“Natural” has become one of the most powerful words in skincare. It sounds gentle. Simple. Safe.
But from an allergy standpoint, natural tells us remarkably little.
As dermatologists who specialize in allergic contact dermatitis, we think about ingredients differently. We don't start by asking whether something came from a plant or a laboratory. We ask: What molecules are actually present? How chemically reactive are they? What happens to them over time? And how likely are they to trigger an immune response in the skin?
Aloe and lavender offer a fascinating example. Both come from plants. Both are widely used in skincare. But their chemistry—and their potential to cause allergic contact dermatitis—could hardly be more different.
First, a little immunology: how does a skincare ingredient become an allergen?
Allergic contact dermatitis is not the same thing as irritation.
An irritant can directly injure the skin barrier. An allergen, on the other hand, triggers a specific immune response—classically a delayed, type IV hypersensitivity reaction.
And here's where chemistry becomes important.
Many contact allergens are small molecules called haptens. On their own, they're too small to generate the full immune response. But chemically reactive haptens can bind to proteins in the skin.
Think:
small chemical + skin protein → new hapten-protein complex
That newly modified protein can be recognized and processed by the immune system. Antigen-presenting cells participate in activation of allergen-specific T cells, creating immunologic memory.
This is sensitization.
Once sensitized, subsequent exposure can recruit those memory T cells back to the skin and produce the red, itchy, sometimes blistering eruption we recognize as allergic contact dermatitis.
But here's an important wrinkle:
The molecule you put on your skin isn't necessarily the molecule your immune system ultimately encounters.
Some ingredients undergo chemical reactions after formulation or exposure to the environment.
Lavender is a beautiful example.
Lavender: when air changes the chemistry
Lavender essential oil contains a complex mixture of volatile molecules. Two particularly important components are the terpenes linalool and linalyl acetate.
Linalool itself is considered a relatively weak sensitizer.
But linalool has an interesting chemical characteristic: its molecular structure makes it susceptible to oxidation.
When exposed to oxygen in the air, linalool undergoes autoxidation and generates new molecules, including linalool hydroperoxides.
In simplified form:
Linalool + O₂ → oxidized linalool → linalool hydroperoxides
And those hydroperoxides are substantially more potent skin sensitizers than the original molecule.
This makes linalool what chemists call a prehapten: a substance that has relatively low allergenic activity initially but becomes a more potent sensitizer after chemical transformation outside the skin.
This isn't just theoretical chemistry.
Linalyl acetate—the major component of lavender oil—undergoes a similar process. In an experimental model, highly purified linalyl acetate was a weak sensitizer, with an EC3 of 25%. After ten weeks of air exposure and oxidation, its EC3 dropped to 3.6%, indicating a marked increase in sensitizing potency. Hydroperoxides were among the most potent oxidation products identified.
And researchers have demonstrated that this chemistry occurs in lavender oil itself.
When lavender oil was exposed to air, its terpenes oxidized and generated the same types of oxidation products seen with the isolated chemicals. As oxidation occurred, the sensitizing potency of the lavender oil increased.
In other words:
The lavender oil in a product today may not be chemically identical to the lavender oil that was originally put into the bottle.
Air matters. Time matters. Chemistry matters.
And this matters in actual patients
This isn't simply something interesting that happens in a chemistry laboratory.
In a study of patients with suspected allergic contact dermatitis who were patch tested with oxidized lavender oil, 2.8% had positive reactions. Interestingly, testing only individual fragrance markers would not have identified every patient who reacted to the oxidized lavender oil.
Hydroperoxides of linalool are now important allergens in contemporary patch testing. A 2022 review found consistently high rates of positive reactions among dermatitis patients tested with oxidized linalool and limonene, while also emphasizing some of the complexities in interpreting weaker patch-test reactions.
And newer research continues to demonstrate that this is a true immunologic phenomenon. In sensitized individuals, repeated exposure to linalool hydroperoxides produced dose-dependent dermatitis accompanied by measurable activation of immune pathways in the skin.
So when we talk about lavender as a potential contact allergen, we aren't saying:
Lavender is bad because it is natural.
We're saying something much more interesting:
The molecular constituents of lavender can undergo chemical transformation, creating compounds with greater sensitizing potential.
Now let's look at aloe
Aloe is also a botanical.
Yet despite its enormous popularity and widespread exposure, allergic contact dermatitis to aloe appears to be remarkably uncommon.
One study patch tested 702 consecutive patients using three different aloe preparations: an oily leaf extract, material from the whole plant, and concentrated aloe vera gel.
Not one patient reacted to any of the preparations.
That doesn't mean aloe allergy is impossible. Case reports exist, as they do for an extraordinary number of substances.
But compare that experience with lavender and its oxidized terpenes, and the contrast is striking.
Why?
Again: chemistry.
“Aloe” isn't even one chemical substance
The part of the aloe plant matters.
The outer portions of the leaf contain anthraquinone compounds, while the inner aloe gel used in many skincare products has a very different composition and is predominantly water with carbohydrate-rich constituents.
The investigators in that 702-patient patch-test study specifically proposed that the carbohydrate-rich composition of aloe gel may help explain its extremely low sensitization potential: carbohydrates are generally unlikely to behave as contact sensitizers.
So compare our two plants.
Aloe vera gel: largely water and carbohydrate-rich material, with remarkably little evidence of contact sensitization despite widespread exposure.
Lavender essential oil: a chemically complex volatile oil containing unsaturated terpenes capable of oxidizing into more potent sensitizers.
Both are “botanicals.”
Immunologically, that classification tells us almost nothing.
This is why “natural vs. synthetic” is the wrong skincare debate
Your immune system doesn't know whether a molecule grew in Provence or was synthesized in a laboratory.
It sees chemistry.
A molecule's ability to cause allergic contact dermatitis depends on factors including its molecular structure, chemical reactivity, concentration, ability to penetrate the skin, exposure frequency, and—in some cases—what that molecule turns into after exposure to oxygen, light or other environmental conditions.
And the skin matters too.
A disrupted skin barrier can increase penetration and inflammatory signaling, which is one reason allergen exposure deserves particular consideration in people who already have eczema or chronically inflamed, “sensitive” skin.
So the relevant question isn't:
Is this ingredient natural?
Or even:
Is this ingredient botanical?
The better questions are:
What molecules are in it?
Are any of those molecules established sensitizers?
Can they oxidize or otherwise transform into sensitizers?
How much exposure are we creating?
And does this ingredient actually need to be in the formula?
This is how we think about formulation at Vetted
At Vetted Dermlab, we don't believe botanicals are inherently bad.
Aloe is a perfect example of why that wouldn't make scientific sense.
We also don't believe synthetic ingredients are inherently good.
Instead, we formulate through the lens we use when treating patients with allergic contact dermatitis: evaluate ingredients based on their chemistry and sensitization potential, not their origin story.
And when an ingredient presents an unnecessary allergen exposure without providing a meaningful benefit to skin, we'd rather leave it out.
Because “natural” is a marketing category.
Allergenicity is chemistry.
And your immune system knows the difference.
