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Friday, 04 March 2011 18:00

Case Studies Illustrating Methodological Issues in the Surveillance of Occupational Diseases

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The documentation of occupational diseases in a country like Taiwan is a challenge to an occupational physician. For lack of a system including material safety data sheets (MSDS), workers were usually not aware of the chemicals with which they work. Since many occupational diseases have long latencies and do not show any specific symptoms and signs until clinically evident, recognition and identification of the occupational origin are often very difficult.

To better control occupational diseases, we have accessed databases which provide a relatively complete list of industrial chemicals and a set of specific signs and/or symptoms. Combined with the epidemiological approach of conjectures and refutations (i.e., considering and ruling out all possible alternative explanations), we have documented more than ten kinds of occupational diseases and an outbreak of botulism. We recommend that a similar approach be applied to any other country in a similar situation, and that a system involving an identification sheet (e.g., MSDS) for each chemical be advocated and implemented as one means to enable prompt recognition and hence the prevention of occupational diseases.

Hepatitis in a Colour Printing Factory

Three workers from a colour printing factory were admitted to community hospitals in 1985 with manifestations of acute hepatitis. One of the three had superimposed acute renal failure. Since viral hepatitis has a high prevalence in Taiwan, we should consider a viral origin among the most likely aetiologies. Alcohol and drug use, as well as organic solvents in the workplace, should also be included. Because there was no system of MSDS in Taiwan, neither the employees nor the employer were aware of all the chemicals used in the factory (Wang 1991).

We had to compile a list of hepatotoxic and nephrotoxic agents from several toxicological databases. Then, we deduced all possible inferences from the above hypotheses. For example, if hepatitis A virus (HAV) were the aetiology, we should observe antibodies (HAV-IgM) among the affected workers; if hepatitis B virus were the aetiology, we should observe more hepatitis B surface antigens (HBsAg) carriers among the affected workers as compared with non-affected workers; if alcohol were the main aetiology, we should observe more alcohol abusers or chronic alcoholics among affected workers; if any toxic solvent (e.g., chloroform) were the aetiology, we should find it at the workplace.

We performed a comprehensive medical evaluation for each worker. The viral aetiology was easily refuted, as well as the alcohol hypothesis, because they could not be supported by the evidence.

Instead, 17 of 25 workers from the plant had abnormal liver function tests, and a significant association was found between the presence of abnormal liver function and a history of recently having worked inside any of three rooms in which an interconnecting air-conditioning system had been installed to cool the printing machines. The association remained after stratification by the carrier status of hepatitis B. It was later determined that the incident occurred following inadvertent use of a “cleaning agent” (which was carbon tetrachloride) to clean a pump in the printing machine. Moreover, a simulation test of the pump-cleaning operation revealed ambient air levels of carbon tetrachloride of 115 to 495 ppm, which could produce hepatic damage. In a further refutational attempt, by eliminating the carbon tetrachloride in the workplace, we found that no more new cases occurred, and all affected workers improved after removal from the workplace for 20 days. Therefore, we concluded that the outbreak was from the use of carbon tetrachloride.

Neurological Symptoms in a Colour Printing Factory

In September 1986, an apprentice in a colour printing factory in Chang-Hwa suddenly developed acute bilateral weakness and respiratory paralysis. The victim’s father alleged on the telephone that there were several other workers with similar symptoms. Since colour printing shops were once documented to have occupational diseases resulting from organic solvent exposures, we went to the worksite to determine the aetiology with an hypothesis of possible solvent intoxication in mind (Wang 1991).

Our common practice, however, was to consider all alternative conjectures, including other medical problems including the impaired function of upper motor neurones, lower motor neurones, as well as the neuromuscular junction. Again, we deduced outcome statements from the above hypotheses. For example, if any solvent reported to produce polyneuropathy (e.g., n-hexane, methyl butylketone, acrylamide) were the cause, it would also impair the nerve conduction velocity (NCV); if it were other medical problems involving upper motor neurones, there would be signs of impaired consciousness and/or involuntary movement.

Field observations disclosed that all affected workers had a clear consciousness throughout the clinical course. An NCV study of three affected workers showed intact lower motor neurones. There was no involuntary movement, no history of medication or bites prior to the appearance of symptoms, and the neostigmine test was negative. A significant association between illness and eating breakfast in the factory cafeteria on September 26 or 27 was found; seven of seven affected workers versus seven of 32 unaffected workers ate breakfast in the factory on these two days. A further testing effort showed that type A botulinum toxin was detected in canned peanuts manufactured by an unlicensed company, and its specimen also showed a full growth of Clostridium botulinum. A final refutational trial was the removal of such products from the commercial market, which resulted in no new cases. This investigation documented the first cases of botulism from a commercial food product in Taiwan.

Premalignant Skin Lesions among Paraquat Manufacturers

In June 1983, two workers from a paraquat manufacturing factory visited a dermatology clinic complaining of numerous bilateral hyperpigmented macules with hyperkeratotic changes on parts of their hands, neck and face exposed to the sun. Some skin specimens also showed Bowenoid changes. Since malignant and premalignant skin lesions were reported among bipyridyl manufacturing workers, an occupational cause was strongly suspected. However, we also had to consider other alternative causes (or hypotheses) of skin cancer such as exposure to ionizing radiation, coal tar, pitch, soot or any other polyaromatic hydrocarbons (PAH). To rule out all of these conjectures, we conducted a study in 1985, visiting all of the 28 factories which ever engaged in paraquat manufacturing or packaging and examining the manufacturing processes as well as the workers (Wang et al. 1987; Wang 1993).

We examined 228 workers and none of them had ever been exposed to the aforementioned skin carcinogens except sunlight and 4’-4’-bipyridine and its isomers. After excluding workers with multiple exposures, we found that one out of seven administrators and two out of 82 paraquat packaging workers developed hyperpigmented skin lesions, as compared with three out of three workers involved in only bipyridine crystallization and centrifugation. Moreover, all 17 workers with hyperkeratotic or Bowen’s lesions had a history of direct exposure to bipyridyl and its isomers. The longer the exposure to bipyridyls, the more likely the development of skin lesions, and this trend cannot be explained by sunlight or age as demonstrated by stratification and logistic regression analysis. Hence, the skin lesion was tentatively attributed to a combination of bipyridyl exposures and sunlight. We made further refutational attempts to follow up if any new case occurred after enclosing all processes involving bipyridyls exposure. No new case was found.

Discussion and Conclusions

The above three examples have illustrated the importance of adopting a refutational approach and a database of occupational diseases. The former makes us always consider alternative hypotheses in the same manner as the initial intuitional hypothesis, while the latter provides a detailed list of chemical agents which can guide us toward the true aetiology. One possible limitation of this approach is that we can consider only those alternative explanations which we can imagine. If our list of alternatives is incomplete, we may be left with no answer or a wrong answer. Therefore, a comprehensive database of occupational disease is crucial to the success of this strategy.

We used to construct our own database in a laborious manner. However, the recently published OSH-ROM databases, which contain the NIOSHTIC database of more than 160,000 abstracts, may be one of the most comprehensive for such a purpose, as discussed elsewhere in the Encyclopaedia. Furthermore, if a new occupational disease occurs, we might search such a database and rule out all known aetiological agents, and leave none unrefuted. In such a situation, we may try to identify or define the new agent (or occupational setting) as specifically as possible so that the problem can first be mitigated, and then test further hypotheses. The case of premalignant skin lesions among paraquat manufacturers is a good example of this kind.

 

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Read 3257 times Last modified on Thursday, 13 October 2011 20:24