David Lerner, an engineering professor at the University of Sheffield in England, has found an interesting use for tampons. He dangles them in streams to see if certain contaminants are present in the water.

The cotton that makes up tampons is unique, in that it’s completely natural and untreated. As a result, the material is able to easily and effectively absorb different chemicals that it comes into contact with and clearly show the presence of these substances. Lerner is using the feminine products, like litmus paper, to test for chemicals known as optical brighteners in freshwater. Found in laundry detergent, shampoos and toilet paper, these chemicals are used to keep items bright and white.

Typically invisible to the naked eye, optical brighteners are only noticeable under ultra-violet light. They are what make people’s white clothing glow during “black light” parties at clubs. When found in bodies of water, the brighteners signify contamination of some kind, including the presence of sewage.

According to Lerner, more than one million households in England have improperly installed sewage systems and, as a result, deposit their sewage directly into a river instead of a treatment plant. “By working back from where pollution is identified and narrowing it down to a particular section of the network, the final step of identifying the source then becomes feasible," he says in a press release. “Our new method may be unconventional—but it's cheap and it works.”

Lerner first read about putting tampons to use in water quality monitoring in a 2004 report written by the Center for Watershed Protection for the United States Environmental Protection Agency. He and his team put the unusual idea to the test in their lab, suspending tampons in liquid. Scanning the tampons with a UV light revealed that they picked up extremely small traces of optical brighteners. The researchers published their findings in a recent issue of the Water and Environment Journal, noting that after only five seconds of exposure to a very diluted amount of optical brightener (0.01 milliliter of detergent per liter of water), the tampon glowed in the dark.

In the field, the group hung tampons in 16 water outlets, openings that connected water runoff from homes to streams and rivers in Sheffield, for three days. Nine of the 16 tampons glowed under UV light after the trial, revealing contamination. In partnership with Yorkshire Water, a water collection, treatment and distribution company, Lerner was then able to study these outlets and trace the pollutants to their source—in one case, to the specific house.

The techniques currently used to locate flaws in sewage systems are complicated and expensive. One of the main strategies involves homeowners placing dye in their sinks and toilets, so the sewage company can follow wastewater and spot any problems. A conductivity and temperature meter examines how quickly water samples conduct a current to pinpoint the presence of pollutants, but it is costly and actually less effective than tampons, according to Lerner's research.

Lerner and his colleagues will be conducting experiments along the Bradford Beck, a river that cuts through the city of Bradford in northern England, to pinpoint and report problem areas to the sewage company. Groups in Manchester, and as far as Maine and Mexico, are interested in learning more about his work and implementing the strategy themselves.

The best part is that, for this purpose, a tampon works as is—in its existing form, no tweaks necessary. “I can’t see any reason to change it,” says Lerner. “It’s simple and effective.”