Every fact on this site, with its evidence grade, its caveats and a link to the source.
We start you on the strong ones: findings that have been replicated or come from one large, well-controlled study. Moderate and weak facts are here too, labeled for what they are. Use “Link to this fact” to share a single card.
Green, natural and unscented / Measured in people or real products
'Natural' oils get no special protection from air: in lab tests, lavender oil's main scent chemicals oxidized just as fast as a synthetic copy made only of those chemicals, and formed the same allergenic hydroperoxides.
Strong evidence: Lab chemistry, neat oil in open flasks over 10-45 weeks; one oil type (lavender), two batches.
Source: Hagvall 2008, Contact Dermatitis Industry-funded Link to this fact
Green, natural and unscented / Animal study
'Natural' doesn't mean allergy-free: essential oils oxidize too; in mouse tests lavender oil exposed to air for 45 weeks became about 8 times more potent as a sensitizer (EC3 36% fresh, 4.4% aged), and oxidized alpha-terpinene from tea tree oil was the most potent of all.
Moderate evidence.
Caveats
Mouse LLNA (groups of 3-4 mice; one 45-week sample) from the primary Hagvall 2008, lab open-flask exposure; aged oil reached 'moderate', not 'strong', potency. Human support: 4/4 oxidized-linalool-allergic patients reacted to oxidized lavender oil (elicitation, n=4). Earlier evidence base figure '11% -> 4.4%' was a misreading.
Information, not medical advice. See also: myths we won’t tell you.