Some perfume and cosmetic products may cause allergic reactions, according to a study.
Researchers at Monash University, Columbia University and Harvard Medical School discovered some compounds contained in cosmetic and perfume products can activate human T cells, which guard our immune system.
The study focused on small chemicals found in many essential oils and botanical extracts.
It also investigated the role of a common protein in the skin, known as CD1a, could play in allergic reactions to cosmetics.
The study, published in the journal Science Immunology, found several chemicals came up as "hits," including farnesol - a compound often used as a fragrance.
An example researchers used was Balsam of Peru, which is an oily tree resin found in many natural and cosmetic products, toothpaste, fragrances and food and drink flavouring.
Researchers said the oil is not chemically manufactured so it is very popular but up to five percent of the population is allergic to it because it contains the small compounds which break down certain individuals immune systems, causing an allergic reaction.
"Normally, many CD1a molecules are filled with natural blockers in our bodies that would prevent an exaggerated immune response.
"Those small compounds basically remove those natural blockers," said researcher Dr Marcin Wegrecki.
Contact dermatitis induced by allergens in personal care products is a common cause of skin rashes.
But despite the known allergy, Balsam of Peru is still used in some products, including toothpaste, sunscreen, face creams and cosmetics.
"Now we know how some of the compounds found in skincare products and cosmetics can directly interact with human proteins," said Dr Wegrecki.