Health surveillance of employees on a lead mine, 1979-1989.

Health surveillance of employees at a lead mine in the northern Cape, employing about 1,400 people, is specifically aimed at early detection of excessive lead absorption, which is the main chemical hazard. Over a period of 9 years the blood lead level distribution showed very few values (2.5%) that exceeded 60 micrograms/100 ml. The predictive validity (calculated according to the method of Alessio) of zinc protoporphyrin (ZPP) levels, at a cut-off level of 4 micrograms/g haemoglobin, to screen exposed workers in order to determine whether their blood lead level would exceed 50 micrograms/100 ml proved to be high (198). In 1988 a significant correlation between ZPP and blood lead levels was found in 195 employees at a low level of absorption manifested by an incidence of only 4% exceeding the cut-off level of 4 micrograms ZPP/g haemoglobin and only 2% exceeding a blood lead level of 50 micrograms/100 ml in that year. Monitoring by ZPP provides a high degree of safety for workers and is a relatively inexpensive, well-accepted and effective method.