You grab your hairbrush for some fast styling, and notice: it's disgusting. It's full of dry, matted hair, hair product, maybe even a little dandruff. And who knows what else?

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If the idea of using an icky hairbrush makes you bristle, know that you usually can't catch anything too serious from a dirty brush, according to Lindsey Bordone, MD, a practicing dermatologist and assistant professor of dermatology at Columbia University. But that doesn’t mean you don’t want to clean it.

What exactly can you catch from a dirty brush?

In a study conducted by the University of Arizona for an antimicrobial products company in Europe, 30 hairbrushes used by women ages 18 to 24 had more bacteria than the plug hole in a bathroom sink (where lots of hair usually collects) or a pet's food bowl. Microbiologists conducting the study found more than 3,400 colony-forming units of bacteria per square inch on those brushes.

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Where do all the bacteria come from? Consider this: A one-millimeter hair follicle can hold 50,000 germs.

The typical result of all those bacteria on your brush is simply a smelly odor and possible irritation of your scalp, experts say.

However, sharing your brush could get you into trouble. If you share your brush with someone who has a fungal infection known as tinea capitis (aka ringworm), you risk getting it. Ringworm usually affects kids, but it can occur on adult scalps, too. Don't share brushes even within the family, Bordone says.

Head lice is another possibility if you share brushes, she says. Children often pick up head lice at day care and school, and lice can spread easily within a household, especially if you share brushes and combs, Bordone says. Everyone needs his or her own brushes and combs, she says. And that means everyone, including each child, no matter how young.

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How to clean a hairbrush

If hair starts to build up on the brush, remove it using a comb (a wide-toothed comb works well) or a pick, Bordone says. "If it's wood, don't run it under hot water. Plastic is more forgiving. If you have dandruff or use styling products, run it under at least warm water [to clean it].''

Brush makers recommend removing the hair, then dipping the bristles into soapy lukewarm water if needed (don’t soak the brush) and gently scrubbing the bristles clean. Shake off excess water and let the brush air dry. If you have a natural bristle brush, do not expose it to direct sunlight. The UV rays can degrade the bristles, according to manufacturers.

When is it time to replace?

Replacing a hairbrush every one to two years is recommended by hairbrush makers, and that is probably reasonable, Bordone says. However, brushes can last longer than that, and Bordone says she’s kept some of hers much longer. The best gauge is wear and tear, not the calendar.

Round brushes, especially if you use them while you blow-dry your hair, tend to have the shortest lifespan, according to brush makers.

Look for signs of wear on the brush, Bordone says. If a plastic-bristle brush is losing the little round balls at the end of the bristles, it's time to toss it. Once a brush starts tangling your hair too much, it's probably time to toss it, according to Bordone. If your hair is very thick, you may wear out the bristles faster than someone with thinner hair.

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Kathleen Doheny is a Los Angeles journalist specializing in health, behavior and fitness topics.