When Sara ordered a zucchini, goat cheese, and broad-bean salad, she had no way of knowing it would almost cost her life. As her airway began to swell after two bites, a shot of adrenaline was the only thing that could save her. Sara was 55 years old at the time and had never had an allergic reaction to goat cheese. A few months before her anaphylaxis she had stopped using a natural moisturizer made with goat’s milk because instead of helping, it only exacerbated her itchy, dry skin. But worse than that, the organic beauty cream ended up putting her in the emergency room.

Sara had a hidden allergy to a compound in goat’s milk. The daily moisturizer she applied built up enough antibodies to cause the flood of immune response she experienced the day she ingested a massive dose of the compound.

Natural beauty products like the one Sara opted for have become very popular: Due in part to the cosmetics market, revenue from essential oils is expected to total $1.5 billion by 2018. And marketing language is often designed to entice consumers by imbuing natural ingredients with wondrous properties–the liquid soap in my bathroom encourages me to experience the soothing fragrance of lavender and chamomile as it “works its magic” on my hands. But as Sara’s case demonstrates, “natural” doesn’t always mean “worry-free.”

“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.

Since the turn of the millennium lavender’s popularity has only grown. Lavender is present in 90 percent of cosmetics products sold in the U.S. It’s found in places you would expect, like detergents and air fresheners, but it is also a common ingredient in less intuitive products, such as adhesives, plasters, and inks. Any scented product, be it cosmetic or stationary, most likely contains lavender. And, because of its proven sleep-inducing effect, products marketed to children—bubble bath, shampoo, lotion—contain lavender 70 percent of the time. It is this omnipresence that provoked the SCA to warn consumers of lavender’s potential harm.

No one knows why some people become sensitive to linalool molecules and others don’t. The exact percentage of the population affected depends on the source: Some studies report 2 percent of people break out in an eczematous rash from contact with lavender, while others claim it's as high as 7 percent. Dr. Donald Belsito, a Columbia Medical Center dermatologist and panel member on two different boards that review ingredient safety, says experiments on mice suggest people’s genes may have the answer. “We are probably all born with whatever allergies we might potentially develop and then it is only with the correct exposures over time that those allergies become manifest,” he says.

Anything someone continually introduces to her body, whether synthesized or naturally occuring, has the potential to someday cause an allergic reaction. Slightly more than half of all linalool produced globally is man-made, but regardless of the source the allergic reaction is the same.

The skin irritation associated with lavender is less extreme than what Sara experienced with goat’s milk, and Cindy Jones doesn’t think the numbers are big enough to warrant a label that would taint lavender’s reputation. “I think one study I was reading in that filing was where they introduced the oxidized linalool to volunteers and they found 1.8 percent developed sensitivity. Which to me just seems low,” Jones says.

Lavender growers worry that a label warning on lavender products will affect sales, precipitating a lavender-free consumer trend. “If you couldn’t use lavender in cosmetics that would be pretty serious,” Jones says. (Or, as a presenter at the recent Society of Cosmetic Chemists annual meeting put it, “When you start to pass a regulation that means you can’t sell Chanel No. 5 anymore, things start to get dicey.”) He was exaggerating—Chanel could still sell its iconic perfume, but the warning label it would have to carry might make people reluctant to spritz themselves.

Stierstoffer cautions that once an allergy does occur it will not go away. “So people have to be forever vigilant once they have an allergy. They have to read labels.” If you have a lavender allergy he points out the importance of buying fragrance-free products (“unscented products” may still contain linalool to mask other odors). And even if you avoid the allergen for a long time re-exposure could lead to a breakout. “The immune system has a very good memory,” Stierstoffer says.