Human testicles may contain high concentrations of microplastics and nanoplastics, substances that can interfere with the reproductive system, a new study suggests. For the study, scientists tested levels of 12 different types of plastic in 23 testes taken from male cadavers that ranged in age from 16 to 88 years old at the time of death. They also tested for the same plastics in 47 testes taken from neutered dogs.