Much like livestock manure, biosolids, or “human manure” is often applied to land as fertilizer. In fact, approximately 50% of all human excrement in the U.S. is spread on various terrains. This is considered a cost-friendly, sustainable method of sewage disposal.

Biosolids, or processed human sewage sludge, are normally treated to remove bacteria and pollutants before they are applied to land. However, some toxicants are resistant to degradation and remain in the treated product. Chemicals commonly referred to as “endocrine disrupting compounds” (EDC), which interact with and potentially alter physiological systems in the body, can be found in both unprocessed and processed biosolids.

Because human fertilizer is often applied to pastures, grazing animals may be affected by EDC in biosolids. If EDC accumulate in the bodies of grazing animals, the compounds could find their way to the human food chain. In recognition of this potential, U.S. and European regulatory agencies have set concentration limits for individual chemicals in biosolids.

However, these regulations overlook the additive effects that could occur when multiple individual chemicals are present. Scientific evidence has suggested that when these compounds are combined they can have synergistic effects even when present at low amounts.

A research team at the University of Glasgow investigated this possibility in a paper published in the Journal of Animal Science.

“The paper [summarizes] the results, to date, of a unique model of real world exposure to environmental chemicals,” said Dr. Neil P. Evans, Professor of Integrative Physiology at the University of Glasgow. “The work highlights some of the shortfalls of laboratory-based studies.”

Multiple studies of sheep grazing biosolid-treated pasture were analyzed. In contrast to studies with laboratory rodents, these studies in sheep more closely reflect human exposure to the range of chemicals found within our everyday environment.

The researchers studied a wide array of physiological systems in the sheep. Overall, they found effects of EDC exposure to be highly variable.

Collectively, EDC exposure yielded more fragile bones than in the control group. Results differed between sexes, but male bone was more sensitive to EDC than that of females. Moreover, fetal bones were relatively unaffected by biosolid exposure in their mothers.

While there were effects on other fetal characteristics such as thyroid weight, testicular development and ovarian development, these changes were not maintained through adulthood.

“Prolonged maternal exposure to biosolids may activate homeostatic mechanisms that protect the fetus,” the researchers wrote.

In contrast, notable changes in the hypothalamic-pituitary axis were observed. The authors also observed “… that early exposure to a cocktail of pollutants, even at very low individual concentrations, can result in alterations in brain sexual development and dimorphism …” These results suggest that EDC have the potential to affect reproductive development and adult reproductive function.

Evans said the variability in results demonstrates the need for further research on the use of biosolids, in part because EDC are widely distributed in the environment of humans and animals alike.

“Biosolids [not] used as fertilizer [are] either buried in landfills or burned,” Evans said. “Some of those chemicals return to the earth in rain, so [burning] does not necessarily get rid of them.”

Conducting studies on biosolids is expensive and labor-intensive. One or two compounds can generally be studied at a time, but there are hundreds or even thousands of chemicals present in EDC .

“We need more work on the effects of chemical mixtures,” Evans said. “We know so little [about biosolids] at the moment.”