Co-exposure to toxic chemicals and the effects of climate change is likely to create a synergistic or synergistic effect that increases reproductive harm, potentially contributing to widespread global fertility declines, a new peer-reviewed study finds.
A review of the scientific literature considers how endocrine-disrupting chemicals, often found in plastics, and the effects of climate change, such as heat stress, are linked to reduced fertility and fecundity across the planet’s species, including humans, wildlife, and invertebrates.
While the reproductive harm of each of these issues has been well studied individually, there is little research into what happens when an organism is affected by both. Suzanne Brander, the study’s lead author and etiquette faculty member at Oregon State University, said the two issues likely pose a greater threat to fertility when taken together, and that the additive effects are “alarming.”
“You’re exposed to two stressors at the same time, not just one, and both can affect your fertility, making the overall effect a little bit worse,” Brander says. The paper reviewed 177 studies.
Shanna Swan, a co-author of the new paper, co-authored a landmark 2017 study that found that sperm levels in men in Western countries had plummeted by more than 50% over 40 years. Other studies have shown that human fertility is also declining at a similar rate.
The University of Washington’s Institute for Health Metrics and Evaluation previously found that the world is moving toward a “low-fertility future,” with more than three-quarters of countries falling below replacement rates by 2050.
The authors of the new paper focused on the effects of endocrine-disrupting chemicals and substances such as microplastics, bisphenols, phthalates, and Pfas. It is believed that these can cause a variety of serious reproductive problems, disrupting hormones and causing decreased fertility.
Brander pointed out that the harm these chemicals cause is often the same for all living things, from invertebrates to humans. For example, phthalates are thought to be associated with changes in sperm shape in invertebrates, spermatogenesis in rodents, and reduced sperm counts in humans. Similarly, Pfa is thought to affect sperm quality, and both are associated with hormonal disruption. Because this chemical is ubiquitous in consumer products, humans are often exposed to it on a regular basis.
Meanwhile, previous research has shown that higher temperatures, lower oxygen levels, and heat stress, among other issues related to climate change, can similarly worsen infertility.
Heat stress is known to affect hormones in humans and is associated with 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 to produce based in part on temperature, but a warming planet “could push it too far in one direction or the other, negating that evolutionary advantage,” Brander said.
Similarly, many endocrine disruptors can alter environmental sex determination.
The study analyzed some of the overlapping effects of chemical exposure and climate change on taxa ranging from invertebrates to humans. For example, individual exposure of birds to high temperatures, Pfas, organochlorines, and pyrethroids can cause abnormal sperm, increased chick mortality, abnormal testes, and population decline, respectively.
“What happens when you’re exposed to more than one of those stressors at the same time? That question has rarely been investigated. Even though there aren’t many studies that have looked at them at the same time, if you have two different factors that cause the same negative effect, they may be additive,” Brander said.
Katie Pelch, a senior scientist at the nonprofit Natural Resources Defense Council, who was not involved in the study, said the authors reviewed high-quality science. He said he would like to see more examples of overlapping impacts, but still agrees with the overall premise.
“[Multiple stressors]are likely to have additive effects, even if they have different mechanisms of harm,” Pelch added.
Solutions to systemic problems include curbing climate change and reducing the use of toxic chemicals. The study cited the global reductions in DDT and PCB usage achieved under the Stockholm Convention as examples of effective measures, but Brander said more needed to be done.
“There is sufficient evidence in both areas to act to reduce our impact on the planet,” she said.
