Antibiotics fed to animals drift in air. (Suspended Drugs).

Drugs added to animal feed can latch onto dust particles that become airborne and float through farm buildings, according to German scientists investigating health risks. Such antibiotics could be toxic if livestock workers inhale them and also could accelerate the evolution of drug-resistant bacteria.

Past studies have found farm-derived antibiotics in waterways (SN: 3/23/02, p. 181) and food products. The air represents a third route of unintended exposure to antibiotics, says microbiologist James Zahn of Iowa State University in Ames.

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Much of the dust in farm buildings that house animals is organic matter from feed, animal skin and excrement, bacteria, and fungi, says Jorg Hartung of the Hannover School of Veterinary Medicine in Germany. Previous research had shown that inhalation of the dust can cause respiratory problems, allergies, and other health effects in farmworkers.

To see whether farm dust contains antibiotics, Hartung and his colleagues analyzed particles that had settled out of the air in a pig-confinement building on a German farm. The researchers looked for chemical evidence of six antibiotics that had been added to feed at low doses to accelerate the animals' growth.

Of samples gathered annually between 1981 and 2000, 18 of 20 contained at least one of the six compounds, says Hannover chemist Gerd Hamscher. Tylosin, the antibiotic found most often, showed up in 16 samples, the researchers report in an upcoming Environmental Health Perspectives.

Tylosin is not prescribed to people because it tends to induce allergies, but it's chemically related to the important medical antibiotic erythromycin. Bacteria that develop resistance to either of those drugs typically have resistance to both. Chloramphenicol, another antibiotic found in the farm dust, can damage DNA in people.

The new study "makes clear the potential for [antibiotic] exposure via inhalation," says pulmonary toxicologist Peter Thorne of the University of Iowa in Iowa City. Besides posing dangers specifically to farm-workers, airborne dust may spread antibiotic resistance, creating a more general public health concern, he says.

Exposing bacteria in the human body to small, steady doses of antibiotics is an ideal way to promote drug resistance, Thorne and the German researchers agree. However, Thorne cautions, researchers still need to determine how much exposure people are likely to get through this route.

The study is the first full report to establish that antibiotics can be spread through the air, says Zahn. At scientific meetings, he has presented data, gathered in pig confinement facilities in the U.S. heartland, indicating that tylosin--and even tylosin-resistant bacteria--can spread through the air. The drug also turns up in air expelled from these facilities by exhaust fans, he adds.

European countries are phasing out the use of antibiotics for promoting growth in animals. Recently, the McDonald's restaurant chain announced plans to reduce its use of meat produced with growth-promoting drugs (http://www.sciencenews.org/ 20030628/food.asp).

Nevertheless, dust-bound antibiotics may remain in farm buildings for some time. Some of the drugs in the German samples had persisted for 20 years, Thorne points out.