A sweeping review of scientific literature reveals that exposure to both toxic chemicals and climate stress at once poses a far graver threat to reproduction than either factor alone, amplifying a global fertility crisis already documented across human and animal populations.
The dual assault works through overlapping biological pathways. Endocrine-disrupting chemicals such as bisphenols, phthalates, and PFAS interfere with hormone systems in ways that reduce sperm counts, alter reproductive development, and lower reproductive success. Heat stress from rising temperatures triggers similar harm by disrupting hormones, impairing sperm production, and destabilizing sex determination in species from fish to birds to mammals.
What makes the new research notable is its focus on the combined effect. While each stressor has been studied in isolation for years, the additive or synergistic damage when organisms face both simultaneously has largely been ignored. The researchers examined 177 studies to make their case that the overlap matters enormously.
"You're not just getting exposed to one stressor 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, a lead author and courtesy faculty member at Oregon State University. She called the combined effect "alarming."
The backdrop for this warning is stark. Sperm counts among men in Western countries have plummeted by more than 50 percent over four decades, according to research co-authored by Shanna Swan, a contributor to the new paper. Human fertility rates have declined at a similar pace globally, with the University of Washington's Institute for Health Metrics and Evaluation projecting that more than three quarters of countries will fall below replacement fertility rates by 2050.
The chemicals in question are everywhere. Phthalates contaminate plastics, cosmetics, and thousands of consumer goods. PFAS, sometimes called forever chemicals, persist in water supplies and food chains. Microplastics permeate drinking water, air, and soil. Humans absorb these substances constantly, often without knowing it.
The reproductive damage they cause spans species boundaries. In invertebrates, phthalates warp sperm shape. In rodents, they impair sperm production. In human males, they lower sperm counts. PFAS damages sperm quality across multiple organisms and disrupts hormonal systems universally. Climate warming produces parallel damage: altered hormones in humans, impaired spermatogenesis in rodents and cattle, and disrupted sex determination in fish, reptiles, and amphibians.
Temperature-dependent sex determination is particularly vulnerable. Many species evolved to produce male or female offspring based partly on environmental warmth. As the planet heats, temperatures can "push it too far in one direction or the other, which overrides that evolutionary benefit," Brander explained. Some endocrine disruptors may accelerate this problem by also altering how sex gets determined.
Take birds as an example. Rising temperatures, combined with exposure to PFAS, organochlorines, and pyrethroids, each independently can cause abnormal sperm, elevated fledgling mortality, testicular deformities, and population decline. The researchers found evidence that these harms overlap across species groups, yet virtually no direct study of what happens when organisms face multiple stressors at once.
"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 at the Natural Resources Defense Council who reviewed the work independently, agreed with the reasoning. "It is likely multiple stressors would have an additive effect, at very least, even if they have different mechanisms of harm," she said.
Fixing the problem demands action on both fronts. The researchers point to the global phase-out of DDT and PCBs under the Stockholm Convention as proof that chemical restrictions can work at scale. But they emphasize that far more action is needed. "There is enough evidence in both areas to act to reduce our impact on the planet," Brander said.
Author James Rodriguez: "The research exposes a dangerous blind spot: we've been studying climate and chemical damage separately when the real crisis lives in the overlap."
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