A Toronto dermatologist is warning about allergic reactions to a common ingredient in permanent hair dye and black henna tattoos that can cause serious symptoms in some people.

Sunnybrook Health Sciences Centre dermatologist Joel DeKoven says reactions to para-phenylenediamine (PPD), often used to intensify colour, can be serious enough to warrant hospital admission.

"Some of the people describe themselves as looking like a monster when they walk into the emergency department," he told CBC News.

Toronto's Samantha Shecter was treated in hospital shortly after dyeing her hair years ago.

"I found out I was allergic. My head around my hairlines started to swell up," she recalled.

Shecter needed intravenous steroids to clear up the redness and swelling.

She later found out the allergy began not with the hair dye, but with black henna tattoos she'd had done on her ankle.

Tattoos sensitize people to PPD

The painted-on temporary tattoos are common at carnivals, festivals and beach resorts, but DeKoven says black henna contains higher concentrations of PPD than permanent hair dye does.

"As a result you're more apt to become sensitized or allergic to it, and then when you go and dye your hair — phooshh," he said, mimicking the sound of an explosion. "You have an unbelievable reaction."

Such reactions are becoming increasingly common according to dermatologists writing in a recent editorial in the British Medical Journal.

Para-phenylenediamine and its chemical cousins called aromatic amines can trigger eczema on the face or around the hairline, and in severe cases, a patient's face swells up leading to painful bruising.

More than two-thirds of hair dyes currently contain PPD or related chemicals, according to the article.

Reactions on the rise

A recent survey in London found the frequency of positive reactions to PPD in patch tests doubled over six years to 7.1 per cent among adults with contact dermatitis, the dermatologists reported.

Market research suggests that the root of the problem is that dyeing hair is popular, especially among young people. The use of temporary tattoos containing high concentrations of PPD is also contributing, the paper's authors said.

Canada's Food and Drug Act forbids the use of PPD in products that come in contact with skin for a long period of time. Health Canada considers hair dye made with PPD to be safe, but it advises people to check if black henna is used in the ink of temporary tattoos.

DeKoven says he'd like to see legislation to prohibit its use in black henna tattoos.

Meanwhile, the European Union allows PPD to make up a maximum of six per cent of the ingredients in hair dyes, but colourists say there is no satisfactory or widely available alternative offering the longevity of hair dyes with PPD.