Occupational skin exposure to cobalt : origin and fate

Abstract

Exposure to cobalt is not without risk. Besides adverse health effects on the respiratory system, cobalt is one of the most important sensitising metals with a contact allergy prevalence of 2.2% in the general population. Sources of exposure to cobalt in the general population are diffuse. Because of its specific uses in for example rechargeable batteries, superalloys, and hard metals, presence of cobalt exposure at work is often more obvious. For that reason, adverse health effects of cobalt have often been studied in occupational settings. The research presented in this thesis was performed in the work environment of hard metal workers and dental technicians, which are occupational groups with a recognised exposure to cobalt. The overall aim was to study skin as target organ for occupational exposure to metals, in particular, to cobalt.

Dental tools and alloys, handled on a daily basis by dental technicians, were tested for release of cobalt with the cobalt spot test, and for nickel release using the Dimethylglyoxime test. Furthermore, release of cobalt, nickel and chromium was quantified in artificial sweat experiments. Concentrations of nickel and chromium for many dental tools and alloys were considered high enough to pose an allergy risk (paper I). In addition, cobalt, nickel and chromium were detected on skin of all participating dental technicians (n=13) measured by acid wipe sampling. Cobalt was also detected in all ten air samples (0.22-155 μg/m3), of which two concentrations exceeded the Swedish Occupational Exposure Limit of 20 μg/m3. Despite skin and respiratory exposure to metals, exposure was not reflected in urine samples of dental technicians (paper II). Although this study had a small sample size and a limited amount of work performed with dental alloys made of cobalt-chromium, the results show that dental technicians are exposed to sensitising metals.

The studies performed within the hard metal industry (paper III and IV) followed the same protocol as was used for dental technicians, but included a larger cohort of workers (n=76) and only assessed cobalt exposure. Evaluation of surface contamination with the cobalt spot test revealed the presence of cobalt on surfaces in the hard metal facilities, even outside production areas. All hard metal workers were exposed to cobalt on skin. In addition, respiratory cobalt exposure was measured by sampling of the inhalable fraction among hard metal workers at production areas. This showed that all workers were exposed to cobalt through air. Cobalt was found in 72% of the urine samples of hard metal workers. Correlations were found between cobalt in air and cobalt on skin before and at end of shift. No significant change was seen in urinary cobalt concentrations over time during 24h. Quantile regression modelling revealed significant associations between urinary cobalt concentrations as the dependent variable, and cobalt skin and respiratory exposure as independent variables, when each exposure route was modelled independently. When modelling the independent exposure variables together, none of the cobalt skin doses were significantly associated with cobalt in urine.

Several theories may explain the observed associations between cobalt skin exposure and concentrations in urine, but from the results in this thesis it was not possible to assess causation. For both dental technicians and hard metal workers, efforts should be made to reduce skin exposure. Examples are the use of disposable gloves, avoiding contamination of other work areas, and no use of private items in work areas.