Rachel Dowley, before and after her severe reaction to hair dye. Caters News / Zuma Press

A 19 year-old British woman nearly died after dying her hair with at-home color.

A day after applying black Garnier Herbashine semi-permanent color to her naturally brown locks, Rachel Dowley's face rapidly became swollen, one of her eyes blackened, and raw red burn marks emerged on her cheeks. "When I looked in the mirror, my face looked really chubby and then it ballooned up gradually every five minutes until I was in a lot of pain," the teen told The Daily Mail.

After checking into Southend Hospital, doctors immediately put the Essex resident on two separate drips, steroids, antibiotics, anti-inflammatories, hydrocortisone cream and painkillers. "While I was in the waiting room, people were moving away from me like I had a contagious disease," remembered Dowley. "The doctor said it was one of the worst reactions he'd ever seen."

The swelling took nearly a month to subside, with Dowley's recovery complicated by a secondary infection that inflamed the burn marks on her face. Doctors warn the teen that if she is exposed to hair dye again, her body could respond with potentially fatal anaphylactic shock, which is the same immune system attack people allergic to bee stings and peanuts experience.

However, Dowley says the reaction didn't come out of the blue. When she was 14, a henna tattoo she received while on vacation on the Greek island of Corfu gave her a mild reaction, and since then, she has found that she has been allergic to dyes. "I did patch tests for tons of hair dyes in the hope of finding one that I wasn't allergic to, but they all reacted," said Dowley. "But when I did a patch test for Garnier Herbashine, I thought I had finally found a hair dye suitable for me. After 46 hours, the test had shown no reaction, and I put the rest of the bottle in my hair," added Dowley.

Patch-testing hair dye is a commonly over-looked step when women dye their own hair at home. Those who do take the time to perform a patch test often only do it the first time they use a certain hair color, though allergic reactions can happen anytime due to changing body chemistry. Others -- like Downey -- don't leave the product on long enough to complete a full patch test.

To perform an allergy test, follow the instructions on the hair dye box to mix a small amount of product to test. Using a cotton swab, apply the product directly to an inconspicuous area of skin that has been freshly cleansed with soap and water -- like the inside of your elbow or behind your ear -- and let it sit for a full 48 hours, advises Julia Youssef, vice president of the L'Oréal USA Technical Center .

If you notice any redness, itching or swelling, wash the dye off, and don't use it. "Women always ask if they can get the area wet with a shower -- you can take a shower, but try not to wash off that one section you've tested. I prefer testing the skin behind the ear so that there's no risk of it getting on your clothes or rubbing off on something," says Youssef.

While Garnier Herbashine doesn't contain the highly irritating ingredient ammonia that can mildly bother many women with scalp itching and burning, it does contain Paraphenylenediamine (PPD), which may cause a severe allergic reaction in individuals who have the rare sensitivity.

Dowley won't be buying hair dye again, but she does urge women to take away an important lesson from her experience. "I just want everyone to do patch tests and make sure they keep them on for the whole 48 hours before application, because no one I know did them until it happened to me. I wouldn't wish the emotional and physical pain on anyone, even my worst enemy."

And if hair dye has you concerned, check out our tips for the most natural and safe hair color options.