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Dermatitis Among Workers Cleaning the Sacramento River After a Chemical Spill -- California, 1991

On July 14, 1991, a train tanker car derailed in northern California, spilling 19,000 gallons of the soil fumigant metam sodium (sodium methyldithiocarbamate) into the Sacramento River north of Redding (Figure 1). The major breakdown product of metam sodium, methylisothiocyanate (MITC), is a known skin irritant at high concentrations (greater than 1%). By July 21, the concentration of MITC in the river, at multiple test sites, measured 20-40 parts per billion (0.01%). On August 6, Shasta County health officials notified the California Department of Health Services (CDHS) of an outbreak of dermatitis among Shasta County jail inmates and crew leaders who had assisted in removing dead fish from the river on July 21-22 in greater than 100 F (greater than 38 C) ambient temperature.

To determine whether the outbreak was related to the chemical spill, during August 12-14, CDC and the CDHS conducted a retrospective cohort study of 42 inmates and crew leaders who participated in the cleanup and 48 state and federal employees who also worked in the river July 21-22. Dermatitis was defined as a self-reported rash on the feet or ankles with onset July 21-August 11 and duration of at least 4 days.

Of the 42 inmates and crew leaders, 27 (64%) had dermatitis; none of the 48 state and federal workers interviewed reported dermatitis. Onset of rash was noted 0-18 days after exposure in the river, peaking at 3-4 days (Figure 2). Rash affected the ankles (89%), feet (74%), legs (56%), hands (15%), and arms (11%). Reported symptoms included redness (96%); itching (81%); scaling (78%); bumpiness (56%); pain, burning, or stinging (37%); warmth (30%); and blistering (26%).

Rash occurred among 25 (76%) of 33 inmates and crew leaders who had lower extremity water contact compared with two (22%) of nine whose feet remained dry (relative risk (RR)=3.4, 95% confidence interval=1.0-11.8). In addition, the risk of rash for inmates and crew leaders was related to time spent in the river (36% for those in the water less than or equal to 3 hours compared with 92% for those in the water greater than 11 hours (chi-square for linear trend=8.0; p=0.005)).

The prevalence of water contact and the duration of time in the river were similar for inmates and crew leaders when compared with state and federal workers. However, of the 31 state and federal workers who had lower extremity water contact, 23 (74%) changed immediately to dry clothing before returning to work; in comparison, of the 33 inmates who had water contact, none changed immediately and nine (27%) changed to dry clothing at the end of the workday--after their feet had been wet up to 10 hours.

By August 14, the dermatitis was resolving for all patients. Reported by: C Busby, S Plank, MD, Shasta County Public Health Dept; R Jackson, MD, Office of Environmental Health Hazard Assessment, California Environmental Protection Agency; L Goldman, MD, R Kreutzer, MD, Office of Environmental and Occupational Epidemiology, California Dept of Health Svcs. Div of Health Studies, Agency for Toxic Substances and Disease Registry. Div of Field Epidemiology, Epidemiology Program Office, CDC.

Editorial Note

Editorial Note: Although laboratory studies indicate that MITC is both a strong primary skin irritant and a mild skin sensitizer (allergen) in animals (1), its irritant and allergenic effects on humans have not been clearly distinguished. Both irritant and allergic contact dermatitis have been reported for MITC, primarily among agricultural workers (2-5). In California, reports to the Pesticide Illness Reporting System for 1990 included six cases of presumed irritant dermatosis associated with exposure to metam sodium that had been sprayed on crops.

Because it hydrolyzes rapidly, metam sodium is transported as a concentrated 33% solution in water. Metam sodium further diluted in water and in the presence of oxygen decomposes principally to the active pesticidal product MITC. In the incident described in this report, decomposition of MITC or of river flora and fauna killed by MITC may have produced other chemicals, concentrations of which were not determined; therefore, an etiologic role for another chemical cannot be completely excluded. The high attack rate of rash and the low probability of previous sensitization to MITC among persons exposed in this episode is consistent with irritant dermatitis.

Irritant contact dermatoses account for 65%-80% of all cases of contact dermatitis (6-8). Risk factors for irritant dermatitis include exposure of skin to humid or wet environments, repetitive friction, heat, soaps or detergents, and skin occlusion (6-8). Among the inmates and crew leaders, dermatitis was associated with lower extremity water exposure and the duration of time spent in water. In addition, based on comparison with the state and federal workers, prolonged water exposure may have promoted the development of dermatitis. Although the prevalence of water contact with workers' upper extremities was high (90%), few inmates reported dermatitis on their hands (15%) and arms (11%) indicating additional factors (e.g., occlusive boots and friction due to weight-bearing) may have contributed to the occurrence of lower extremity rash.

To minimize the risk of irritant dermatitis, special precautions are necessary during prolonged exposure to water--particularly in the presence of concentrations of a potentially irritant chemical. Such precautions include maintaining dry skin in areas of substantial friction (e.g., by wearing watertight waders or boots of appropriate height and gloves) and changing immediately to dry clothes.


  1. Nihon Schering KK and Shionogi and Co., Ltd. Summary of toxicity data on methyl- isothiocyanate (MITC). Journal of Pesticide Science 1990;15:297-304.

  2. Richter G. Allergic contact dermatitis from methylisothiocyanate in soil disinfectants. Contact Dermatitis 1980;6:183-6.

  3. Schubert H. Contact dermatitis to sodium N-methyldithiocarbamate. Contact Dermatitis 1978;4:370-1.

  4. Jung VH. Occupational dermatoses due to pesticides (German). Dt. Gesundh-Wesen 1979;34:1144-8.

  5. Jung VH, Wolff F. Occupational contact dermatitis caused by nematin (vapam) in agriculture (German). Deut. Gesundheitsw 1970;25:495-8.

  6. Rietschel RL. Diagnosing irritant contact dermatitis. In: Jackson EM, Goldner R, eds. Irritant contact dermatitis. New York: Marcel Dekker, Inc. 1990:167-71.

  7. Bruze M, Emmett EA. Occupational exposures to irritants. In: Jackson EM, Goldner R, eds. Irritant contact dermatitis. New York: Marcel Dekker, Inc. 1990:81-106.

  8. NIOSH. Prevention of occupational skin disorders: a proposed national strategy for the prevention of dermatogical disorders. American Journal of Contact Dermatitis 1990;1:56-64.

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