Why does semen glow in the dark?

Answers to your questions about the news.
Aug. 26 2011 7:02 PM

Sunshine of Your Love

Why does semen glow in the dark?

Hotel room being cleaned. Click image to expand.
There's a way to check what's on that hotel bed ... and carpet ... and wall

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.

Got a question about today's news? Ask the Explainer.

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.

Forrest Wickman is a Slate staff writer. 

TODAY IN SLATE

Frame Game

Hard Knocks

I was hit by a teacher in an East Texas public school. It taught me nothing.

Yes, Black Families Tend to Spank More. That Doesn’t Mean It’s Good for Black Kids.

Why Greenland’s “Dark Snow” Should Worry You

If You’re Outraged by the NFL, Follow This Satirical Blowhard on Twitter

The Best Way to Organize Your Fridge

Politics

The GOP’s Focus on Fake Problems

Why candidates like Scott Walker are building campaigns on drug tests for the poor and voter ID laws.

Sports Nut

Giving Up on Goodell

How the NFL lost the trust of its most loyal reporters.

Iran and the U.S. Are Allies Against ISIS but Aren’t Ready to Admit It Yet

Farewell! Emily Bazelon on What She Will Miss About Slate.

  News & Politics
Foreigners
Sept. 16 2014 4:08 PM More Than Scottish Pride Scotland’s referendum isn’t about nationalism. It’s about a system that failed, and a new generation looking to take a chance on itself. 
  Business
Moneybox
Sept. 16 2014 4:16 PM The iPhone 6 Marks a Fresh Chance for Wireless Carriers to Kill Your Unlimited Data
  Life
The Eye
Sept. 16 2014 12:20 PM These Outdoor Cat Shelters Have More Style Than the Average Home
  Double X
The XX Factor
Sept. 15 2014 3:31 PM My Year As an Abortion Doula
  Slate Plus
Slate Plus Video
Sept. 16 2014 2:06 PM A Farewell From Emily Bazelon The former senior editor talks about her very first Slate pitch and says goodbye to the magazine.
  Arts
Brow Beat
Sept. 16 2014 5:07 PM One Comedy Group Has the Perfect Idea for Ken Burns’ Next Project
  Technology
Future Tense
Sept. 16 2014 1:48 PM Why We Need a Federal Robotics Commission
  Health & Science
Science
Sept. 16 2014 4:09 PM It’s All Connected What links creativity, conspiracy theories, and delusions? A phenomenon called apophenia.
  Sports
Sports Nut
Sept. 15 2014 9:05 PM Giving Up on Goodell How the NFL lost the trust of its most loyal reporters.