Should I add BHT? "It probably doesn't do any harm" is checkable

About a 6 minute read.

Nine days ago, a well-framed question:

"Is discoloration or degradation due to oxidation really a thing that I should care for? I am only making fragrances as a hobby. They're strictly for personal use in small enough batches (50ml bottles) that would only last a year or so if I keep 2 or 3 in rotation. But I do share with friends.

I do a lot of reading in this hobby and it's a steady refrain that when volatile materials like aldehydes and citrus oils as well as those materials prone to discolor from oxidation (Vanillin anyone?) need some extra care.

BHT pops up most frequently (Along with UV protection - Benzophenone). Should I buy me some BHT (et al)? If yes, choose one:

A. Include it in dilutions of relevant materials. B. Include it as at the combined maceration stage (Concentrate plus SD40B). C. It depends (A and/or B) Read some more. D. Chill, have a drink and decant my stuff to amber or opaque colored bottles."

Seven replies, disagreeing. One of them does something none of the others do:

"I would go with option B, though I do it to prevent things from going rancid rather than to stop discolouration. I actually like it when they go that darker colour. There was one that I made a year or so back that started out almost completely colourless, but even after adding BHT it still turned a pinky orange - I thought it made it look more appealing." (Tolerable-DM)

He has separated two things: discolouration and degradation. Most of what follows is about why that separation matters.

Another reply is the most specific on dosing:

"I'm honestly not 100% sure but i don't think it matters much. You probably want to use it in hedione since that can go bad pretty easily (you'll notice a cheese like smell if it does) but i just do about 0.1% in all of my materials. And i predilute my alcohol with 0.1% bht and 0.2% vitamin e, i use that alcohol for my dilutions and my perfumes." (kdoughboy12)

Somebody asked:

"Is there a specific reason you use both BHT and tocopherol?" (kriebelrui)

"Not really, i figure it probably doesn't do any harm so might as well." (kdoughboy12)

That reply was voted to -1. And "it probably doesn't do any harm" is checkable.

BHT has an odour type in the database

butylated hydroxytoluene, CAS 128-37-0, 52 suppliers. Its category field reads "antioxidants and preservatives for fragrance and flavor".

And its odour fields are not empty:

Field	Value
Odour type	phenolic
Odour	phenolic; camphoreous
Strength	low (rank 1)
Substantivity	400 hours (20% in dipropylene glycol)
Odour description	"at 100.00%. mild phenolic camphor"
Melting point	70-73 C (a solid)
logP	5.30 (est)
Water solubility	0.6 mg/L @ 25 C (exp)
GHS	H410 very toxic to aquatic life with long lasting effects
Oral toxicity	rat LD50 890 mg/kg
Occurrence	garlic bulb; rue oil China @ 0.05%

Three things worth pausing on:

One, it smells. The strength field says low at rank 1, so it is not a big smell. But it is not zero, and its substantivity is 400 hours, the ceiling value in this library. It does not leave. Putting 0.1% into every material and every bottle of alcohol, as kdoughboy12 does, means that phenolic accumulates in the base.

Two, it is a solid. Melting at 70 to 73 C, colourless crystals. Adding it means dissolving it first, and with a water solubility of 0.6 mg/L and logP 5.30 that means ethanol or an oil phase.

Three, its only GHS statement is environmental. H410, very toxic to aquatic life with long lasting effects. No skin statement, no inhalation statement, just that one.

So "it probably doesn't do any harm" has three concrete costs: an odour, a dissolving step, and an environmental classification. None large, none zero.

The database has a flag for only one of the two problems

The asker put discolouration and degradation in one sentence, and Tolerable-DM separated them. The database sides with Tolerable-DM, indirectly.

There is a flag hidden in the solubility field: non-discoloring in most media.

249 records carry it. Of those, 149 have 30 or more suppliers, and the library has 817 materials with 30 or more suppliers, so only 18.2% carry the flag.

The flag can only be used positively. Its absence does not mean a material discolours; it means nobody filled it in.

And the flagged list has some familiar names:

Carries the "non-discoloring" flag	Suppliers
geraniol	198
linalool	135
geranyl acetate	130
benzyl alcohol	125
citronellol	119
linalyl acetate	117

Linalool, geraniol, citronellol, linalyl acetate.

Those four are the most notorious autoxidisers in the contact allergy literature. As set out in The bottle you opened is no longer the same material, pure linalool shows no sensitising potency in the local lymph node assay while air-exposed samples do, and the hydroperoxides formed by oxidation are the strongest sensitisers.

They do not discolour, and they do go bad.

That flag describes appearance, not stability.

And it fails the other way too

The asker named vanillin himself ("Vanillin anyone?"). It is the classic browning case.

I checked its solubility field. Nothing about discolouration at all.

193 suppliers, appearance "white to off white crystalline powder", the standard storage sentence, 24 months shelf life. The database cannot warn you that this one turns brown.

So the field fails in both directions: having the flag does not mean stable, and lacking it does not mean it discolours.

Is oxidation itself worth caring about

Hagvall and colleagues ran a direct experiment in Contact Dermatitis in 2008, and its title is the conclusion: "Lavender oil lacks natural protection against autoxidation, forming strong contact allergens on air exposure."

They had previously studied the sensitising potency of linalyl acetate, linalool and beta-caryophyllene, lavender oil's main constituents, on air exposure as single compounds. This paper asked whether the other naturally present constituents in the whole oil would protect them.

They do not.

• The terpenes in air-exposed lavender oil oxidised at the same rates as the pure compounds exposed to air
• The same oxidation products were identified
• The sensitising potency of lavender oil increased accordingly (local lymph node assay)
• Patients with patch test reactions to oxidised linalool also reacted to air-exposed lavender oil

Several dozen other constituents in a whole natural oil did not stop the main ones oxidising.

Karlberg and Lepoittevin reviewed a hundred years of this in the same journal in 2022. It is not new: turpentine causing occupational contact dermatitis in painters in the early twentieth century is the same mechanism (the culprit in Scandinavian turpentine being Δ3-carene hydroperoxides). Their conclusion is that monoterpene hydroperoxides are a common cause of contact allergy in the population, and that a century of chemical and clinical work should be sufficient to meet the evidence requirement.

So oxidation is real, and it is a different thing from discolouration.

But neither paper answers his question

To be clear: Hagvall's paper did not test whether adding an antioxidant helps.

It tested whether an oil's own contents protect it (they do not). Whether added BHT works is a different experiment, and I did not find it.

So what I can say is: the problem is real, and I have no evidence about whether BHT solves it. Neither do the seven replies. Nobody ran a control.

Answering his four options

• A (in dilutions) and B (at maceration): the database cannot separate them. BHT is lipophilic (logP 5.30, water solubility 0.6 mg/L), so it dissolves in ethanol at either stage. B is less work because you do it once.
• C (it depends): an honest answer, but nobody said what it depends on. What can be said is whether your materials are terpenes. For linalool, geraniol, citronellol and limonene, Hagvall's paper shows they do not protect themselves.
• D (amber bottles): the only option with direct evidence behind it, and it adds nothing to the formula. Hagvall measured air exposure, not light. But excluding light and reducing headspace are two independent, zero-cost actions.
• His own conclusion ("no harm no foul, I'll probably use some just as part of my learning through experimentation") is reasonable, as long as he knows that 0.1% carries an odour, a dissolving step and an environmental classification.

What you can actually do

• Establish first whether you fear discolouration or degradation. They have different material lists.
• Do not use the "non-discoloring" flag to judge stability. Four of the top six flagged materials are the best-known autoxidising terpenes.
• Absence of the flag does not mean it discolours either. Vanillin has no entry in that field at all.
• BHT smells, is a solid, and carries an environmental classification. Know those three before putting 0.1% into everything.
• Using two antioxidants together should have a reason. The person on the forum says his does not.
• Amber bottles and less headspace are free. Do those first.
• Treat terpene materials separately (linalool, geraniol, citronellol, limonene, linalyl acetate). They will not protect themselves.

What this doesn't establish

• I ran no oxidation experiments. This is database fields plus two papers.
• I could not find research on whether added antioxidants work for fragrance materials. Hagvall measured the absence of natural protection, not the effect of an addition. That is the biggest gap here, and the forum does not fill it either.
• The "249 records" is string matching. Differently worded entries are missed, so it is an undercount.
• The 18.2% is computed within the 817 materials having 30 or more suppliers. A different threshold changes it.
• The database does not document who filled the flag in or on what basis.
• BHT's "low" strength is the database's judgement. Whether it is detectable at 0.1% I did not test.
• The 400-hour substantivity was measured at 20% in dipropylene glycol and is not comparable with a 0.1% use level.
• Hagvall 2008 used a mouse local lymph node assay and human patch testing, measuring sensitisation rather than odour degradation. The two are related but are not the same endpoint.
• Karlberg 2022 is a review, and what I cite is its judgement about the weight of evidence.
• I did not look at benzophenone (UV protection) at all. The asker raises it, and this article does not cover it.