The effect of lead on male fertility: a time to pregnancy (TTP) study

Growing attention has been paid in recent decades to the effects on male reproduction of occupational exposures to toxic agents. There is strong evidence that high level exposure to lead, i. e. blood lead level (PbB) > 70 microg/dl, is associated with male infertility and some reports suggest an effect even at lower PbB (i. e. < 50 microg/dl). The aim of this study is to shed more light on the postulated association between occupational exposure to relatively low levels of inorganic lead and reduced fertility in men estimated by the length of time taken to conceive: time to pregnancy (TTP).

A survival analysis of TTP of the last pregnancy was performed adopting the Kaplan Meier methodology. The target population included 782 lead-exposed workers and 165 controls. 251 lead workers and 119 controls were finally eligible and interviewed. Lead-exposed subjects were distributed into four exposure levels according to their blood lead concentration (i.e. < 20; 20-29; 30-39, and >/= 40 microg/dl). The Cox model was adopted to estimate the Relative Risk of unsuccessful waiting time to pregnancy associated to the exposure to lead.

A statistically significant difference in fecundability (shorter TTP) in favor of exposed subjects was detected. Nevertheless, longer TTP was associated within the exposed group to higher levels of PbB, even though the gradient is not statistically significant. The exposed workers revealed an average number of children larger than those not exposed, and a clear gradient of the same variable was evident from the lowest to the highest PbB level. Focusing on subjects with one child only, the Cox model confirmed no significant difference in fecundability between exposed and not exposed, whereas a statistically significant longer TTP was associated to the exposure level >/= 40 microg/dl.

It is not easy to assert or to deny the effect of inorganic lead on male fecundity, quantitatively estimated by TTP, with the data available for this study. In fact, while the general data seem to exclude effects of Pb on male fecundability a more detailed analysis suggests an unfavorable effect at relatively high levels of exposure but some confounding attributable to personal and social conditions of the workers cannot be ruled out. Further investigations with a better control of confounding are needed.