The researchers studied how propylene oxide affects the testicles of rats when injected into their bellies. They gave different doses of propylene oxide to the rats for several weeks. The highest dose caused a decrease in sperm count and an increase in abnormal sperm. Even lower doses had some effects on sperm quality. However, the hormone testosterone levels and the cells in the testicles did not change much. This study is the first to show how propylene oxide can harm the testicles in a dose-dependent manner.