What started as a simple hair transformation quickly turned into a terrifying medical emergency.

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A 19-year-old student named Estelle was left unrecognisable after suffering a severe allergic reaction to hair dye — with her head swelling dramatically overnight.

The French teen had decided to switch up her look, going from blonde to brunette using a dye containing para-phenylenediamine (PPD), a chemical commonly found in hair colouring products, per Business Insider.

At first, everything seemed routine.

Estelle did carry out a patch test beforehand — a standard safety step recommended before using hair dye — but crucially, she didn’t follow the full guidance. Instead of waiting the advised 48 hours, she left the test for just 30 minutes before going ahead with the application.

Within hours, things began to change.

Her scalp started to itch, and despite being advised by a pharmacist to use antihistamines and cream, the reaction rapidly escalated.

By the next morning, the situation had become alarming.

Estelle woke up to find her head had swollen significantly — growing to a circumference of 25 inches (63 cm), compared to the average 22 inches (56 cm).

“My forehead had doubled in volume,” she told Le Parisien. “I looked like a lightbulb.”

The swelling was so severe that her mother rushed her to the emergency room, where doctors quickly treated the reaction.

Medical staff later attributed the extreme swelling to PPD — a substance found in around 90% of hair dyes, though its concentration has been regulated since 2013.

While reactions to PPD are known to occur, doctors noted that cases as severe as Estelle’s are relatively rare.

She was given an adrenaline shot and kept in hospital overnight for observation, as doctors monitored her condition and waited for the swelling to subside. She was also placed on an IV drip of corticosteroids and antihistamines.

For Estelle, the experience was deeply frightening. “Before arriving at the hospital, you just don’t know how long it will take for you to suffocate, if you have the time to get to the hospital or not,” she said.

Thankfully, the swelling eventually reduced and her head returned to its normal size — but the incident has highlighted just how serious allergic reactions to everyday products can be.

Experts stress that proper precautions are essential.

The FDA recommends applying “a small amount of dye on a quarter-size area of skin in the bend of your elbow” and leaving it for a full 48 hours before using any hair dye. If any reaction occurs during that time, the product should not be used.

Hair professionals also warn that reactions can happen even in salons.

“If you feel any irritation, let your colorist know so they can rinse the color out immediately,” said master colorist Stephanie Brown. “Allergies can be mild to serious. If anything feels weird, you must let someone know.”

Another key piece of advice is to check product ingredients carefully.

“Read labels first before applying any new dye product or chemical,” Miko Branch, CEO of Miss Jessie’s, told Teen Vogue. “Identify if there are any ingredients that are harmful to avoid a reaction.”

While some people may only experience mild sensitivity, others — like Estelle — can face far more serious consequences.

Featured image credit: World Manual (Created with AI to help illustrate this story).