This shouldn’t be surprising. 1)Oils oxidize 2)Oxidation products are altered chemically 3)Changing stuff changes how skin reacts. The fact that these oils are such chemical stews should tip folks to wonder what happens when they degrade.
clipped from

Essential oils can become allergens on contact with air and skin, says researcher
By Katie Bird, 05-Feb-2009

Essential oils may become allergenic on contact with oxygen in the air and certain skin enzymes, according to recent research.

Formulators should be aware that certain aromatic oils are altered by environmental factors and can cause contact allergies in sensitive individuals, even if the oil itself is not an allergen in its pure form, said researcher Lina Hagvall from the University of Gothenburg, Sweden.

Hagvall looked at lavender oil and geraniol oil (the major component of rose oil) as part of her pHD thesis.

  blog it