Short answer
IFRA's limits for skin-sensitizing materials come from a Quantitative Risk Assessment, now in its second version (QRA2). It starts from a benchmark dose that is not expected to induce sensitization (the NESIL, in micrograms per square centimeter of skin), divides it by a sensitization assessment factor (SAF) to get an acceptable exposure level, and divides that by how much of each product type consumers put on a square centimeter of skin per day. The result is a maximum concentration per product category, which QRA2 then reduces for products used together on the same body area (aggregate exposure). That is why one material has a different limit in a body lotion, a perfume and a candle.
Why a perfumer should understand it
You do not calculate IFRA limits yourself — the Standards publish them, and the method for checking a formula against them is in how to check IFRA limits. But knowing where the numbers come from explains things that otherwise look arbitrary: why categories exist, why a body mist can face tighter limits than an eau de parfum, why candles are unrestricted for sensitization, and why limits move when new data arrive. What the Standards are, and how they relate to law, is covered in IFRA Standards explained.
The four quantities
| Term | Meaning | Units |
|---|---|---|
| NESIL | No Expected Sensitization Induction Level — the dose per area of skin not expected to induce sensitization, derived from experimental and human data | µg/cm² |
| SAF | Sensitization assessment factor — a safety factor covering differences between people, the product matrix and the conditions of use | None |
| AEL | Acceptable exposure level = NESIL ÷ SAF | µg/cm² |
| CEL | Consumer exposure level — how much product reaches a square centimeter of skin per day in the product type that drives the category | mg/cm²/day of product |
A use is considered acceptable when the exposure to the material stays at or below the AEL. Since exposure to the material is the product exposure multiplied by the material's concentration in the product, the maximum concentration is:
maximum concentration = AEL ÷ product exposure = NESIL ÷ (SAF × CEL)
A worked example from IFRA's own guidance
IFRA's Guidance for the use of the IFRA Standards (51st Amendment) works the method through for coumarin, a weak sensitizer with a NESIL of 3,500 µg/cm². Taking two categories from its tables:
| Step | Category 4 (fine fragrance) | Category 9 (rinse-off; driven by bar soap) |
|---|---|---|
| SAF | 100 | 300 |
| AEL = 3,500 ÷ SAF | 35.0 µg/cm² | 11.7 µg/cm² |
| CEL of the driving product | 2.21 mg/cm²/day = 2,210 µg/cm²/day | 0.2 mg/cm²/day = 200 µg/cm²/day |
| Maximum concentration = AEL ÷ CEL | 35.0 ÷ 2,210 = 1.58% | 11.7 ÷ 200 = 5.83% |
| Aggregate adjustment factor | 0.95 | 0.50 |
| QRA2 upper limit | 1.58 × 0.95 = 1.5% | 5.83 × 0.50 = 2.9% |
The arithmetic was checked against the guidance's table. These are the guidance's teaching figures for the sensitization endpoint only. The limit printed in a Standard is the lowest of the QRA2 result and any limit from other endpoints, such as systemic toxicity, so never take a number from this example as coumarin's current limit — read the Standard.
Why categories exist
Every product type has its own exposure. IFRA groups product types into twelve categories, and the limit for each category is driven by the product in it with the highest exposure. According to the guidance, the driving products include lip products (Category 1), deodorants and antiperspirants (Category 2, which also takes body sprays and mists by default), fine fragrance (Category 4), hair sprays (Category 7), bar soap (Category 9) and hand dishwashing detergent (Category 10). Category 12 — candles, incense, automated air fresheners — involves negligible skin contact, so for sensitization its concentrations are not restricted.
Two consequences for formulators:
- Choose the category from the product's real use. A body mist labeled for use anywhere on the body falls in Category 2, with lower limits than Category 4; labeled not for the underarms, it can be assessed under Category 4. See body mist formula.
- Higher SAFs follow vulnerable or occluded skin. The guidance's SAFs are 300 for underarm products, baby wipes and rinse-off body products, for example, against 100 for fine fragrance.
What QRA2 added
QRA was adopted for IFRA Standards with the 40th Amendment in 2006. QRA2, used from the 49th Amendment, brought two refinements:
- Aggregate exposure. People use many fragranced products on the same body area every day. QRA2 uses a model based on consumer habits surveys (the Creme RIFM model) to estimate combined exposure per body area and reduces each product's limit by an adjustment factor until the combined exposure is acceptable. That is the 0.95 and 0.50 in the table above.
- Revised safety factors and redefined categories that better reflect how products are used.
What the QRA does not do
- It addresses induction, not elicitation. It aims to prevent people becoming sensitized. Someone already allergic can react at lower levels, which is why allergen labeling exists alongside the Standards; see the fragrance allergens list.
- It covers one endpoint. Other Standards rest on phototoxicity, systemic toxicity or other concerns.
- It does not assess your product. A formula within every limit still needs a safety assessment where the law requires one; see what a safety assessor needs.
Why limits change
A new NESIL from better data, a change in exposure estimates, or a recategorized product type changes the output. That is one reason each amendment can tighten or loosen limits, and why a formula compliant last year needs rechecking after an amendment is notified. The procedure is in IFRA Standards explained.
Sources
- IFRA — Guidance for the use of the IFRA Standards (51st Amendment, June 30, 2023): §3.1 on QRA and QRA2, Table 6 (SAF, CEL and aggregate adjustment factors), Table 7 (coumarin example), Table 8 (categories); checked October 10, 2026.
- IFRA — Using the Standards, checked October 10, 2026.
Frequently asked questions
Can I calculate my own limit for a material that has no Standard?
Not credibly without a validated NESIL and exposure data. Absence of a Standard is not proof of safety; it is a question for a safety assessor.
Why is the fine fragrance limit sometimes higher than the body lotion limit?
Because a lotion puts much more product on much more skin, so its exposure per square centimeter per day is higher, and the same acceptable exposure is reached at a lower concentration.
Does a lower SAF mean IFRA trusts a product type more?
It means the conditions of use add less uncertainty. Occluded skin, damaged skin and vulnerable users get larger factors.
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.