Thioglycolic Acid in Relaxers and Perms: Hotlist Rules

Perm and relaxer formulas sit in an unusual spot on the Hotlist compared to most restricted ingredients, because it's not just a maximum percentage you're checking. Thioglycolic acid and its salts come with layered conditions, concentration, product type, and pH all interacting at once, and missing one of them is easy if you're used to simpler single-number limits elsewhere.

What thioglycolic acid actually does

Thioglycolic acid and its various salts, ammonium thioglycolate being the most common in professional and home perm kits, work by breaking disulfide bonds in hair keratin. That's the whole chemistry behind both perms and relaxers, you break the bonds that hold hair's shape, reshape it around rods or with tension, then use a neutralizer to reform the bonds in the new configuration. It's an effective and long-established technology, and it's also exactly the kind of ingredient that needs careful concentration control, since the same bond-breaking action that reshapes hair can damage hair and irritate scalp skin if concentration or exposure time run too high.

Why the Hotlist treats it as conditional, not flat-banned

The Cosmetic Ingredient Hotlist lists thioglycolic acid and related thioglycolates as restricted rather than prohibited, meaning they can be used in specific product types under specific conditions rather than being banned outright. The conditions generally hinge on:

• Product category. Home-use perm and relaxer products are typically held to different concentration ceilings than professional, salon-applied products, on the logic that a trained operator applying a product under controlled conditions is a different risk picture than an untrained consumer at home.
• Concentration of free thioglycolic acid. This is calculated as the free acid, not just the total salt weight, which is a detail that catches people who only check the raw ingredient percentage on their supplier's spec sheet without converting to free acid equivalent.
• pH range. Perm and relaxer chemistry only works, and only stays within a reasonably safe irritation profile, inside a specific pH window. Outside that window, either the product won't perform as intended or it becomes considerably harsher on skin and scalp.

Because the exact percentage ceilings and pH ranges are the kind of detail that gets refined by Health Canada over time, and because home-use versus professional-use limits differ, this is a case where you genuinely need to pull the current Hotlist entry directly rather than relying on a number you remember from a past formulation. Treat any specific percentage you've seen quoted elsewhere as a starting point for verification, not a final answer.

What this means for your filing

If you're formulating or private-labeling a relaxer or perm product, a few things to nail down before you file a Cosmetic Notification Form:

1. Confirm which salt form you're using and get its correct INCI name from your supplier, since ammonium thioglycolate, glyceryl thioglycolate (more common in acid perms and some relaxer systems), and other salt forms are distinct entries.
2. Get the free acid percentage, not just the salt concentration, from your supplier's documentation. This is the number that actually gets checked against the Hotlist limit.
3. Confirm your pH falls within the permitted range for your product category, and keep that pH data on file as part of your formulation records.
4. Classify correctly as home-use or professional-use, since this affects which concentration ceiling applies to your specific product.

A quick reference on what to check

Factor	Why it matters
Salt form (ammonium, glyceryl, etc.)	Determines correct INCI name and may carry distinct limits
Free thioglycolic acid %	The actual figure checked against Hotlist limits, not total salt weight
Product pH	Perm and relaxer chemistry is pH-dependent; limits are often tied to a specific range
Home-use vs professional	Different concentration ceilings typically apply

The bigger picture

Thioglycolates are a good example of why the Hotlist isn't just a simple banned-substances list. A lot of restricted entries carry these layered, conditional structures, concentration tied to product type, tied to intended use, sometimes tied to pH or particle size for other ingredient classes entirely. Reading a Hotlist entry in full, rather than pattern-matching from memory, is the safer habit across the board, and it matters especially here given how directly this chemistry interacts with skin and scalp exposure.

If you're managing a haircare line that includes a perm or relaxer product alongside gentler leave-on items, Cosmetic Comply screens each formula against the current Hotlist with a confidence score and has a real reviewer look at anything that needs a closer read, which is particularly useful for exactly this kind of conditional, multi-factor restriction.