Rounding Rules When an Allergen Sits Near the Threshold

Somebody always finds the edge case. You run the math on a fragrance oil, multiply it by the use level, and get an allergen concentration of 0.0009%. The threshold is 0.001%. Do you round up and declare it, or round down and skip it?

This comes up constantly once List 1 and List 2 fragrance allergen disclosure become mandatory in Canada, first for List 1 on April 12, 2026, then List 2 on August 1, 2026. The thresholds themselves are simple: above 0.001% (10 ppm) in a leave-on product, above 0.01% (100 ppm) in a rinse-off product. What's not simple is what happens when your calculated number sits close enough to that line that rounding decides the outcome.

Why this happens more than you'd expect

Allergens like Limonene, Linalool, Citronellol, Geraniol, Eugenol, and Coumarin rarely get added to a formula on their own. They arrive as trace components of essential oils and fragrance blends. A lavender essential oil might carry linalool at a certain percentage of the oil itself. You use the oil at 0.5% in your lotion. The linalool concentration in the finished product is the oil's linalool percentage multiplied by that 0.5% use level, and that math often lands you somewhere inconveniently close to the trigger point rather than comfortably above or below it.

Add multiple fragrance components from more than one essential oil in the same formula, each contributing a little bit to the total for a given allergen, and you can end up with a cumulative figure that changes meaningfully depending on how many decimal places you carry through the calculation.

The core principle: calculate first, round last

The safest practice is to do your arithmetic with full precision and only round the very final answer, not each intermediate step. If you round the essential oil's declared linalool percentage before multiplying by use level, and then round again after multiplying, you can shift a result that's genuinely above threshold down to something that looks like it's under, purely from accumulated rounding rather than any real change in the formula.

A cleaner sequence looks like this:

1. Get the supplier's actual percentage of the allergen within the raw material (fragrance oil, essential oil, or blend), not a rounded label value if you can get the underlying data.
2. Multiply by the raw material's use level in your finished formula.
3. If more than one raw material contributes the same allergen, sum all contributions before rounding anything.
4. Round only the final combined figure, and round it to enough decimal places that you're not accidentally erasing the difference between "just above" and "just below" the threshold.

When the number is genuinely borderline

If your final, carefully calculated figure is still sitting right at the edge, close enough that measurement uncertainty in the supplier's data could plausibly put you on either side, the practical answer is to declare it. Declaring an allergen that turns out to be technically just under threshold costs you a line on an ingredient list. Failing to declare one that was actually over threshold is a compliance gap that surfaces at the worst possible time, usually after a complaint or an audit.

There's also a second reason to lean toward disclosure in borderline cases: supplier documentation on natural essential oils isn't always precise to four decimal places, and batch-to-batch variation in a natural raw material is real. If your calculation depends on trusting a supplier's typical value to the fourth decimal place, you're already in territory where declaring is the more defensible choice.

A worked example

Say a body lotion uses 0.8% of a fragrance blend, and that blend's certificate of analysis lists geraniol at 1.2% of the blend. The finished product concentration works out to 0.8% times 1.2%, which is 0.0096%, expressed as a decimal that's 0.000096, or 0.0096% written as a percentage. That's below the 0.01% rinse-off threshold but comfortably above the tighter 0.001% leave-on threshold. So the same blend, at the same use level, is a clear "no disclosure needed" for a rinse-off shampoo and a clear "must disclose" for a leave-on lotion. This is exactly why product type, not just concentration, decides the outcome, and why redoing the calculation for every product format matters even when the fragrance blend itself is identical.

Where this fits into filing

Getting allergen math right matters because it feeds directly into what goes on the CNF and eventually onto the label itself once the mandatory dates hit. Cosmetic Comply's ingredient matching carries supplier blend percentages through to the finished-product level automatically, so the multiplication step that trips people up on paper gets handled consistently, with a real reviewer checking anything that lands near a threshold before the filing goes in.