When prosecutors in Manhattan filed to drop charges against Dominique Strauss-Kahn on Monday, they revealed more than the flimsiness of their evidence. As the New York Times’ City Room blog pointed out, the 25-page motion also detailed how DSK’s $3,000-per-night suite was stained with the semen of at least three additional men. A forensics expert in the article explained that semen stains are smeared “all over the place” in many other hotels, the traces visible from their telltale glow under UV lamps. Wait, does semen really glow in the dark?

Sort of. Semen won’t give off light like a glow-in-the-dark sticker, but it does fluoresce. In other words, it absorbs ultraviolet light and re-emits that energy as visible light. The same holds for many organic substances, and most bodily fluids—including sweat, saliva, and urine—will shine when you put them under an ultraviolet “black light.” Semen happens to glow the brightest, however, on account of the particular mix of chemicals it contains.

Criminal investigators use black lights to detect semen because they’re portable and easy to use. Semen stains can also be detected by sight, by touch (feeling for crusty residue or crunchiness in fabrics), and chemical testing, but UV is rapid and hands-off. Still, fluorescence from ultraviolet light does not prove the presence of semen—the splotch on the wall or bed cover might come from a fluorescent detergent or thick saliva—so it’s usually followed by more conclusive testing. For example, forensic investigators often test for prostatic acid phosphatase, a type of enzyme that is much more common in semen than in other bodily fluids.

Roomers may find dried semen stains disgusting (and they might expect more from a $3,000-per-night hotel), but they present no serious health risks. Sperm cannot survive for long outside the body and will have stopped swimming long before occupants are likely to come into contact with dried stains. HIV virions also perish at a rapid pace, most of them dying in a matter of hours as the fluid dries. The U.S. Centers for Disease Control and Prevention report there have been no confirmed cases of HIV infections from environmental surfaces. You should be safe from sperm in any hotel room—just leave your black light at home.

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Explainer thanks David Foran of Michigan State University and Lawrence Kobilinsky of the John Jay College of Criminal Justice at the City University of New York.