A new study found that testicles of both human men and dogs contained tiny little shards of plastics. Called microplastics, this debris is thought to come from the chemicals in our environment. This includes PVC and phthalates used to make everyday items such as plastic water bottles, food storage containers, and toys.

Previous research has found microplastics all over the human body—from the brain to the bloodstream to the lungs. They’ve also been found in semen, so the fact that a new study finds them in testicles is not surprising. Still, it adds to concerns about the impact of our environment on reproduction.

For the study, researchers from the University of New Mexico examined the testicles of 23 humans and 47 dogs. Dog testicles were provided fresh by the local vet who takes them out during routine neutering procedures. Human testicles were from cadavers ages 16 to 88 and had already been preserved.

The researchers explained that they included dogs because the way dogs make sperm closely matches the way humans do. Also, dogs live in our homes. This means they are exposed to many of the same chemicals we are.

The researchers weighed and measured each testicle and then dissolved the biological tissue leaving only solids. About 75% of the solids that remained were determined to be microplastics. This is more than found in other parts of the body.

There is good reason to believe that microplastics are bad for our reproductive health. But science has yet to determine exactly what they do to us or what health issues they cause. Earlier research found that phthalates decrease sperms’ ability to swim and cause fragmented DNA in their heads. Other theories of how microplastics might interfere with reproduction include altering hormones during fetal development, causing cancer, and triggering damaging inflammation. And—because of all of this—microplastics might  be behind well-documented  decreasing sperm counts. 

The researchers did find that sperm count was lower in those dogs that also had higher rates of the chemical PVC. But this is only a correlation. The study can’t tell us whether PVC caused the decrease in sperm count. The researchers also couldn’t determine sperm count in the human testicles because they came from cadavers.

We are surrounded by plastics and are just starting to learn the ways in which this might be harmful to our bodies. This study adds to the growing sense that microplastics are impacting our reproduction. It will take more research to confirm these suspicions and figure out how.

While research continues to look for answers, we could all limit our individual exposure to plastics. We can do this by avoiding plastic water bottles, skipping take-out food, and not microwaving in plastic containers. But experts agree that our dependence on plastics is not a problem that can be solved on the individual level.