When Do Unused Bath Bombs Expire?

Do Unused Bath Bombs

Suppose you’re digging through the depths of your bathroom cupboard and discover a bath bomb, still in its wrapping. Visions of taking the world’s most relaxing bath fill your head. But there’s just one problem—you haven’t bought or made a bath bomb for a very long time, so this specimen is decidedly old. Can you still use it? Do unused bath bombs expire?

Bath bombs have expiration dates, usually within one year.  After six months, a bath bomb may begin to lose its fizz although it’s still safe to use. However, added bath bomb ingredients like essential oils or oatmeal or flower petals can cause a bath bomb to get moldy or rancid within one year. Proper storage in a cool, dry environment is vital.

Probably everyone has heard of the term ‘shelf life’. Taken in context, it is pretty self-explanatory. Shelf life describes the amount of time that products can be stored and still remain usable. 

Typically, a product’s shelf life is the same as the composite ingredient with the shortest shelf life. This is a general rule, and it is, therefore, applicable to bath bombs. 

So, let’s look at the ingredients that comprise a bath bomb and their individual shelf lives. Additionally, we’ll look at what happens to a bath bomb that is past its expiration date and how to store your bath bombs to extend their shelf lives. 

Primary Bath Bomb Ingredients And Their Shelf Lives

bath bomb ingredients

For a bath bomb to be a bath bomb, it needs to have three primary ingredients: (also called sodium bicarbonate or bicarbonate of soda) baking soda, citric acid, and cornstarch.  When you learn to make a basic bath bomb yourself, you become familiar with how these ingredients interact. Looking at the shelf life of each ingredient will help us answer the question “Do unused bath bombs expire?”

Baking Soda

Baking soda has a shelf life of approximately six months. This is the opened shelf life, which applies to bath bombs, even if the bath bomb is still in its packaging. It also refers to the ideal temperature and air conditions (cool and dry)in which it’s stored. After six months, baking soda won’t go bad in the sense of growing mold or becoming rancid; it just loses its chemical effectiveness.

Citric Acid

The shelf life of citric acid is approximately three years. This is the shelf life under cool and dry environmental conditions. Once again, the shelf life is applicable, even if the bath bomb is still in its packaging. Citric acid also does not become rancid or grow mold after these three years. As with baking soda, it just loses its chemical effectiveness.


The shelf life of cornstarch is almost indefinite, even after it has been opened. It should be kept cool and dry so as not to be compromised, but it will not expire or lose effectiveness. 

Based on the above information and the shelf life rule, the shelf life of a basic bath bomb is six months because this is the shortest shelf life of one of the primary bath bomb components, namely baking soda.

Bath Bombs Have A “Best Before” Date

Packing in mold

Technically, a basic bath bomb won’t be dangerous if you use it after the six-month mark, but it will certainly not create the fizzing effect, which is the essence of a bath bomb. Is it worth using a non-fizzing bath bomb? You can decide.

Why don’t bath bombs that are beyond their “best before” date fizz? To understand that, you need to know what produces the fizz in the first place. (It’s a bit of chemistry, but it’s not too confusing, so bear with me). 

How Bath Bombs Work

The bath bomb fizz is a chemical reaction between the baking soda and citric acid. This chemical reaction requires water to catalyze it; this is why bath bombs do not bubble and fizz as soon as you mix the ingredients together. It’s also why storing bath bombs in a dry place is vital. 

The Chemical Reaction: Baking Soda and Citric Acid

When you combine baking soda and citric acid, a chemical reaction occurs that produces carbon dioxide. The release of the carbon dioxide causes the fizz. You can make bath bomb embeds to put in the center of a bath bomb. 

An embed is like a mini-bath bomb without the cornstarch. And without the cornstarch you get a faster chemical reaction. You place the bath bomb embed inside a bath bomb to make it spin and fizz like crazy. Watch the video below for more information on how this chemical reaction works.

Make Your Bath Bombs Spin and Fizz Like Crazy

The Role of Cornstarch

But what about cornstarch? What’s the purpose of the third primary bath bomb ingredient?

Cornstarch slows down the reaction between the bicarbonate of soda and the citric acid by binding to each of these substances and slowing down (but not stopping) their dissolution in the water. What would typically be over in seconds can be stretched to last a few minutes through the simple addition of cornstarch. 

So thank you, cornstarch, for making my bath bombs so much fun!


Why Unused Bath Bombs Lose Their Fizz Over Time

Colored Water

The reason that bath bombs lose their effectiveness is that when they are sitting in the back of your bathroom cupboard, they are fizzing away but at a much, much slower rate than they do when you drop them in the bathtub. Why? Because they are exposed to some amount of air (even when stored well).

Air contains moisture, so it contains molecules of water. The baking soda and citric acid react in the presence of these air-borne water molecules. The bath bomb slowly loses carbon dioxide gas and, therefore, its ability to effervesce when you drop it into your tub. 

Unless your bath bomb is kept in a very humid environment, you probably will not be able to see any visible change in the bath bomb. However, you might notice that your old bath bombs look more porous and crumble more easily than your new ones. This is because the chemical structure has been altered by the moisture in the air.

So while a bath bomb can lose its ability to shock and awe, this won’t cause an unused bath bomb to expire.


Secondary Bath Bomb Ingredients


If you’re an avid bath bomb user, or even if you have simply walked past Lush or a similar store, then you know that bath bombs can come in many shapes, sizes, smells, textures, and colors. You can attribute all the fun, delightful and intriguing variations in bath bombs to the secondary bath bomb ingredients. These secondary ingredients can also alter the shelf life of the bath bomb. And this is where we will find more of the answer to our question, “Do unused bath bombs expire?”

Essential Oils

A common secondary bath bomb ingredient is essential oils. These provide a scent to the bath bomb and add the element of aromatherapy to your bath experience. Different essential oils have different shelf lives ranging from one to eight years. 

Since one year is still longer than the shelf life of baking soda, essential oils should not shorten the shelf life of your bath bomb. However, there are other factors that you have to consider when it comes to essential oils: 

First, essential oils are not harmless just because they are natural substances. They actually have to be treated as you would treat a commercial chemical. Essential oils are considered safe for use while they are within their expiration date. After they expire, they have the potential to cause skin irritation and rashes. Read more about how to safely use essential oils in the tub.

Second, you have to be careful because when essential oils are exposed to light, moisture, or heat, they can change into substances that are potentially unsafe to use even before they expire. Once again, proper storage of bath bombs is vital. 

If you are making your own bath bombs, check out the 10 best essential oils for homemade bath bombs here.

Bath Salts

Many bath bombs are made with salts such as Epsom salts, Himalayan salts, or Dead Sea salts. These salts have various healing properties. Many people claim that soaking in a tub with these salts can improve the cardiovascular system, boost immunity and decrease stress. Bath salts do not expire.

The introduction of ingredients like oatmeal, butter, peel, milk, flower petals, and other fresh ingredients can reduce the shelf life of a bath bomb as these ingredients have the potential to become rancid or even grow mold. 

Can Bath Bombs Grow Mold Or Become Rancid?

Without ingredients like oatmeal, butter, milk, peel, and flower petals, bath bombs are unlikely to go rancid. However, they can grow mold if they are exposed to spores, water, or oxygen. It seems odd, but remember, bathroom mold can even grow on walls. 

If you see mold spots or any discoloration on your bath bomb, throw it out; do not use it. 

How To Properly Store Bath Bombs

Gift Wrapped Bath Bombs

Proper storage of bath bombs preserves the potency of the primary ingredients, maintains the integrity of secondary ingredients, and minimizes the chance of mold growth. The correct storage conditions for bath bombs are dark, dry, and cool. Keep your contained bath bombs away from sources of heat: sun, radiators, hot water pipes, underfloor heating, heating vents, etc. 

Golden rule: don’t store bath bombs in the bathroom, even if it is still in its packaging. Moisture and heat are inevitable in a bathroom, making it a less than ideal storage location.  

Keep your bath bombs in the packaging they come in, even if it’s not as aesthetic as bare bath bombs in a mason jar. Some bath bombs are sold ‘naked’. You should put these into an air-tight container as soon as you purchase them.  

Packaged or unpackaged, it’s a good idea to store your bath bombs in air-tight containers. These should have an uncompromised silicone or rubber seal. Glass or plastic doesn’t really make a difference; neither are porous, but make sure the containers have no cracks. You can also try zip-lock bags, but press as much of the air out as possible before sealing. 

For ideas on how to package homemade bath bombs, read my post 3 Fun Ways to Wrap DIY Bath Bombs.

So Do Unused Bath Bombs Expire?

3 bath bombs on a bathroom shelf

Unused bath bombs do expire after six months. That said, a standard bath bomb should not be harmful to use after its expiration date because the primary bath bomb ingredients do not go bad like food ingredients can. 

However, this can change depending on what secondary ingredients are added to the bath bomb. And since most store-bought bath bombs do contain essential oils, which do go rancid and can become harmful, it’s a good rule of thumb to discard a bath bomb if you suspect you’ve had it for a year or longer.

Finally, a bath bomb that shows discoloration or any signs of mold should not be used at all. But don’t despair if you determine it’s time to throw out your old bath bomb. There are so many other fun and healing baths to try! How about a bath with Epsom salts and essential oils or a detox bath to really destress and lift your mood.

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