||Outbreak of ethyl mercaptan exposure and health assessments in a community
||Institute of Environmental and Occupational Health
mass psychogenic illness
結果：本次暴露事件中通報衛生局的人數為41位，皆來自於一國民小學，其中包含40位學生與1位老師。平均年齡為10歲，最常見的症狀包括頭暈（87.5%）、全身無力（50%）、噁心（47.5%）、呼吸困難（10%）與嘔吐（10%）。其臨床特徵和實驗室檢查結果在性別上並無顯著差異，暴露事件一星期後之追蹤並無發現持續症狀與後遺症。在121位的受訪居民中，有10位聞到氣味，其中6位在A區，4位在B區，盛行率越隨靠近逸散源有上升趨勢(p = 0.009)；其中2位居民自訴症狀，盛行率亦隨靠近逸散源具上升趨勢(p = 0.041)。然而，此學校是位於B區，此區內並無其他通報衛生局的個案，受訪者中也無症狀。雖然此事件具群體心因性疾病的特徵，但它具有明確且可被偵測之物質，因此並不全然符合可被接受之定義。
Background: Ethyl mercaptan is a volatile liquid and may cause irritation of the eye, skin, and mucous membranes and lead to nausea and headache in humans in low-level exposures. Because of its extremely strong garlic-like odor with an odor threshold of 1 ppb, ethyl mercaptan is widely used as an odorant for liquefied natural gas. In an outbreak of exposure to ethyl mercaptan in natural gas in the Tainan City of Taiwan, a cluster of symptomatic victims were observed in an elementary school where the exposure level was low. Therefore, we conducted an investigation to study association between the health effects and the exposure to ethyl mercaptan.
Methods: We reviewed the medical records of all victims reported to the Bureau of Health of the Tainan City and collected the data on clinical characteristics and laboratory tests. The data on exposure levels were retrieved from Environmental Emergency Response Team, which were measured using a photoionization detector. A telephone follow-up was conducted on each reported victim one week after the incident to identify persistent symptoms and sequelae. In addition, we divided the area surrounding the exposure source into Zones A, B, and C from near to far from the source and divided each zone further into four regions according to the directions relative to the source. Ten residents were interviewed in each region after the incident to collect information for further analysis.
Results: A total of 41 victims were reported to the bureau, and all were from an elementary school, including 40 pupils and a teacher. The average age of the pupils was 10, and the most common symptoms were dizziness (87.5%), general weakness (50%), nausea (47.5%), dyspnea (10%), and vomiting (10%). There were no significant differences of clinical signs and biochemical test results between two genders. Neither persistent symptoms nor sequelae were reported one week after the incident. Ten of the 121 interviewees reported sensing the smell, including 6 in Zone A and 4 in the Zone B, and the prevalence had an increasing trend toward the sources (p = 0.009). Two additional symptomatic victims were identified among the 121 interviewees, and the prevalence also had an increasing trend toward the sources (p = 0.041). However, the school was located in Zone B, and there were no victims reported to the bureau, nor symptomatic victims identified in this zone. Whereas the incident had some features of mass psychogenic illness (MPI), it did not meet the generally accepted criteria, because there was an identifiable agent.
Conclusions: In this incident, although ethyl mercaptan was clearly the causal agent, the symptoms reported by the victims could not totally be attributed to the exposure to this agent, indicating effects of psychological factors. Traditionally, MPI is defined as a spectrum of symptoms without any identifiable actual causal agent. However, in many cases, like in this incident, the observed health effects cannot be explained solely by the exposure to the causal agent. We propose using the term of “secondary mass psychogenic illness” to include incidents in which there are identifiable agents but the exposure to the agent can account for only a part of the health effects. The recognition of the psychological factors through using this term can highlight the importance of treating these factors in the management of such incident.
1. Background 1
1.1 Physical and chemical characteristics of ethyl mercaptan 1
1.2 Psychological effects of chemical exposures 2
1.2.1 Variable psychological phenomena 2
1.2.2 Impact on society 3
1.3 Presentation of chemical incident 4
2. Methods 6
2.1 Study subjects 6
2.2 Measurement of exposure 6
2.3 Measurement of health outcomes 7
2.4 Statistical analysis 8
3. Results 9
3.1 Health assessments of exposed victims 9
3.2 A one-week-later follow up 10
3.3 Interview assessment of residents 10
4. Discussion 11
4.1 Contradiction of the symptoms 11
4.2 Characteristics of MPI 12
4.3 Incompatible with MPI 13
4.4 Secondary mass psychogenic illness (SMPI) 14
4.5 Two variants of psychological effects 15
4.6 Odor assessment 16
4.7 Management and prevention of MPSI 18
4.8 Strength and limitations 19
5. Conclusions 21
6. References 22
7. Appendix 36
7.1 Review chart for medical records of all victims 36
7.2 Questionnaire of telephone follow-up for all victims after one week 38
7.3 Questionnaire of follow-up for residents interview after one week 40
7.4 Telephone Interview Record 42
7.5 Interview of the teacher 46
7.6 News of the chemical accident 54
7.7 Material Safety Data Sheet (MSDS) of ethyl mercaptan 55
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