Secondary sex ratio in relation to exposures to polychlorinated biphenyls, dichlorodiphenyl dichloroethylene and methylmercury

Effects of hazardous chemicals on secondary sex ratio: A comprehensive review

The secondary sex ratio (SSR), defined as the ratio of male to female offspring at birth, has garnered significant scientific interest due to its potential impact on population dynamics and evolution. In recent years, there has been a growing concern regarding the potential consequences of environmental chemicals on the SSR, given their widespread exposure and potential enduring ramifications on the reproductive system. While SSR serves as an indicator of health, ongoing research and scientific inquiry are being conducted to explore the potential relationship between chemicals and offspring ratio. Although some studies have suggested a possible correlation, others have yielded inconclusive results, indicating that the topic is intricate and still needs to be elucidated. The precise mechanism by which chemical agents exert their influence on the SSR remains ambiguous, with disruption of the endocrine system being a prominent justification. In light of the complex interplay between chemical exposure and SSR, the present review aims to comprehensively examine and synthesize existing scientific literature to gain a deeper understanding of how specific chemical exposures may impact SSR. Insights into chemical hazards that shift SSR patterns or trends could guide prevention strategies, including legislative bans of certain chemicals, to minimize environmental and public health risks.

Paternal occupational exposure to chemicals and secondary sex ratio: results from the Japan Environment and Children's Study

BACKGROUND

Declining proportions of male births have been reported in several industrialised countries. Previous reports have shown that exposure to certain chemical substances might influence the secondary sex ratio (SSR). We assessed the associations between paternal occupational exposure to chemicals and the SSR of their children using the Japan Environment and Children's Study (JECS), a large-scale birth cohort study.

METHODS

Data on paternal occupational exposure to various agents and other covariates were collected using a self-administered questionnaire to partners of pregnant female participants enrolled in the JECS. After adjusting for potential confounders, multivariable modified Poisson regression models were used to evaluate associations between paternal occupational exposures and the SSR of their children. This study was registered in the UMIN Clinical Trials Registry, number UMIN000030786.

FINDINGS

The JECS study gathered data on 103 062 pregnancies, 104 065 fetuses, and 51 898 partners of pregnant women. Among 50 283 children with data on paternal occupational exposures, 25 657 were male and 24 626 were female. The proportion of boys whose fathers were regularly occupationally exposed to insecticides was 0·445 (males, n=293; females, n=366; 95% CI 0·406-0·483), which was lower than the proportion of boys whose fathers were not exposed to insecticides. After adjusting for confounding factors, regular paternal occupational exposure to insecticides (adjusted relative risk 0·86, 95% CI 0·78-0·96) and medical disinfectants (0·95, 0·90-1·00) were significantly associated with lower SSRs among their offspring compared with the offspring of fathers not exposed to these substances.

INTERPRETATION

A declining proportion of boys could potentially be due to fathers working in environments in which they are exposed to chemicals. The associations between poorer semen quality and levels of reproductive and thyroid hormones require investigation.

FUNDING

Ministry of the Environment of Japan.