Allergy to hair dye is soaring in numerous countries as more and more young people color their locks, according to a medical journal.

The culprit is para-phenylenediamine (PPD) and its cousins in a chemical family called aromatic amines, the mainstay of hair dyes for more than a century, the British Medical Journal (BMJ) warns.

Allergic response to PPD is well-documented, causing eczema on the face or around the hairline, but in some cases the reaction is so severe that the victim's face swells up and causes painful bruising, needing hospital treatment.

The BMJ points to case reports from dermatologists in Britain, Belgium, Portugal, Denmark, Germany and Singapore which, together, suggest incidence of PPD allergy is soaring.

One London clinic reported that between 1965-75 it treated only between five and 11 patients with the allergy each year.

Now the figure tops the 40 mark, and that is likely to be a big underestimate. Only a small fraction of people with the allergy seek medical help, and an even smaller number are tested to identify the source.

Screening of individuals in Bangkok, using a test kit to see if their skin was allergic to a patch with PPD, suggests that more than a million Thais may be sensitive to PPD, the BMJ said.

In Germany, the figure could be 1.3 million.

The BMJ says there's no point in covering up the roots of the problem: the fashion for hair dyeing, especially among young people.

In Japan, for example, 13 percent of of female high school students, six percent of women in their 20s and two percent of men in their 20s used hair dye in 1992.

In 2001, the proportions were 41 percent, 85 percent and 33 percent respectively.

"Cultural and commercial pressures to dye hair and, perhaps, the widespread obsession with the 'culture of youth' are putting people at risk and increasing the burden on health services," the BMJ said sternly.

It admitted, though, that vanity and peer pressure meant the outlook for change was dark -- or midnight blue, as the hairdye makers themselves might prefer to describe it.

"It may not be easy to reverse these trends... as some patients have continued to use such dyes even when advised that they are allergic to them and risk severe reactions."