Q: How can I remove mold and orange-ish scum from the grout and other surfaces in my shower?

A: Grout, a cement-based material, is porous. In a shower, it absorbs moisture and typically stays damp for long periods. Add in soap residue, and it’s an ideal environment for mold or mildew and for pink or orange scum, especially where the caulk isn’t completely smooth and properly angled to allow water to drain quickly.

Mold and mildew are often used interchangeably; both terms refer to fungi that grow on surfaces that stay damp for long periods, especially when the surface is smeared with some of the many things the fungi can feed on, such as soap scum and residue from shampoos and conditioners. The scum, often called “pink mold,” is a type of bacteria, Serratia marcescens. You might find both of these growths on grout or on the lower part of a shower curtain.

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Mold and bacteria can trigger health issues, but the small amounts that grow in grout and on shower curtains tend to be more of a cosmetic issue. That’s not insignificant, though. No one wants to start the day by staring at it.

You may be able to get rid of pink or orange scum by simply pouring on some soda water, then brushing the grout with an old, damp toothbrush. The bubbly water helps eject the bacteria. For even more cleaning power, you could use hydrogen peroxide in the 3 percent concentration.

But bubbly water and hydrogen peroxide probably won’t be enough to get rid of mildew. Because you are dealing with both issues, you might want to use chlorine bleach, which kills both types of organisms and erases the color. The challenge is getting the bleach solution into all of those little sections of grout and keeping it there for at least a few minutes, so it can do its work.

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Choose a day when you can open a window to reduce the amount of bleach fumes you inhale. Wearing eye protection, gloves and old clothes that you don’t mind getting spattered with droplets of bleach, you should make a cup or two of a cleaning solution using 1 teaspoon of bleach to 1 cup of water. Use an old toothbrush to apply the solution to the grout, starting at the top and working down and crosswise, in a stairstep pattern. Wait at least 10 minutes before rinsing.

If you have areas that look especially grungy, keep the bleach solution pooled there by wadding up short lengths of toilet tissue and pressing them against the stained areas with the toothbrush when it’s loaded with the bleach solution. The damp tissue should stick in place. Leave it for a half-hour or so, then scrape off the wads with the toothbrush and collect them in a container that you can empty into the toilet and flush away, along with any leftover bleach solution. Don’t worry if you have a septic tank and have been warned not to use bleach. The amount you mixed, even if you were to pour all of it down the drain, shouldn’t cause a problem.

You can use plain water to rinse bleach residue off the grout, but soda water can work even better. When you’re done, close the bathroom door but leave the window open to help disperse the smell.

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If the stains persist, you might be dealing with something else, such as sticky residue from oily cleaners or hair conditioner. The Tile Council of North America recommends using an alkaline cleaner, such as Spic and Span or Mr. Clean, rather than an acid-based product, because acids degrade the cement in grout. The association also suggests trying an enzyme cleaner, which will attack stains just as it does on laundry. (You can use the same formula on both.) The association warns not to use oily cleaners, such as Murphy Oil Soap or Pine-Sol, because they can leave a residue.

Even alkaline cleaners can leave a residue if not thoroughly rinsed. But how can you do that without getting water all over the bathroom floor? “The absolutely best way to clean grout is to apply the cleaner and then vacuum (‘shop vac’) up the dirty water,” the association says. “This lifts the dirt off the joint. Apply rinse water and vacuum that water up. This lifts off any remaining soap film.” Be sure to use a wet-dry vacuum designed for handling liquids, though; it’s probably not the one you use to clean floors. Read the association’s full primer on how to clean tile and how to seal the grout after you get it clean by going to its website, tcnatile.com.

Have a problem in your home? Send questions to localliving@washpost.com. Put “How To” in the subject line, tell us where you live and try to include a photo.

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