Short answer
Bath bombs, bath salts and bath oils are IFRA Category 9 — products added to bathwater, treated with soaps and shower gels as rinse-off. The fragrance problems are physical rather than chemical: fragrance oil floats on bathwater unless something disperses it, any water in the fragrance or the binder starts a bath bomb's fizz too early, and salts clump or sweat. Weigh the fragrance against the whole batch, check it against Category 9, use a dispersant where oil would otherwise sit on the water, and test in a real bath.
What IFRA's guidance says
The guidance places "bath gels, foams, mousses, salts, oils and other products added to bathwater (such as bath bombs)" in Category 9. Foot soaks — products for a foot bath — are in Category 9 as well. Category 9 limits are usually more generous than leave-on ones, but every restricted material still has a limit there, and the check is done on the product as sold, not on the diluted bathwater. The method is in how to check IFRA limits.
Bath bombs
A bath bomb is mainly sodium bicarbonate and citric acid, which react and fizz when wet. The fragrance goes into the dry mix with oils and a binder, and three things follow from that:
- Moisture starts the reaction. Fragrance oils are normally anhydrous, but some contain water-miscible solvents or a little water, and so do some botanical extracts. A mix that "grows" or crumbles in the mold has often taken on moisture from somewhere. Test each new fragrance in a small batch.
- Oil floats. In the bath, fragrance and butters released from the bomb float on the surface and can coat the tub and the bather's skin in a thin film of more concentrated fragrance. A dispersing agent — polysorbate 80 is commonly used — helps spread the oils through the water instead.
- Color and discoloration. Vanillin and some other materials discolor over time; in a pale bomb that shows. Colorants used in bath products are themselves regulated cosmetic ingredients in most markets; see adding color to perfume for the colorant rules.
The fragrance level is the fragrance's weight over the finished batch: 15 g of fragrance in a 1,000 g batch is 1.5%. Bath bombs are often scented at visibly higher levels than lotions because the bath dilutes them, but the Category 9 check applies to the bomb itself.
Bath salts
- Salts are blended with fragrance (sometimes premixed with a little oil or a dispersant) until evenly coated, then left to absorb before packing.
- Fine salts take up fragrance more evenly than coarse crystals; coarse salts can leave fragrance pooling in the jar.
- Some salts, Epsom salt in particular, draw moisture from the air and clump; airtight packaging helps.
- The fragrance level is fragrance over the total, as for bombs.
Bath oils
A bath oil is either floating — a plain oil that spreads as a film on the water — or dispersing, with an emulsifier or dispersant that makes the water milky as it is poured in. A floating oil leaves its fragrance in the surface film, in direct contact with the skin at the waterline, which is a reason to keep its fragrance level conservative and avoid irritant materials. A dispersing oil spreads the fragrance through the water. Whichever you make, the fragrance dissolves in the oil phase; the arithmetic is the same as for perfume oils, but the category is 9, not 4.
Materials to be careful with
Bathwater is warm and the skin is soaking in it, so a few materials that are unremarkable in a perfume deserve a second look:
- Spice oils such as cinnamon bark and clove, and other materials known as irritants, are best kept low or out of floating oils and undispersed bombs.
- Expressed citrus oils are phototoxic on skin later exposed to sun. IFRA's guidance treats Category 9 as rinse-off for phototoxicity, which is generally less restrictive than leave-on, and its newer Standards set rinse-off limits for some phototoxic materials; read the Standard for each oil you use. See IFRA phototoxicity calculations.
- Glitter and other insoluble decorations bring their own rules in some markets, unrelated to fragrance.
Testing
- Make a small batch with the fragrance and one without, from the same mix.
- Watch the scented batch for growth, cracking or early fizz over 48 hours.
- Store samples warm, cold and in light for four weeks; check odor and color against the control (stability testing).
- Use one in a real bath: does the scent bloom, is there an oily film, does it stain the tub, how does the skin smell afterwards?
Records
- Fragrance code and version; level as a percentage of the batch; Category 9 check.
- Dispersant and its level; colorants and their identity.
- Batch weight, date, and test results.
Keeping each bath product as its own formula linked to the parent fragrance is described in adapting a fragrance to a different product base; the load arithmetic for other products is in fragrance load calculation.
Sources
- IFRA — Guidance for the use of the IFRA Standards, Table 12 (bath products and foot soaks: Category 9) and Table 5 (phototoxicity considerations by category); checked October 11, 2026.
Frequently asked questions
Why do my bath bombs expand in the mold?
Moisture has started the acid–base reaction. Common sources are humid air, a wet binder, or a fragrance or extract that contains water or water-miscible solvent. Test fragrances one at a time in small batches.
Do I need polysorbate in a bath bomb?
Not legally, but without a dispersant the fragrance and oils float on the water. Whether that is acceptable depends on the fragrance level, the materials and how the product is used; many makers include one.
Is a bath oil a perfume oil?
No. A perfume oil worn on the skin is fine fragrance, Category 4; an oil poured into a bath is Category 9.
Written and reviewed by the RUŌOD Lab team. This article is general education about perfume formulation and record-keeping; it is not legal, regulatory or safety advice, and the examples are illustrations, not validated commercial formulas. How we write and check these guides.