The perfect coffee mug to keep at work is lovingly protected from thieves in the office break room and is often treasured by its owner for years, if not decades. But with so much talk about environmental pollutants and “sick” buildings, is it possible that the cherished coffee mug could be harboring nasty germs, or worse? One expert, Jeffrey Starke, a professor of pediatrics at Baylor College of Medicine in Houston and until last year was the director of infection control at Texas Children’s Hospital for 22 years, weighs in on the potential hazards of an old mug of joe and when washing that ceramic cup is bad for your health.

Where Germs Lie

The surface of a typical coffee mug isn't terribly conducive to harboring germs, says Dr. Starke, but when you fill it with coffee, sugar, cream—then touch it with lips and hands “you provide the opportunity for organisms and bacteria to fester,” he says.

Many microbiologists argue that nursing a cup of coffee just 45 minutes after pouring it is unhealthy, since standing liquid can encourage environmental pathogens, especially mold, to grow. But Dr. Starke says that these concerns are unfounded. “If I went and cultured the average unwashed coffee cup, of course I’m going to find germs,” the infectious disease specialist says. “But remember the vast majority came from the person who used the cup.”

Very few viruses can exist for more than an hour on an inert object, says the specialist in pediatric infectious diseases. Moreover, it is difficult to reinfect yourself with a virus you’ve already contracted. “However, during cold and flu season, I would guard my mug and make sure no one else is using it. Don’t share,” says Dr. Starke.

If you leave cream or sugar in your mug over the weekend, that can certainly cause mold to grow, says Dr. Starke, and “if your mug had obvious signs of mold, you wouldn’t want to drink from it.”

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