A new peer-reviewed review led by researchers at Oregon State University found that simultaneous exposure to endocrine-disrupting chemicals and climate change stressors likely produces an additive or synergistic effect that increases reproductive harm and may contribute to the global decline in fertility. The paper synthesized findings from 177 studies examining how chemicals in plastics and effects such as heat stress are each linked to reductions in fertility and fecundity across humans, wildlife and invertebrates.

While the reproductive harms of endocrine-disrupting chemicals and climate change impacts in isolation are well-studied, the review focused on their combined effects. The authors focused on endocrine-disrupting chemicals including microplastics, bisphenol, phthalates and Pfas, which are ubiquitous in consumer goods.

"You're not just getting exposed to one – but two – stressors at the same time that both may affect your fertility, and in turn the overall impact is going to be a bit worse," said Susanne Brander, courtesy faculty at Oregon State University. Phthalates have been linked to altered sperm shape in invertebrates, spermatogenesis in rodents and reduced sperm counts in humans. Pfas are thought to impact sperm quality, and both Pfas and phthalates are linked to hormone disruption.

Warming temperatures, lower oxygen levels and heat stress associated with climate change may exacerbate infertility. Heat stress has been found to affect human hormones and is linked to spermatogenesis in rodents and bulls. Temperature plays a role in sex determination in fish, reptiles and amphibians, and many endocrine disruptors may alter environmental sex determination. In birds, exposure to increased temperature, Pfas, organochlorines and pyrethroids individually may each cause abnormal sperm, increased fledgling mortality, abnormal testes and population decline.

"It is likely [multiple stressors] would have an additive effect, at very least, even if they have different mechanisms of harm," said Katie Pelch, a senior scientist.

A 2017 study co-produced by Shanna Swan found sperm levels among men in western countries had fallen by more than 50% over four decades, and research has shown human fertility has been diminishing at a similar rate. The University of Washington's Institute for Health Metrics and Evaluation found the world was approaching a low-fertility future, with more than three quarters of countries projected to be below replacement rate by 2050.

The review describes the solution as reining in climate change and reducing the use of toxic chemicals, and cites the global reduction of DDT and PCBs achieved under the Stockholm convention as an example of an effective measure. "There is enough evidence in both areas to act to reduce our impact on the planet," Brander said.