Dimethylformamide-induced liver damage among synthetic leather workers
IN NOVEMBER 1986, a 26-y-old male synthetic leather worker was hospitalized at the National Taiwan University Hospital. During the 2 mo that had preceded his admission, he experienced recurrent epigastric pain, nausea, fatigue and abnormal liver function. His serum tested negative for (a) hepatitis B surface antigen (HBsAg), (b) IgM antibody against hepatitis B core antigen, and (c) IgM antibody against hepatitis A virus. A liver needle biopsy revealed multiple zonal necrosis, which was present mainly in the central zones, accompanied by scarce cell infiltrates. After 2 wk in the hospital, his alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels dropped from 901 U/I and 390 U/I to 61 U/I and 32 U/I, respectively. After it was determined he did not abuse alcohol and drugs and that his job involved exposure to dimethylformatide (DMF)–a well-known hepatotoxic agent [1-5]–DMF-induced hepatitis was highly suspected. The authors attempted to confirm this diagnosis and to determine the prevalence of liver damage in synthetic leather workers by conducting a study of his co-workers.
With the exception of alcohol, [6,7] very little has been reported about synergism between exposure to hepatotoxic solvents and hepatitis B carrier status. Because the prevalence of hepatitis B carriers can be 15-20% in Taiwan, [8,9] we had the opportunity to evaluate any possible interactive effects that occurred in the liver.
Materials and methods
Workplace evaluation. The factory at which the patient had been employed for 2 mo produces mostly synthetic leather and select polyurethane-coated fabrics. Two processes, one wet and one dry, are used to prepare the materials. The wet process entails using a large amount of DMF; lesser quantities of other solvents, e.g., ethyl acetate, methylethyl ketone (MEK), butanone, acetone, methylene dichloride, and toluene, are used. Solvents and various coloring agents are mixed with polyurethane components or with other resins in a mixer, after which they are pumped onto fabric, which is then dipped in water. The dry process involves more MEK and less DMF and toluene. After a complete mixing, the solvent-resin mixture is pumped and hand-ladled onto the moving fabric. The finished product–rolls of synthetic leather and polyurethane-coated fabric–are used to make handbags and other items.
After an initial walk-through survey, we decided to measure air concentrations of DMF, ethyl acetate, and MEK–the three solvents consumed in the greatest quantity during the two processes. Even though toluene and methylene dichloride had not been used during the preceding 3 wk, we nonetheless measured their concentration in air.
A total of 19 random samples of organic solvents were conducted: 8 samples were obtained with charcoal tubes, and 11 were obtained with silica get tubes. With the exception of 1 measurement, all levels of ethyl acetate were within the threshold limit values (TLVs) recommended by the American Conference of Government Industrial Hygienists (ACGIH).  Toluene and methylene dichloride were undetectable. All acetone and butanone levels were within TLVs.
Factory areas were designated by exposure categories, which were determined by air concentration of DMF and potential dermal contact with solvents. Random, 10-min samples obtained with silica get tubes were analyzed, and air concentrations of DMF ranged from 9 to 60 ppm. Administrative and maintenance positions and other jobs in which individuals did not experience direct exposure to solvents, and where the air concentration of DMF was less than 10 ppm, were assigned an exposure index of 0. Jobs that entailed dry process material mixing and separation and a DMF concentration of 10-40 ppm (i.e., DMF was not used as the major solvent) were assigned an exposure index of 1. Exposure index 2 included wet processing and mixing jobs in which DMF was used as the major solvent, and direct skin contact with the solvent occurred more frequently than during the two job categories described above. Air concentrations of DMF in exposure index 2 ranged from 25 to 60 ppm.
Charcoal and silica gel tubes connected to personal samplers were used for 10-min random air samples, which were analyzed by gas chromatography according to the method recommended by the National Institute of Occupational Safe and Health (NIOSH).  The factory’s operations continued uninterrupted throughout the day; therefore, we assumed that the random samples were representative of the air typically inhaled by workers.
Health evaluations. We asked all 240 employees to participate in the study. Complete physical examinations, liver function tests, and serum creatinine phosphokinase (CPK) and HBsAg assays were conducted. Each worker provided a detailed history of (a) work, including potential skin contact with solvents, (b) alcolhol consumption, (c) cigarette smoking, and (d) blood transfusions. Symptoms of headache, dizziness, anorexia, nausea, epigastric pain, or general weakness, if present, were also noted for each worker, Physical examinations were performed by two doctors who were instructed to take particular note of chloracne or neurological symptoms.
Statistical analysis. Data were analyzed using the Mantel-Haenzel summary procedure  and the Mantel extension for the test of trends.  Logistic regression analyses (EGRET) statistical package ) were conducted to estimate the independent and interactive effects of hepatitis B carrier status and exposure to DMF.
Of the 183 workers examined, none showed abnormal neurological signs or chloracne. There was a high turnover rate at the factory, and most of the workers were young (Table 1). Most workers who were exposed directly to DMF did not drink much alcohol, a result most likely influenced by the fact that alcohol use was a precursor to nausea and flushing of their faces and hands. Two workers had a history of blood transfusions; therefore, they were excluded from the liver[TABULAR DATA OMITTED]
function test analysis so that confounding by non-A and non-B hepatitis would be prevented.
There was a significant association between a higher prevalence of abnormal liver function and a higher index of exposure to DMF (Table 2). Logistic regression analysis revealed that high concentrations of DMF were associated with elevated ALTs (p = .01), whereas[TABULAR DATA OMITTED]
HBsAG carrier was slightly but independently associated with an elevated ALT (p = .07). Some workers had normal values of ALT (i.e., < 35 IU/I); however, the exposed individuals in this group had higher mean values of ALT, and among the non-HbsAg carriers, a higher mean value of AST (Table 3).
Mean values of transaminases were generally higher among HbsAg carriers. There was, however, no similar trend for serum amylase, albumin-globulin ratio, gammaglutamyl transferase (GGTR), and alkaline phosphatase. As well, no similar trend was noted for indicators of renal damage, i.e., serum creatinine and urine protein.
Fatigability, anorexia, nausea, and epigastric pain occurred more frequently among workers who were exposed to high concentrations of DMF, but the associations were not statistically significant (Table 4). Workes exposed to a higher concentration of DMF were more likely to have an elevated CPK, but HBsAG carriers did not have a higher frequency of abnormal CPK levels (Table 5). Only 143 of the 183 workers examined were available for further analysis of CPK isoenzyme. There appeared to be no association between higher exposure levels of DMF and elevated MB function of CPK isoenzyme (Table 6). An analysis of variance demonstrated that there was no difference in mean level of MB fraction among the different exposure groups.
Even though we found 1 case of suspicious chemical hepatitis, and 15 additional workers had abnormal liver functions, it does not necessarily follow that these effects were caused by DMF. The authors, however, provide[TABULAR DATA OMITTED]
he following arguments in support of an association between DMF and the effects noted.
First, the possibility that non-A/non-B hepatitis accounted for the effects is almost nonexistent because workers who had histories of blood transfusions were excluded (Table 2).
None of the workers who experienced considerable DMF exposure were alcoholics (Table 1). Conversely, alcohol consumption (> 24 g/d) was inversely related to the DMF exposure index and to abnormal liver function. Most of the workers who had abnormal liver function
Table 4. — Prevalence of Symptoms among Workers Exposed to Different Levels of Dimethylformamide (DMF)
DMF exposure index
0 1 2 p value (*)
Workers examined (n) 75 82 24
Fatigability (%) 10.3 2.59 16.7 .11
Dizziness (%) 1.3 9.9 12.5 .032
Anorexia (%) 3.8 7.4 14.3 .12
Nausea (%) 1.3 2.5 4.3 .38
Epigastric pain (%) 5.4 5.2 20 .12
(*) Mantel-Extension for trend.[TABULAR DATA OMITTED]
had higher ALTS than ASTs, which differs from alcohol-induced liver function abnormality.  We conclude that alcohol was not responsible for liver damage. At the time of study, none of the workers examined were taking any specific drugs other than vitamins and antacids prescribed by their physicians for treatment of liver problems and epigastric pain; therefore, hepatotoxic drugs provide an unlikely explanation for liver damage.
Despite the use of many solvents at the factory, air concentrations of solvents–ethyl acetate excepted–were all below TLVs. Ethyl acetate is rarely hepatotoxic, and it probably was not associated with a high prevalence of abnormal liver function in our study. Furthermore, both toluene and methylene dichloride were not in use and were not detected at the workplace.
Whereas chronically active HBsAg carriers are numerous in Taiwan,  noncarrier workers had abnormal liver functions that correlated with exposure to higher levels of DMF (Table 2). Exposed workers who had a normal value of ALT still had increased activities of ALT and AST, especially in those who were not HBsAg carriers (Table 3). Therefore, hepatitis B is not singly responsible for liver damage among the factory workers. We checked the e antigen for stronger viral activity among all HbsAg carriers and found that 9 of 43 (21%) were positive; only 3 of 9 had abnormal elevations of ALT (1 was found in each DMF exposure index of 0, 1, and 2, respectively).
We conclude that DMF was responsible for the high prevalence of abnormal ALT and elevated mean amino-transferase activities among synthetic leather workers.
Perhaps HBsAg carriers, who are more susceptible to liver change by alcohol, [6,7] are more susceptible to hepatotoxic solvents, e.g., DMF. Logistic regression demonstrated an independent effect between HBsAg carrier status and effect of DMF exposure on prevalence of abnormal liver function (Table 2). HBsAg carriers had an increased mixed-function oxidase enzyme function.  Whereas DMF is metabolized in the liver, it probably requires this enzyme during demethylation to monomethyl formamide (MMF).  HBsAg carriers probably generate MMF more rapidly and in larger quantities. No study has been conducted in which the hepatotoxicity of DMF and MMF in humans has been compared, but animal data reported by Lundberg et al.  suggest that MMF is more toxic to the liver than is DMF. The higher susceptibility of HBsAg carriers to DMF might be explained by a more rapid conversion of DMF to MMF that results from an elevated cytochrome p-450 enzyme function. This prompts speculation that other organic solvents or chemicals might have a similar enhanced hepatotoxic effect on HBsAg carriers via this pathway. Our study was relatively small, and the effect of HBsAg only showed a borderline statistical significance; therefore, no strong inference could be drawn. More epidemiological studies and animal studies on duck HBsAg carriers and noncarriers are needed to elucidate the mechanism whereby infection by the hepatitis B virus enhances the hepatotoxic effect of some organic solvents.
Epigastric pain, nausea, and loss of appetite have occurred after exposure to 20 ppm of DMF. [1,18,19] A few studies have reported suspicious liver or lung damage at DMF levels less than 50 ppm. [1,2,17] We found similar symptoms and liver injury at DMF levels of 10-60 ppm (especially at levels greater than 25 ppm) (Tables 2 and 4). Moreover, exposed workers who had normal ALT values had significantly increase transaminase activity (Table 3).
Even though only 11 random, 10-min air samples were obtained, they are representative of the air normally present in the factory. Our on-site observations of work practices and occupational settings confirmed this. Whereas workers who are exposed to higher concentrations of DMF in air are also more likely to have direct skin contact, we are unable to determine what portion of exposure is attributed to inhalation. Nevertheless, we recommend that the 10 ppm TLV for DMF be re-evaluated in the future to insure a proper margin of safety. Biological monitoring and assessment of skin exposure should also be performed and considered in future standard-setting activities.
To date, the detrimental effects of DMF on human muscle tissue have not been reported. Animal studies suggest that DMF damages the myocardium in animals,  and that the final metabolite, formamide, may be toxic to muscle fibers in rats and frogs. [1,20] Although our study revealed a clear, linear trend that associated an abnormal level of CPK with degree of DMF exposure (Table 5), further analysis of the isoenzyme of CPK did not show any specific elevation of MB or BB fractions (Table 6). Perhaps damage to muscles is nonspecific. Because CPK is an indicator of local muscle strain in occupational settings,  and exposure categories 0, 1, and 2 were associated with a concomitant increase in muscle effort, we tentatively conclude that our findings may indicate that muscle strain occurred more frequently in individuals who were in these exposure categories.
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