A new study has found that animals that grazed on land fertilized with human sewage often give birth to female offspring with compromised fertility. Although their research is still in its early stages, the study’s researchers think eating meat from these animals may similarly affect human fertility.

Since the 1990’s, much of United States farmland has been fertilized using human waste, commonly referred to as sewer sludge. The waste, which is treated and processed into easy-to-use pellets, is cheap, abundant, and safe. Or at least, that’s what we thought. According to the study, now published online in Scientific Reports, chemicals in human sewage may have a striking effect on pregnant ewes, causing them to produce female offspring that possess eggs that are more unhealthy than healthy. Although the adverse effect was seen regardless of what stage of pregnancy the sheep were in when exposed to the sludge, sheep that fed on lands fertilized with sewage during their last two to three months of pregnancy were affected the most.

Closer study may explain the root cause of the sheep’s fertility issues. The chemicals appeared to disrupt the developing ovaries within unborn female sheep, causing them to produce lower quality eggs. Like all female mammals, sheep develop their life supply of eggs while in the womb, so this reaction would have a lifelong effect on the sheep’s fertility.

The sewage-sludge fertilizer may not be confined to female fertility. A 2005 study showed that males born to sheep that grazed on sewage-fertilized fields experienced reductions in the types of cells necessary for the secretion of the male sex hormone testosterone, lead author, Dr. Richard Lea told Newsweek.

What’s more, Lea explained in a recent statement that the effects of chemical contaminants in the fertilizer may not stop at the animals, and could also pose a threat to reproductive development in humans who consume products made of sheep milk.

“There is a concern that by eating the meat from the sheep, we’re taking onboard these chemicals,” Lea told Newsweek.

This is not the first time that the use of human sewage sludge as fertilizer has been criticized. In addition to possibly messing with human and animal fertility, Food News Safety reported that the sludge regularly tests positive for containing heavy metals, flame retardants, pharmaceuticals, and many other harmful chemicals.

The news sounds grim, but researchers are looking for a way around it. Professor Paul Fowler of the University of Aberdeen, who helped coordinate the study, believes that changing the time period in which pregnant sheep are exposed to these chemicals is better than doing nothing, but “steps to reduce contamination of sewage sludge-derived fertilizer” may be best.

Source: Lea RG, Amezaga MR, Loup B, et al. The fetal ovary exhibits temporal sensitivity to a ‘real-life’ mixture of environmental chemicals. Scientific Reports . 2016