Clove, carnation, pumpkin spice, gingerbread: a lot of autumn and winter candles lean on two molecules, eugenol and isoeugenol. They have the same formula and almost the same shape, and on your fragrance's safety data sheet they sit next to each other. On your candle label they behave very differently. A few tenths of a percent of isoeugenol can do what takes ten percent of eugenol. Here's why, and how to see it coming.

Two molecules, one double bond apart

Both are C10H12O2: a guaiacol ring (a phenol with a methoxy group) carrying a three-carbon side chain. The only difference is where the double bond sits in that chain. In eugenol (CAS 97-53-0) it's at the end of the chain. In isoeugenol (CAS 97-54-1) it's moved one position in, next to the ring.

That small shift changes the smell:

  • Eugenol is the smell of cloves: warm, spicy, a little medicinal. It's the main component of clove bud oil, usually three quarters or more of it. One analysis published in Molecules in 2016 (Xu et al.) measured 76% eugenol in clove bud oil.
  • Isoeugenol is softer and sweeter, leaning from the spice towards the flower. Perfumers use it as the backbone of carnation accords, and in small amounts to round off spice, oriental and floral blends.

It also changes how strongly each one sensitises skin, and that's what the label rules care about.

How the EU classifies each one

We checked ECHA's Classification & Labelling Inventory on 29 September 2026:

  • Isoeugenol (EC 202-590-7) has a harmonised classification in CLP Annex VI, index 604-094-00-X, last updated in the 13th adaptation (ATP13): Skin Sens. 1A, H317, with a specific concentration limit of 0.01%. Signal word "Warning", exclamation mark pictogram.
  • Eugenol (EC 202-589-1) has no harmonised classification. Suppliers classify it themselves, and the headline entry ECHA shows from those notifications is Skin Sens. 1B, H317 and Eye Irrit. 2, H319.

Those two lines make a hundredfold difference. In a finished candle, a Skin Sens. 1B substance triggers H317 from 1%. Isoeugenol's specific limit triggers it from 0.01%. The EUH208 "Contains…" line kicks in at a tenth of each: 0.1% for eugenol, 0.001% for isoeugenol.

Eugenol's eye irritation doesn't matter at candle doses. A mixture only becomes Eye Irrit. 2 from 10% of such ingredients, far more eugenol than any candle holds.

IFRA agrees on the potency, but not for candles

The IFRA Standards show the same gap in potency. For fine fragrance (Category 4), IFRA allows up to 2.5% eugenol in the finished product but only 0.11% isoeugenol.

For candles, which IFRA places in Category 12, both standards say "No restriction". So a candle fragrance can legitimately contain more isoeugenol than a perfume could. The IFRA certificate won't flag it. The CLP label will.

A worked example: a spiced pumpkin candle

Take a hypothetical spiced pumpkin fragrance oil whose SDS lists 8% eugenol and 0.3% isoeugenol, used at a 10% load. The finished candle contains 0.8% eugenol and 0.03% isoeugenol.

  • Eugenol at 0.8% is under its 1% limit. On its own it gives no pictogram, only the EUH208 line.
  • Isoeugenol at 0.03% is three times its 0.01% limit. The candle is classified H317, with the exclamation mark and "Warning".

So the ingredient present at 0.3% of the oil decides the pictogram, and the one at 8% doesn't. Because the candle is now classified as a sensitiser, eugenol is still named on the label, alongside isoeugenol, since it's above its 0.1% declaration level.

Now flip the question: how much of each can the oil contain, at a 10% load, before the candle needs a pictogram?

  • Eugenol: up to just under 10% of the oil.
  • Isoeugenol: up to just under 0.1% of the oil.

Dropping the load doesn't help much with isoeugenol. At 6%, this oil still puts 0.018% isoeugenol in the candle, above the limit. You'd need a load under about 3.3% to get below it, which is too weak for most spiced scents. The practical fix is a fragrance with less isoeugenol, not a lighter hand.

Eugenol goes the other way when clove is the star. A blend that is 15% clove bud oil, at roughly three quarters eugenol, already carries about 11% eugenol. At a 10% load that's 1.1% in the candle, over the 1% line, and H317 comes from eugenol alone. Read your essential oil components on the SDS, not just the headline name.

What to do with this

  • Look for isoeugenol first in any clove, carnation, spice or gingerbread fragrance. At a 10% load, anything from 0.1% in the oil means a pictogram.
  • Don't treat the IFRA certificate as a label check. "No restriction" in Category 12 says nothing about your CLP classification.
  • Check which eugenol classification your supplier uses. It's self-classified, so SDSs can differ. Most show Skin Sens. 1B, and a plain Skin Sens. 1 has the same 1% limit.
  • Watch the small print on similar molecules. Cinnamaldehyde works the same way as isoeugenol: a harmonised Skin Sens. 1A with a 0.01% limit, and it often sits in the same spiced blends.

Molekule reads every component from your fragrance's SDS, checks each one against ECHA's harmonised and industry classifications, applies specific concentration limits like isoeugenol's, and shows you the load at which each line appears on your candle label. Check your spiced candle's label in Molekule.