“A lot of people think if you buy a natural product then you are not going to have any allergies to it. No, that's not what natural means,” says Dr. Cindy Jones, a biochemist and natural-beauty formulator. In fact, two of the most reputedly benign ingredients—the magic-makers chamomile and lavender—are known allergens.

Chamomile can soothe, but for those with an allergy to this family of plants, which also includes daises and ragweed (responsible for common springtime allergies), the potential for hives and swelling hinders relaxation.

Lavender induces tranquility too, but it can also cause enough skin irritation that in May 2014 the Swedish Chemicals Agency (SCA) proposed a health warning on lavender products. The European Union is now considering labeling lavender, “May Be Harmful if Inhaled.” More specifically, a lavender allergy is caused by a compound within lavender extract called linalool. Linalool produces lavender’s fragrance and reacts with air to form the skin irritant. The natural extract of a lavender plant contains 20 to 40 percent linalool, depending on the plant variety, and chemists can synthesize linalool at a purity of 97 percent.

The more people use natural products, the more likely they are to develop an allergy to them, since reactions often occur with regular contact. These types of allergens are called sensitizers.

“People often think that when they become allergic to some thing it has to be something new,” says Dr. Michael Stierstoffer, a dermatologist practicing in the Philadelphia area. “But often it’s something that they have been repetitively exposed to and then at some point in time the immune system just decides to become allergic to it.”

Some types of allergies induce hay fever and asthma as the immune system dumps histamine and other inflammatory response chemicals into the blood stream in response to the allergen. A Type 1 allergy, as it is known, can be fatal if the inflammation is so severe that the airway swells to the point of closing (called anaphylaxis). A less extreme allergy (Type 4) occurs when lymph nodes absorb an allergen and tag it as suspicious. Continued exposure assures the immune system of the allergen’s ill will and, eventually, contact with the allergen results in a scaly rash. Both types of allergies can exhibit this sensitization lag time, though it’s more common with Type 4.

Stierstoffer says because of the frequent consumer assumption that natural equals better, more people encounter chamomile and lavender than in the past and thus more people react to them. “The more you get exposed to an allergen, the higher the chance that your body’s immune system will see it as something it doesn’t like and react to it.”

The best example of this is a sensitization study conducted with a natural product over nine years in Japan. Researchers analyzed the low-dose exposure of 1483 patients to lavender oil. The study showed that between 1990 and 1998 the rate of allergy among participants increased from 1 percent to 14 percent, with a spike in 1997 when aromatherapy became trendy.