To beef up their herds, many cattle ranchers give their livestock steroid growth hormones. Manure-laden runoff from cattle feed yards carries these endocrine-disrupting compounds into the environment, where they can adversely affect fish and other wildlife—and taint drinking water supplies. But these compounds also stick to dust, and a new study finds that airborne particulate matter may be a significant source of steroids from beef cattle feed yards in arid regions (Environ. Sci. Technol. 2015, DOI: 10.1021/acs.est.5b01881).
Several years ago, Philip N. Smith, an ecotoxicologist at Texas Tech University, was duck hunting in the dry, windy flats around Lubbock, Texas, downwind from a nearby cattle feed yard. The ducks weren’t flying, and there was so much dust and debris in the air that it coated his teeth. He began to think about what was in it. Cattle excrete steroids and their metabolites, and these compounds bind to manure and soil that is kicked up and carried by the wind from the surface of the pen yards. He says he came away with a hypothesis: “Can we detect these compounds in the stuff that’s hitting me in the face?”
So he and his colleagues, including Brett R. Blackwell, now at the Environmental Protection Agency, set up air sampling equipment at five feed yards in Texas and Oklahoma, collecting particles on filters and making measurements over the course of two years. By extracting steroids from the filters and analyzing the compounds using liquid chromatography with tandem mass spectrometry, the researchers determined the concentrations in airborne particles of seven steroids cattle can excrete after treatment with the commonly used growth hormones estradiol, trenbolone acetate, and melengestrol acetate. They detected steroids at all feed yards, and the most abundant one was the estrogen 17α-estradiol, which was found on 94% of the filters at a median concentration of 21-ng/g particulate matter.
The researchers then estimated the emission of each of the compounds on particles from these feed yards using a standard model. They found that the largest feed yard in the study, with 56,000 head of cattle, emits 63 mg of 17α-estradiol per day on airborne particles. Shannon Bartelt-Hunt, a civil engineer at the University of Nebraska, Lincoln, who was not a part of the study, says that this amount is similar to what might be transported by runoff from a feedlot of that size on the basis of a previous study by her team (Environ. Sci. Technol. 2012, DOI: 10.1021/es202680q).
Blackwell says steroids on particulate matter probably pose the greatest risk to aquatic wildlife because the compounds would dissociate from particles that reach nearby water bodies. The median diameter of particles the team sampled was about 30 μm—large enough that people would not inhale them, and that they would not travel very far. On a very windy day, particles at the median size might travel more than 10 miles, whereas on a calm day, they might blow only a mile, Blackwell says. Ranch workers and nearby residents could potentially inhale smaller particles carrying the steroids, the researchers say, but the health impacts of such exposure are not well-known.
Bartelt-Hunt comments that previous studies of steroid release from animal production activities have focused on sources of runoff from feedlots or manure applied as fertilizer, and this work shows that particulate matter is important too. “Especially in arid climates, this may constitute a significant pathway,” she says.