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Some pretty, fizzy bath bombs have a dirty secret

They can damage plumbing. But not all of them are bad.

Plop. Fizz. Relax. Adding a fragrant bath bomb to your tub can transform a routine wash into a relaxing, spalike soak. Although these popular bundles of stress reduction are especially welcome these days, they can pose problems for your plumbing. But not all bath bombs are bad; it depends on what’s in them and how you use them.

Birnur Aral, director of the health, beauty, and environmental sciences lab at the Good Housekeeping Institute, walked me through this quick science lesson. Basically, two ingredients make bath bombs fizz: baking soda (sodium bicarbonate) and citric acid. When an object combining them is dropped into warm bath water, you witness an acid-base reaction between the two, and CO2 (carbon dioxide) gas is released, causing the fizz and foam. Think of it as a less-volatile version of that third-grade volcano science project, where you mixed baking soda and vinegar.

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Some bath bombs substitute cream of tartar for the acid. Aral also notes that titanium dioxide, commonly used in cosmetics and sunscreen, is on some ingredient lists. She surmises that it's to "give weight and a hefty feel" to an otherwise lightweight product, and it may also add some opacity, so the bath bomb's colors are stronger.

Besides an acid and a base, most bath bombs also contain Epsom salts — known for soothing muscle aches — and oils, such as coconut, soybean, or olive, to form a cohesive mass, much like adding an egg allows meatballs to hold their shape. Then there are essential oils for imparting a scent to your bath. There could also be extras, such as colorants, pieces of dried flowers, confetti, and, especially in products targeting younger consumers, oh-so-messy glitter.

Aaron Mulder, operations manager and co-owner of Mr. Rooter Plumbing in San Antonio, gets questions about bath bombs more often than one might expect. He categorizes them as FOG: fats, oils, and grease, which, in plumber parlance, are things you try to avoid introducing into your plumbing system.

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Although you may bathe in hot water, Mulder said, after the first 10 feet of your drain, the pipes start to cool substantially, especially if you live in colder climates, and anything in the outflowing water congeals quickly.

In addition, if the Epsom or other salts do not dissolve completely, they may resolidify, he said. And there’s even more: “Drains are slimy and sticky to begin with, so they catch things like flower petals, glitters, and other add-ons that aren’t biodegradable,” he said.

In theory, baking soda, citric acid, and the salts are all water-soluble, Aral said. “But since we are dealing with added ingredients, such as the oils, it is possible that some of these only get partially dissolved in the bathwater,” she said. “I think the biggest culprit is titanium dioxide, which is insoluble in water. Though this is a minor component in these formulas, it is possible that over time, it can get accumulated in the nooks and crevices of your pipes or mingle with other debris, such as hair and dead skin that gets sloughed off, and contribute to clogs.”

One total no-no: bath bombs and jetted tubs. Mulder said this is a recipe for disaster. Hot tubs and spalike tubs use a pump to suck water into and force water out of their jets. “These systems can’t really filter foreign material found in a bath bomb,” he said. “If you damage the main pump system, you may have to pull out the entire tub — a costly consequence.”

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This isn’t to say that bath bombs are taboo. You just have to be smart when using them. Even Mulder lets his young daughter indulge. “Anything that helps me get my 5-year-old into the tub is a plus,” he said.

Mulder and Aral both suggested using a screening material, such as a mesh jewelry bag or pantyhose secured with a rubber band, to hold the bath bomb. It will still activate when dropped into the water, but the mesh will catch larger particles and foreign materials. For tubs with a removable stopper, consider using a strainer; it will catch chunks of salt and other drain-clogging debris, which can be tossed in the trash.

Take extra care post-bath when wiping down your tub, especially if your bath bomb contains glitter. Aral said some glitter particles can leave scuff marks and/or scratch your bathtub’s finish. “To be safe, I would first gently pick any obvious glitter particles from the tub by hand before taking a cleaning product and brush to them,” she advised.

Aral suggests using bath bombs infrequently, because they are not necessarily gentle to the skin. The colorants, fragrance, and potentially the salts, combined with a hot bath, could cause dryness and irritation for sensitive skin. She added: “I think one can get a pleasant experience with a plain old bubble bath and bath oils, and hence eliminate the glitter and colorants that can be messy in the bathtub, let alone in the pipes.”

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If you do want to add some fizz to your bath, look for a bath bomb with the smallest number of ingredients. The fewer the ingredients, the less mess it creates in your tub and pipes.

Or consider the DIY approach. “There are so many bath-bomb formulas floating out there,” Aral said. “Find a basic recipe, and turn it into a fun science project for you and your kids.”