Simultaneous exposure to toxic chemicals and climate change’s impacts likely generates an additive or synergistic effect that increases reproductive harm, and may contribute to the broad global drop in fertility, new peer-reviewed research finds.

The review of scientific literature considers how endocrine-disrupting chemicals, often found in plastic, coupled with climate change’s effects, such as heat stress, are each linked to reductions in fertility and fecundity across global species – including in humans, wildlife and invertebrates.

Though the reproductive harms of each of these issues in isolation are well-studied, there is little research on what happens when living organisms are subjected to both. Together, the two issues likely pose a greater threat to fertility, and the additive effect is “alarming”, said Susanne Brander, a study lead author and courtesy faculty at Oregon State University.

“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,” Brander said. The paper looked at 177 studies.

Shanna Swan, a co-author on the new paper, co-produced a groundbreaking 2017 study that found sperm levels among men in western countries had plummeted by more than 50% over four decades. Human fertility has been diminishing at a similar rate, other research has shown.

The University of Washington’s Institute for Health Metrics and Evaluation previously found the world was approaching a “low-fertility future”, with more than three quarters of countries below replacement rate by 2050.

The new paper’s authors zeroed in on the effects of endocrine-disrupting chemicals and substances, including microplastics, bisphenol, phthalates, and Pfas. These are thought to cause a range of serious reproductive issues, disrupt hormones, and be a potential driver of the fertility drop.

Brander noted how these chemicals’ harms are often the same across organisms, from invertebrates to humans. Phthalates, for example, have been linked to altered sperm shape in invertebrates, spermatogenesis in rodents, and reduced sperm counts in humans. Similarly, Pfas are thought to impact sperm quality, and both are linked to hormone disruption. The chemicals are ubiquitous in consumer goods, so humans are often regularly exposed.

Meanwhile, previous research has shown how warming temperatures, lower oxygen levels and heat stress, among other issues associated with climate change, similarly may exacerbate infertility.

Heat stress has been found to affect human hormones, and is linked to spermatogenesis in rodents and bulls. Research shows that temperature also plays a role in sex determination in fish, reptiles and amphibians. The species has evolved to choose which sex it produces in part based on temperature, and the heating planet can “push it too far in one direction or the other, which overrides that evolutionary benefit”, Brander said.

Similarly, many endocrine disruptors may alter environmental sex determination.

The study broke down some of the overlapping effects of chemical exposure and climate change on taxonomic groups, from invertebrates to humans. For example, birds’ exposure to increased temperature, Pfas, organochlorines, and pyrethroids individually may each cause abnormal sperm, increased fledgling mortality, abnormal testes, and population decline.

“What happens if they’re exposed to more than one of those stressors at the same time? There has been little exploration of that question. Even if there have not been a lot of studies looking at these simultaneously, if you have two different factors that both cause the same adverse effect, then there’s a likelihood that they are going to be additive,” Brander said.

Katie Pelch, a senior scientist with the Natural Resources Defense Council nonprofit, who was not part of the study, said the authors reviewed high-quality science. She said she wants to see more examples of the overlap in impacts, but still concurred with the overall premise.

“It is likely [multiple stressors] would have an additive effect, at very least, even if they have different mechanisms of harm,” Pelch added.

The solution to the systemic problems would involve reining in climate change and reducing the use of toxic chemicals. The study cites the global reduction of the use of DDT and PCBs achieved under the Stockholm convention as an example of an effective measure, but much more is needed, Brander said.

“There is enough evidence in both areas to act to reduce our impact on the planet,” she said.