Total Reduced Sulphur

A Brief Summary of Health Effects

Prabjit Barn, Tom Kosatsky

What is total reduced sulphur?

Total reduced sulphur (TRS) is a mixture of several compounds which contain a sulphur component in the reduced form. The most common TRS compounds are: (1) hydrogen sulphide (H2S), (2) methyl mercaptan, (3) dimethyl sulphide and (4) dimethyl disulphide.1 Much of the research on TRS has focused on H2S, which is considered to be the largest fraction and most toxic component of the mixture.1,2 Here, H2S will be used to describe TRS-related effects.

TRS and H2S are typically emitted from industrial sources, including natural gas plants, petroleum refineries and pulp and paper mills. H2S is also associated with municipal sewers and sewage treatment plants.

In BC, TRS and H2S are measured at some monitoring stations.

BC air quality guidelines

Table 1. Summary of BC quality guidelines for TRS and H2S.

Compound3 Standard/guideline* AveragingTime Value+
TRS BC Level A 1-hr 7 µg/m3 (5 ppb)
TRS BC Level A 24-hr 3 µg/m3 (2 ppb)
TRS BC Level B 1-hr 28 µg/m3 (20 ppb)
TRS BC Level B 24-hr 6 µg/m3 (4 ppb)
H2S BC Level A 1-hr 7.5 -14 µg/m3 (5 ppb)
H2SBC Level A24-hr4 µg/m3
(3 ppb)
H2S BC Level B 1-hr 28 – 45 µg/m3 (20 – 32 ppb)
H2S BC Level B 24-hr 6 – 7.5 µg/m3 (4 – 5 ppb)

BC Level A = objective for new and proposed discharges and, within the limits of the best practicable technology, to existing discharges by planned staged improvements for these operations.

BC Level B = intermediate objective for all existing discharges to reach within a period of time specified by the Director, and as an immediate objective for existing discharges, which may be increased in quantity or altered in quality as a result of process expansion or modification.

+Unit conversions for TRS and H2S: 1 ppm = 1000 ppb; 1 ppm= 1.39 mg/m3; 1 mg/m3 = 1000 µg/m3

H2S Inhalation toxicity

H2S is readily absorbed across the lungs.4 Once in the body, H2S primarily acts by preventing cellular uptake of oxygen through the inhibition of cytochrome oxidase.5,6 Target organs are those with the highest oxygen demands, including the respiratory, nervous, and cardiovascular systems.5,6

H2S Health effects

Case studies in occupational settings and animal studies inform the majority of the knowledge on H2S-related health effects, particularly those related to acute exposures. A few studies have also been conducted in H2S affected communities. This information is briefly summarized below.

Acute health effects due to inhalation exposure

H2S has a characteristic “rotten egg” smell which can be detected at low levels (0.001-0.13 ppm).6 Continued exposure or exposure to higher levels (e.g. 100 pm between 2 -15 min) can lead to olfactory nerve fatigue making odor itself a poor indicator of the presence of H2S.7

Eye irritation and inflammation occur at levels above 100 ppm within a period of 2-15 minutes of exposure.6 Although eye irritation has also been reported at lower concentrations (10 ppm), it is not clear if these effects are due to H2S alone, or in combination with exposure to other gases.6 Severe irritation of the nose, throat and lungs can occur at levels at or above 100 ppm. Because of its strong smell, olfactory Pulmonary oedema (fluid accumulation in the lungs) may occur at levels above 250 ppm.6 Above 500 ppm, exposure can cause loss of consciousness, also known as “knockdown.” 8,9 At higher levels (500-1000 ppm), central nervous system depression, tissue hypoxia, cardiovascular effects, and respiratory arrest can occur, which can result in death.6,8

Long term health effects due to acute H2S exposures

A variety of long term, persistent health effects have been reported by individuals who have experienced acute exposure to H2S resulting in “knockdown” in occupational settings. Typical symptoms include neurological effects (headaches, impaired memory, problems with focusing), respiratory effects (wheeze, shortness of breath), as well as ocular dysfunction (corneal abrasions).5,8 Damage to brain structures, including to the basal ganglia and cortex, have been reported during follow-up of individuals occupationally exposed to acute levels of H2S.5

Long term health effects due to chronic exposure to low H2S levels

Occupational studies have found evidence for adverse health effects, including bronchial hyper responsiveness and mood disorders. Unfortunately, many of these studies lack accurate exposure data making it difficult to assess the actual level of exposure. 4

A few studies have investigated long-term health effects in communities exposed to short-term periods of high or long-term periods of low industrial-based emissions.10-14 These studies have found some evidence of respiratory and central nervous system- related effects in residents exposed to ambient TRS and/or H2S. Symptoms include breathlessness, cough, eye and nasal irritation and nausea. These studies have several limitations, which make it difficult to assess the level of health risk posed to community members exposed to ambient TRS and/or H2S. Due to the potential chronic nature of exposure in communities, it is difficult to separate acute and chronic exposure-related effects. Exposure classification is an issue with many of the studies. Some studies did not collect monitoring data, while others used mean daily or annual concentrations to define low and high exposures to community members which may not be representative of actual exposures. Additionally, although participants were not told which exposure period (high versus low exposure days) corresponded to collection of questionnaire data, the presence of a detectable odor very likely alerted participants to elevated levels, which consequently could have led to potential reporting bias. In studies where self-reporting of symptoms was compared between residents of an exposed versus reference community, knowledge of particular industry sources could have also led to reporting bias.

Study findings are summarized in table 2.

 Exposure reduction measures

Individuals concerned about their exposures to TRS or H2S during an air quality advisory can follow some measures to help reduce exposures, including:

  1. Avoiding areas with known industrial emissions of TRS and/or H2S;
  2. For individuals experiencing symptoms like respiratory irritation, staying indoors until levels decrease; it is important to reduce indoor sources of air pollution during this time;
  3. Continuing to control medical conditions such as asthma and chronic respiratory disease. If symptoms continue to be bothersome, individuals should seek medical attention.

Table 2. Summary of studies investigating the health impacts of community level exposures to TRS and/or H2S


Study Study Design Pollutant concentrations Health outcomes measured Findings
Acute Exposures
et al
1992 13
Self-administered health questionnaires were completed by residents during an “exposure” (n=60) and “reference” (n=66) period 4 months apart in the same community.24-hr and1-hr averages were collected for ambient levels of H2S and SO2 during each period.24-hour averages of ambient TRS measurements collected continuously throughout study period. Exposure period:
H2S 24-hr avg: 35 ug/m3 and 43μg/m3SO2 1-hr avg: 3 μg/m3
Reference period:
H2S 4-hr avg: 0. 1-3.5

SO2 1-hr avg: 3 μg/m3
Respiratory and central nervous system effects Significantly higher prevalence of symptoms during exposure period:- Breathlessness (35% vs 2% for exposure and reference periods, respectively ),
– mental symptoms (10% vs 0%),
– eye symptoms (22 % vs 2 %),
– cough or pharyngeal irritation (15 % vs 5 %)
– nausea (14 % vs 3 %).Nasal symptoms were higher during reference (20%) vs. exposure (8%) period.Participants who completed questionnaires during both periods reported more breathlessness in the exposure period.
Martila et al. 1995 14 Series of self-administeredquestionnaires were completed by residents in a TRS-affected community (n=81).A baseline survey was completed during a lowexposure period, and 6 follow up surveys were completed 1 day following a day categorized as low, medium, or high exposure. Low exposure:
10 μg/m3
Medium exposure:
High exposure: >30-82 μg/m3
Respiratory and central nervous system effects Mean daily intensity of some symptoms was significantly higher on medium and high exposure days compared to low exposure days, including eye and respiratory symptoms, as well as headaches.Probability ratios were calculated to assess likelihood of experiencing symptoms on higher versus lower exposure days; all were higher for medium and high days, and increased with increasing exposure. For example, probability ratios for eye symptoms were: 3.17 (95% confidence interval: 1.21,7.47) for medium exposure and 5.0% (95% CI: 1.66, 12.65).
Campagna et al. 2004 12 The number of hospital visits for children and adults in the daysfollowing high and low exposure to H2S and TRS were investigated in two cities.High exposure was defined as at least one rolling daily average of H2S or TRS > 30 ppb while low exposure was defined as all rolling daily averages of H2S or TRS < 30 ppb.Mean percent changes (MPC) between the number of hospital visits following a day of high exposure versus a low exposure day were calculated, as a measure of association between exposure and hospital visits. Levels ranged from < 30 ppb to > 30 ppb( individual levels not reported) Respiratory and digestivedisease-related hospital visits For children less than 18 years of age, a positive association was found between asthma hospital visits and 1-day lagged TRS levels (MPC = 25%; 95% CI:9–44). For adults, a positive association was found between asthma-related hospital visits and H2S levels on the previous day (83%; 95% CI: 4–221).A positive association also was found between hospital visits for all respiratory diseases, and H2S and TRS levels on the previous day, at some stations, for children (e.g. station 7: 26%; 95%CI: 5–51).No association was found between contaminant levels and hospital visits for all digestive diseases.Levels of H2S and TRS in communities were not provided.
Chronic Exposures
Partti-Pellinen et al. 1996 11 Self-administered health questionnaires were completed by 336 residents of an exposed community, and 380 participants in a reference community. Exposed area:
TRS: Mean annual: 2-3µg/m3 24-hr: 0-56 µg/m3Max: 1-hr: 155 µg/m3
SO2: Mean annual: 1 µg/m324-hr: 0-24 µg/m3
Max 1-hr: 152 µg/m3
Reference area:
SO2: Mean 1 µg/m3Max: 30 µg/m3
Respiratory and central nervous system effects Residents of the exposed area reported higher occurrences of eye and nasal symptoms, as well as cough and headache for the previous 4 week and previous 12 month periods. Corrected odds ratios (OR) for all symptoms were calculated. For example, the ORs of reporting a headache in the exposed community were 1.83 (95 %confidence interval= 1.06 – 3.15) and 1.70 (95 % confidence interval = 1.05 – 2.73) for the previous 4 week and 12 month periods respectively.
Inserra et al.2004 15 Computer assisted tests wereperformed by participants (n=171 in exposed area, 164 in reference area).Air modeling was conducted, based on community-wide H2S monitoring, and highest 1-hr concentrations were used to classify each of 14 residential monitoring locations. Levels not reported. Neurobehavioural performance No significant differences in testperformances by residents in exposed and reference areas were found.No information on other potential sources, including employment at the mill, was collected.
Legator et al.2001 10 Questionnaires completed byresidents in two exposed communities (n=223) and three reference communities (n=170). No ambient monitoring conducted. Respiratoryand central nervous system effects The number of symptoms reported byresidents in exposed communities was significantly higher than those in reference communities.Symptom categories with tge highest odds ratios were central nervous system (OR =12.7, 95% CI: 7.59, 22.09), respiratory (OR= 11.92, 95% CI: 6.03, 25.72) and blood(OR = 8.07, 95% CI: 3.64, 21.18).Reporting bias was likely since residents in exposed categories were recruited according to their proximity to a large industrial source of H2S.




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