Dr. Wesley, who also reviewed the list of some 200 ingredients posted on the website, said that some of them, including aminomethyl propanol, a product stabilizer, do penetrate the skins uppermost layers. And depending on where a hair is in the growing cycle, it can be open or closed to product penetration, making it likely that some cleanser or cream has infiltrated the stem cell region of the follicle, Dr. Wesley said.

Maryanne Senna, a dermatologist and the director of the Hair Academic Innovative Research Unit at Massachusetts General Hospital in Boston, said that products dont need to reach the hair bulb the onion-shaped swelling you see if you pull a hair out by the root to cause hair loss. The superficial inflammation that products may cause can contribute to hair loss and increased breakage.

Inflammation around the follicle can also cause it to harden, affecting hair texture and, in the case of curly hair, curl pattern. What ingredient, specifically, may be causing the irritation? There are multiple possibilities, both doctors and a cosmetic chemist said after reviewing the ingredient list.

Dr. Senna and Perry Romanowski, a cosmetic chemist and founder of TheBeautyBrains.com, a site where scientists examine product ingredients and industry claims, separately suggested that fragrance could be to blame.

Its a huge sensitizer, said Dr. Senna, who also teaches at Harvard Medical School. Dr. Wesley pointed to a solvent called propylene glycol, which causes redness, and said that with such a lengthy list of ingredients, some instances of scalp sensitivity are not surprising.

Asked about these scientists assessments, DevaCurl responded with a statement from Ms. Smith reiterating that DevaCurls products do not penetrate the scalp or affect the hair bulb. Hair loss, she added, can result from many unrelated factors and there is zero evidence that our products are contributing to that process.

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Did a Cult Hair-Care Line Cause Thousands of Women to Lose Their Hair? - The New York Times