[Nature of the black zones in hair following ingestion of thallium].

A total of 510 hairs with Widy's zones from 6 cases of poisoning were microscopically studied, both with light from above and from below. In every instance when the light came from below the zones appeared lighter in colour when pressure was applied. This lighting-up in color was reversible. This effect is incompatible with the opinion, still prevalent today, that the zones may be explained by hyperpigmentation. Quite frequently the zones became lighter during a prolonged exposure to water while simultaneously gas bubbles developed. However, even after extensive exposure to thioglycolic acid the zones did not disappear entirely in all cases. We explain these phenomena with the total reflection of the light at numerous bordering surface of substances with differing indices of fraction. We measured the thallium concentration in the urine and in the hair during the time interval between two thallium poisonings with the result that in those parts of the hair which had grown during a period when no thallium level was measured in the urine we found discernibly to strongly increased thallium concentrations. The results of our study indicate that, considering the most recent knowledge, proof of multiple thallium poisoning ought not to be attempted by means of fractional chemical investigation of the hair alone.