1. A major issue that has to be taken into account while interpreting animal studies is the variation in response to Pb between and within species. Already in 1971 the experiments of Schroeder and Mitchener pointed out that mice are more vulnerable to the toxic eVects of lead on reproduction than rats.47 Also on variation within species Der et al12 suggested that gene diVerences might be an important factor of diVerence in response to foreign compounds, when they compared their own findings in Sprague-Dawley rats with those of Hilderbrand et al11 on Sesco rats. The SpragueDawley rat strain seemed to be more resistant to the toxicity of Pb than the Sesco strain
2. The cross sectional study by Alexander et al in Canada is probably the best published survey available evaluating semen quality and serum concentrations of reproductive hormones in workers exposed to Pb. As with other findings, the research showed that PbB >40 μg/dl may aVect spermatogenesis by reducing sperm concentration and total sperm count. No association was found between exposure to Pb and sperm morphology, motility, or reproductive hormones. The strengths of the study were mainly the size and careful selection of the study population, availability of historical Pb monitoring data, the control for all the relevant confounding factors—for example, age, smoking, alcohol consumption, period of abstinence, and blood concentrations of other metals such as Cd and Zn—the statistical analysis, and the validity of seminal analysis. The major limitation of the study was the poor participation rate, as in other studies. This research provided further evidence of a direct toxic eVect of Pb on spermatogenesis.57
3. On the other hand, a French cohort study on live births of 229 workers exposed to Pb (mean PbB 46.3 μg/dl) compared with 125 unexposed subjects showed no association between exposure to Pb and fertility, and did not provide clear evidence of the adverse eVects of occupational exposure to Pb on male fertility as studied by recording live births. While interpreting this research, it should be noted that it had a small sample size, the possible existence of uncontrolled confounding factors could not be ruled out, and the quality of exposure information was limited.72
4. The scarce data on testicular Pb concentrations seem to indicate interstrain diVerences in the capacity of accumulating Pb in the testes. In the high dose experiment of ThoreuxManlay et al PbB of 1700 μg/dl after 35 days of exposure resulted in a testicular Pb concentration of 0.8 μg/g in Sprague-Dawley rats,32 whereas the Charles Foster rats, used in the experiment of Chowdhury et al in 1987, reached testicular Pb concentrations of 2.0, 1.6, 2.6, and 4.3 μg/g with PbB of 56, 91, 196, and 332 μg/dl respectively after 30 days of exposure.20 Apparently Sprague-Dawley rats accumulate less Pb in their testicles than other rat strains. Unfortunately, few studies have focused their assessment of exposure on the cumulative capacity of Pb