Andrea Salinas for years used lavender-scented deodorant, figuring the “all-natural” product was healthier than commercial brands. She also used lavender oil as a body spray.

Recently, Ms. Salinas, a clinical social worker in San Francisco, started breaking out with a red, itchy rash that started in her underarm area and spread across her torso. Tests by a dermatologist turned up an unexpected culprit: She had developed an allergy to lavender.

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