If you make orange, lemon, grapefruit or "fresh" candles, limonene is almost certainly the biggest single ingredient in your fragrance oil. It's the molecule that makes up most of citrus peel oil. It also explains why citrus candles fade fast, why your label says H317, and why two SDSs for the same molecule can give your candle different pictograms.

What limonene is

Limonene is a monoterpene, C10H16, and it comes in two mirror-image forms. (R)-(+)-limonene, also called d-limonene, is the one pressed out of orange and other citrus peel. (S)-(−)-limonene, or l-limonene, is much less common. Textbooks say the first smells of orange and the second of pine or turpentine.

The reality is less tidy. A 2021 study in the Journal of Chemical Education (Kvittingen et al.) ran odour tests with analytical-grade enantiomers and found that few people linked (R)-limonene with orange or (S)-limonene with lemon. The same paper collects perfumery sources describing very pure limonene as weak and harsh, with d-limonene fresh and slightly citrusy. The bright "orange" you smell in a peel comes from limonene plus small amounts of much stronger-smelling minor components. Limonene is the bulk and the lift, not the whole character.

Why citrus fades first

Limonene is small and light. It boils at about 176 °C and has a vapour pressure of around 0.19 kPa at 20 °C, which makes it one of the most volatile materials in a typical fragrance oil. In perfumery terms it's a classic top note.

  • Cold throw (the unlit candle) is often strong at first, because limonene leaves the wax surface easily.
  • That same volatility means it's the first thing to go. A citrus candle stored uncovered for weeks, or poured too hot, can lose its sparkle while the heavier notes stay.
  • Hot throw (the lit candle) gets a quick burst of citrus as the melt pool forms, then heart and base notes carry the scent.

That's why citrus blends are usually anchored with heavier materials, and why lids, cool pour temperatures and fresh oil matter more for citrus than for vanilla or woods.

Limonene oxidises, and that's the allergy problem

Fresh limonene is a weak sensitiser. The trouble starts when it sits in air. It oxidises into limonene hydroperoxides, which are strong contact allergens. Karlberg and Dooms-Goossens showed in 1997 (Contact Dermatitis) that air-exposed d-limonene caused patch-test reactions in dermatitis patients at rates comparable to allergens in the standard test series.

The fragrance industry's response is an IFRA specification, not a usage limit: limonene and natural oils rich in it should be kept as free of peroxides as practical, for example with an antioxidant such as 0.1% BHT or tocopherol, and should have a peroxide value below 20 mmol/L. For you, that means buying from suppliers who certify IFRA compliance, keeping oil bottles closed, cool and away from light, and not using citrus oil that has been open for a long time.

Limonene is also one of the original 26 cosmetic fragrance allergens, and it stays on the expanded EU allergen list of Regulation 2023/1545. Candles aren't cosmetics, though. For a candle, what counts is its CLP classification.

Limonene's CLP classification: check the CAS number

We checked ECHA's database on 29 September 2026. There are two different harmonised entries in CLP Annex VI, and which one applies depends on the CAS number your SDS gives:

  • d-limonene, (R)-p-mentha-1,8-diene, CAS 5989-27-5, EC 227-813-5, index 601-096-00-2. Revised by the 17th ATP, applied since 17 December 2022: Flam. Liq. 3 (H226), Asp. Tox. 1 (H304), Skin Irrit. 2 (H315), Skin Sens. 1B (H317), Aquatic Acute 1 (H400, M-factor 1) and Aquatic Chronic 3 (H412). Signal word "Danger".
  • Dipentene; limonene, the racemic mixture, CAS 138-86-3, EC 205-341-0, index 601-029-00-7: Flam. Liq. 3 (H226), Skin Irrit. 2 (H315), Skin Sens. 1 (H317), Aquatic Acute 1 (H400) and Aquatic Chronic 1 (H410). Signal word "Warning".

So H304 is part of d-limonene's harmonised classification, not just an industry self-classification. It's why a citrus fragrance oil is often labelled "Danger". A solid candle doesn't inherit it, as we explain in our post on the H304 aspiration hazard in candles. The bigger difference for candle makers is the aquatic line: Chronic 3 for d-limonene, Chronic 1 for dipentene. That's a factor of 100 in CLP's summation method.

A worked example: 40% limonene at 8% and 10%

Take a hypothetical sweet orange fragrance oil whose SDS lists 40% limonene. In the candle, that becomes:

  • 10% load: 4.0% limonene in the finished candle.
  • 8% load: 3.2% limonene.

Skin sensitisation. Both forms of limonene trigger H317 in a mixture at 1% or more. At 4.0% and 3.2%, the candle is H317 with the exclamation mark and "Warning", and limonene is named on the label as one of the substances responsible. Because it's already classified, it doesn't also go on an EUH208 "Contains…" line. You'd only land on EUH208 for limonene with 0.1% to under 1% in the candle, which here means a load under 2.5%.

Skin irritation. Skin Irrit. 2 ingredients only classify a mixture from 10% combined. Limonene alone at 4% doesn't get there, but it adds to any other irritants in the oil.

Aquatic hazard, if the SDS says CAS 5989-27-5 (d-limonene). Aquatic Acute 1 with M-factor 1 needs 25% to classify the candle, so no H400. Aquatic Chronic 3 also needs 25% on its own. At 4% or 3.2%, limonene adds only a small amount to the aquatic total. No aquatic statement from limonene.

Aquatic hazard, if the SDS says CAS 138-86-3 (dipentene). Now it's an Aquatic Chronic 1 ingredient, which counts ten times over. With an M-factor of 1, 2.5% or more in the candle makes it Aquatic Chronic 2. At both 4.0% and 3.2%, the candle is H411 with the environment pictogram. You'd need to drop the load below 6.25% just to get down to H412 with no pictogram.

Same oil, same load, same molecule in the jar, and one label has a dead tree and fish while the other doesn't. The deciding factor is the identifier in Section 3 of the SDS. Real citrus oils also contain citral, linalool and other terpenes that add to these totals, so the full calculation always matters.

What to do with this

  • Read the CAS number next to limonene on your fragrance SDS. If it says 138-86-3 for an orange-derived material, ask your supplier whether 5989-27-5 is the correct identifier.
  • Expect H317 on most citrus candles. At typical loads, 40% limonene in the oil is far above the 1% trigger.
  • Protect the oil from air. Fresh, closed, IFRA-compliant citrus oil smells better and carries fewer hydroperoxides.

Molekule reads each component and its CAS number from your fragrance SDS, matches it to ECHA's live harmonised entries and runs these calculations for your candle at the load you choose. Check your citrus candle's label in Molekule.