Doing the Allergen Math on an Essential Oil Soap Blend

You blend three essential oils for a soap's scent, and now you need to know what to disclose as fragrance allergens. The instinct is to look up each oil's name and see if it's "on the list." That's not how this works, and treating it that way is how makers end up under-disclosing without realizing it.

Essential oils are not single ingredients for allergen purposes

An essential oil is a complex mixture of dozens of naturally occurring compounds. Several recognized fragrance allergens, Limonene, Linalool, Citronellol, Geraniol, Eugenol, and Coumarin among them, occur naturally within many essential oils as constituent compounds, not as something separately added. Lavender essential oil naturally contains linalool. Citrus oils naturally contain limonene. This is just the chemistry of the plant material, not a formulation choice.

So the question isn't "is bergamot oil a fragrance allergen." It's "what percentage of this specific batch of bergamot oil is made up of named allergen constituents, and what does that work out to once I use this oil at my recipe's fragrance load."

The two-step calculation

Step one: find the constituent percentage within the oil.

Your supplier's certificate of analysis or technical data sheet should show a breakdown of the oil's major constituents, often as a percentage range. This is the number you need, not the oil's overall name or a generic assumption based on what the oil is "known for." Batches vary, and suppliers who sell to cosmetic and soap makers regularly should have this documentation available or be able to get it to you.

Step two: carry that percentage through your recipe.

Multiply the constituent's percentage within the oil by the oil's use rate in your finished soap. If an essential oil is documented at, say, 40% linalool content, and you're using that oil at a 2% fragrance load in your recipe, the linalool ends up at a much smaller fraction of your finished bar, specifically 40% of 2%, which works out to 0.8% of the recipe by that oil alone. If you're blending multiple oils that each contain overlapping allergens, like linalool showing up in both a lavender and a bergamot component of your blend, add those contributions together before checking against the threshold.

Checking against the right threshold

Soap is generally a rinse-off product, so the relevant disclosure trigger is above 0.01% (100 ppm) in the finished product, higher than the leave-on threshold of above 0.001% (10 ppm) that applies to things like lotions and leave-in conditioners. Run your calculated allergen concentration against the 100 ppm rinse-off threshold, not the leave-on one, when you're working specifically with soap.

A worked example

Essential oil in blend	Use rate in recipe	Documented linalool % in oil	Contribution to finished soap
Lavender oil	2%	35%	0.7% (well above 0.01% threshold)
Bergamot oil	1%	5%	0.05% (above threshold)
Combined linalool contribution			0.75%, needs disclosure

Note how small individual percentages of an oil in the recipe can still add up to an easily-disclosable allergen concentration once you do the constituent math. This is the trap of eyeballing it instead of calculating it.

Keeping this manageable across a product line

If you make several soap variants with overlapping essential oil blends, it's worth building a reference table once per essential oil, its documented constituent breakdown, so you're not requesting fresh certificates of analysis every time you tweak a recipe. Update it if you switch suppliers, since constituent percentages can vary meaningfully between suppliers and even between harvests for some botanicals.

With both List 1 and, from August 2026, List 2 allergens now mandatory for disclosure on the CNF and label, getting this constituent math right per blend matters more than it used to. This is exactly the calculation Cosmetic Comply automates as part of preparing a Canadian filing, expanding each essential oil in your recipe into its allergen constituents and carrying the math through to your finished product's real concentrations, rather than leaving you to track spreadsheets across every blend by hand.