventive and mitigating barriers described above are difficult to apply in facilities associated with sewer system operations (see Figure 5). Figure 5 presents emergency scenarios related to the maintenance of selected sewer system components. Here, regulating parameters that influence SRB growth is virtually impossible.

In sewer systems, most of the conditions necessary for microbial activity and H₂S formation are present year-round: constant water presence, suitable pH, appropriate temperature, and the availability of nutrients such as organic material from households (food scraps, fecal matter), decaying leaves, grass, and plants entering via street drains.

Organic material may sediment within the sewer system, creating anaerobic environments and triggering microbial activity. If disturbed, this sediment may suddenly release dangerous H₂S concentrations. The amount of organic matter in sewers can increase significantly during the use of garbage disposals in households (common in some countries) or due to certain industrial activities and heavy rainfall events that wash large quantities of organic material into retention or overflow basins. Thus, it is practically impossible to prevent the creation of optimal conditions for microbial activity in sewer systems, and preventive barriers cannot always be effectively applied (see Figure 5).

The most effective way to prevent intoxication during maintenance and cleaning of sewer system components is to entirely prevent worker exposure to areas where dangerous H₂S concentrations might form. This means organizing work procedures to avoid the risk of intoxication altogether.

Sewer system components (eg, sewers, manholes, overflow chambers, pump station shafts) are generally classified as confined spaces with elevated injury risk. In many countries, legal regulations mandate the development of work permit systems for operations in such spaces, including documented work planning, ventilation methods and duration, and mandatory gas monitoring¹⁷⁾.

United States regulations, for example, specify that hazard assessments must identify potential toxic or flammable gas risks in confined spaces. Verification measurements must be conducted prior to entry, in the following order: oxygen concentration, flammable gases, and toxic gases. Measurements must be carried out for sufficient time (considering instrument response time and sample extraction times for long probes). Gas concentrations must continue to be monitored throughout the work process¹⁸⁾.

Rescue operations constitute a special chapter. As shown by the analysis presented earlier, some accidents resulted in multiple fatalities precisely due to improperly conducted rescue attempts¹⁹⁾. Rescuers must be properly trained, equipped, and prepared for confined-space rescue operations. Without these conditions met, workers should not attempt rescue efforts.

However, some incidents reveal that dangerous gas concentrations can rise very suddenly, rendering previous safety measures ineffective. One such case is described¹⁵⁾, though the report is relatively brief. The accident occurred during regular cleaning of a settling tank integrated into a sewer system. The tank had a volume of 30 m³ and a depth of 5 meters. The company reported that workers were equipped with personal protective equipment (PPE) and gas detection devices. The report mentions the formation of a “gas pocket” of unspecified concentration.

“Gas pockets” refer to localized areas of high gas concentration. In the case of gases heavier than air, such as H₂S, these pockets can remain trapped in depressions or uneven terrain and may also form beneath sediment surfaces. Disturbing the sediment (eg, through mixing or pumping) can suddenly release accumulated gases. Similar phenomena have been documented in other accidents as well²⁰⁾. When working in confined spaces within sewer systems, this risk must always be considered.

Others

Use of PPE

The use of PPE, specifically respirators, is considered a mitigating barrier. Within the hierarchy of injury prevention measures, it ranks as the last line of defense. However, special attention must be paid to the selection of appropriate PPE, particularly regarding the type of respirator and filter used²¹⁾. If an unsuitable respirator or filter is chosen, PPE may become ineffective at higher concentrations of H₂S, losing its protective function. According to American Industrial Safety guidelines, if the concentration of H₂S is below 100 ppm, an air-purifying full-face respirator may be used, provided that the filter cartridge is specifically rated for H₂S²²⁾. For higher concentrations, a fully self-contained breathing apparatus is necessary. Recommendations regarding the selection of appropriate respirators are also provided by the United States National Institute for Occupational Safety and Health²³⁾. Moreover, proper training and practical instruction of workers are essential. This includes establishing formal policies for the selection and use of respirators during specific activities conducted by the company.

Ignoring safety measures

Nearly three million workers die each year due to occupational accidents and diseases²⁴⁾. A significant proportion of these incidents could be prevented simply by adhering to established safety regulations. For example, many accidents result from failure to use protective helmets or goggles during manual work²⁴⁾. Noncompliance with occupational health and safety rules is a persistent issue that can be addressed through a combination of awareness-raising efforts, technological solutions, and improvements in organizational culture. Prevention is consistently more cost-effective and efficient than dealing with the consequences of accidents.

Analysis of accident records reveals that in several cases, exposure occurred because safety measures were ignored — either intentionally or unintentionally — by the workers involved. Examples of such incidents are reported in several studies and reports^{10,25,26,27,28)}.

There are many reasons why workers may ignore safety protocols. For instance³⁰⁾, describe how repeated exposure to hazards can lead to habituation, where workers become desensitized to warning signs. Employees performing the same routine tasks daily may cease to perceive the risks associated with their activities. According to Safe Work Australia²⁹⁾, 22% of laborers, machinery operators, and drivers admitted to violating safety rules to meet deadlines, despite being aware of the risks at their workplaces. Furthermore, 28% of workers acknowledged that they willingly accepted workplace risks. Time pressure is often cited as a major reason why employees disregard safety regulations, prioritizing meeting demanding deadlines over maintaining safe practices. Traditional training methods are not always effective, as they may fail to influence automatic attention processes. However, experiential learning using virtual reality (VR) has shown promise in enhancing hazard awareness. In one study³⁰⁾, simulated accidents in VR significantly improved participants’ attention to hazards even one week after the training. This effect is attributed to the fact that realistic simulations activate preventive attention mechanisms and increase sensitivity to potentially risky situations. Research suggests that experiencing dangerous scenarios in a controlled environment can lead to deeper internalization of safety rules and improved recognition of warning signs in real-world settings.

Conclusion

The analysis indicates that several accidents show characteristics of major industrial incidents, particularly due to multiple fatalities or injuries. Fatal intoxication occurred in 82% of all recorded cases, yet the health consequences for survivors can also be severe and often permanent, including reduced working capacity or the need for long-term medical treatment. Reports from available databases describe cases of neurological damage and lasting respiratory impairment among affected workers.

Although no statistically significant trend in case frequency was identified, the prevention of such events remains essential. The Bow-Tie method proved useful for analyzing accident scenarios and identifying barriers to reduce their occurrence. The primary preventive measure is to influence physical or chemical conditions to limit the activity of sulfate-reducing bacteria, which are responsible for hydrogen sulfide formation. Key parameters include temperature, pH, oxygen solubility, and redox potential, all of which affect bacterial metabolism. However, practical knowledge regarding the feasibility and cost-effectiveness of such interventions remains limited, and implementing measures like cooling or chemical dosing may pose financial challenges.

In some facilities, particularly in sewer networks and similar infrastructure, modifying environmental conditions is not possible. In these cases, a better understanding of bacterial development under varying conditions would make it possible to assess the level of hydrogen sulfide risk. Based on such assessments, personnel access to these environments could be managed more effectively to prevent future incidents.

Limitations

The use of incomplete datasets may represent a limitation of this study. The analyzed occupational accident records do not constitute a comprehensive overview of all incidents related to hydrogen sulfide exposure, particularly with respect to older events, which are often insufficiently documented or no longer retrievable. Moreover, differences in reporting systems, legislative requirements, and the degree of digitalization across countries and overtime may introduce bias into the dataset. For these reasons, the presented data cannot be used for reliable assessment of temporal trends.

However, the primary objective of this study was the identification of typical risk scenarios and contributing factors associated with microbiologically generated H₂S. From this perspective, the use of a heterogeneous and incomplete dataset is considered appropriate and informative, as it enables a qualitative analysis of accident mechanisms and safety-relevant patterns.

Conflicts of interest

The authors declare that there are no conflicts of interest.

Funding

This work was supported by the Internal Grant Agency of the Mendel University in Brno – Faculty of Agronomy, No. IGA25-AF-IP-030, Determining the effect of selected factors on the formation of hazardous hydrogen sulfide concentrations in the working environment, financed by IGA AF MENDELU.

Supplementary material

This article contains supplementary material, which is available in the online version.

Supplementary eTable 1

Supplementary eTable 2

Supplementary eTable 3