People who live in areas with high natural gas production have worried about the chemicals used in the fracking process for years. But according to a report from the Examiner, they may have real reason to be concerned. A recent study from researchers at the University of Missouri in Columbia shows that at least 40 percent of the chemicals used in the fracking process have been found to lower sperm count in men, increase testicle size, and increase the overall levels of testosterone in the blood.

The research was published in the October 13 edition of the journal Endocrinology, and shows that the chemicals used in hydraulic fracturing processes had endocrine-disrupting properties, meaning that they affected the balance of hormones in the body.

The study exposed female mice to various chemicals and recorded the impact they had on levels of estrogen, androgen, progesterone, glucocorticoid, and thyroid hormones in their offspring. They picked chemicals used by gas companies in Garfield County, Colorado.

Female mice were exposed to 23 of the fracking chemicals for an 11-day time window, leading up to the day when the mice gave birth. The level of exposure in the experiment was commensurate with what wild animals in gas production areas would experience.

The researchers compared the offspring of the exposed mice to a control group that hadn’t been exposed to any fracking chemicals. They found that the male mice who were born to exposed mothers showed lower sperm counts, larger testicles, and higher levels of blood testosterone as adults.