Material guide: treemoss absolute — the thing people react to may not come from the lichen

About a 5 minute read.

The short version

• Name: treemoss absolute, CAS 68648-41-9
• Species: Pseudevernia furfuracea, but the INCI name is Evernia furfuracea — one organism, two genus names in circulation
• Odour type: mossy; odour: woody, dry, forest, seaweed, herbal, green
• Strength medium (rank 2); substantivity 400 hours; dark brown semi-solid
• Suppliers: 29 (oakmoss absolute has 42)
• Notes field: "Do not use on any part of the body because of sensitizing potential."
• Three papers point at one thing: part of what sensitises people comes from the tree it grew on

It is not a moss, it is a lichen, and it grows on pines

Oakmoss (Evernia prunastri) grows on oaks. Treemoss (Pseudevernia furfuracea) grows on conifers. Both are lichens — a symbiosis of fungus and alga — not bryophytes.

And where it grows is not a taxonomic footnote. It is the whole point.

Paulsen and Andersen's 2019 title says it: Fragrant and sticky allergens from the pinewood: Cohabiting and coreacting. Their opening:

Tree moss (Pseudevernia furfuracea [L.] Zopf.), a lichen growing on conifers, is a frequent fragrance sensitizer. Previous studies have shown two subgroups of tree moss-allergic patients: a group sensitized to common allergens of tree and oak moss (Evernia prunastri), and another group sensitized to colophonium-derived allergens, which may contaminate tree moss extract.

"Contaminate" means: harvesting the lichen scrapes the bark, and pine bark carries colophony (rosin). Colophony's main allergens are resin acids — abietic and dehydroabietic acid.

Three papers, one line

2002: the problem shows up on the oakmoss side first.

Johansen and colleagues studied 885 consecutive patients. Their starting point was already established:

It has been shown that the oak moss patch test material from one producer contained resin acids which are ingredients of another lichen, tree moss.

Which means the oakmoss in the fragrance mix — the standard tool for diagnosing perfume allergy — may have carried treemoss-derived rosin components. That points the instrument the wrong way: it measures colophony allergy and reports fragrance allergy. Their results showed a significant relationship between colophonium and the fragrance mix (p < 0.001), and the relationship held even when all oakmoss reactions were disregarded.

2012: split the treemoss-positive patients in two.

Uter and colleagues re-analysed surveillance data from 2003–2010, taking the 3,030 patients tested with all three antigens:

Antigen	Positive
Tree moss E. furfuracea	6.37%
Oak moss E. prunastri	5.71%
Colophonium	4.82%

The point is in the crossing: concordance between oakmoss and treemoss was good only in patients not co-sensitised to colophonium (Cohen's κ = 0.681). And treemoss-sensitised patients showed strong concomitant reactions to colophonium significantly more often when they were not co-sensitised to oakmoss.

Conclusion: two subgroups — those sensitised to (oxidised) resin acids, and those not.

2019: put a number on it.

Paulsen and Andersen added treemoss extract to the baseline patch test series. Of 632 patients, 22 (3.5%) reacted. Opening that up:

• Of the 8 sensitised to treemoss only among fragrance allergens, 75% had relevant colophonium reactions
• Of the 14 also sensitised to other fragrance allergens, that figure was 28.5%

Their clinical advice is worth copying down:

If tree moss is not included, patients with positive colophonium reactions should be informed of possible (false) cross-reactivity to tree moss to avoid [needlessly avoiding] this labeled fragrance allergen.

"False cross-reactivity" — because you are not allergic to the lichen. You are allergic to the resin stuck to it.

How the database's fields put it

The first sentence of notes is a prohibition:

Do not use on any part of the body because of sensitizing potential. By alcohol washing of Treemoss Concrete, an absolute is obtained which is a viscous and dark colored product.

"Do not use on any part of the body" — I counted that sentence in the angelica root oil article. Few records carry it, and the reason differs each time. For angelica root oil it was phototoxicity. Here it is sensitisation.

And the field does not say which sensitiser. Which is exactly what the three papers above are about: which one changes what you should do.

The same notes hides one more thing. The TSCA definition it quotes reads:

… evernia furfuracea and usnea barbata, usneaceae.

Two different lichens under one CAS. (Different things sharing a CAS is something I measured here.)

A number that looks like a unit error

The toxicity_oral field:

oral-rat LD50 4330 ml/kg (Food and Cosmetics Toxicology Vol. 13, p. 915, 1975)

The unit is ml/kg, not mg/kg. For a 250 g rat, 4,330 ml/kg works out to more than a litre. That is not possible.

Older toxicology literature did use ml/kg for liquids. But this record's appearance field says dark brown semi-solid. I cannot tell from the fields whether this is the original paper's notation or a transcription error — only that you should not compute with this cell.

(The usable one is the dermal cell: skin-rabbit LD50 > 5000 mg/kg, same 1975 source. Acute toxicity is not this material's problem. Sensitisation is.)

There is "seaweed" in the odour field

The odor field: woody, dry, forest, seaweed, herbal, green. And odor_descriptions adds a line:

Warm, mossy, resinous, fir balsam with an almost mint twist.

"Resinous" and "fir balsam." That is what colophony gives you. What the evaluator smelled is what the papers measured in the patch tests.

And "seaweed" is unusual within the mossy group — oakmoss's description does not carry it.

How to use it

• Not on skin. The notes field says so outright, and all three papers are about sensitisation.
• Check IFRA. Both oakmoss and treemoss are restricted, and the restriction is tied to atranol/chloroatranol content specifications.
• If you or a client reacts to colophony (rosin, sticking plasters, rosin soap), be especially careful here — it may not be a coincidence.
• It is not interchangeable with oakmoss. Their common allergens only partly overlap, and treemoss brings resin acids on top.
• Substantivity 400 hours, medium strength, used in the fougère/chypre base. The oakmoss and veramoss articles cover other answers to the same position.

What this doesn't establish

• I have not smelled it. This is fields, whole-database statistics, and three papers.
• All three papers measured patch test reactions, not exposure in a formula. Patch concentration (absolute at 1% pet.) and the concentration in your finished product are different things.
• I have no figure for how much contamination there is. No paper here reports resin acid content in treemoss absolute, and I do not know how much it varies between suppliers.
• I did not check the current IFRA text for treemoss, only that it is on the restricted list.
• I cannot say whose error the ml/kg cell is. I did not go to the 1975 source.
• Atranol/chloroatranol were covered in the oakmoss article. This one does not touch their levels in treemoss at all.
• "Seaweed is unusual in the mossy group" is an impression from reading descriptors, not a whole-database word frequency count.