Allergic hair dye reactions are increasing as more people - particularly the young - colour their hair, researchers have warned.
Cancer-causing compounds have been removed from hair dyes
In the British Medical Journal, they warn this can lead to facial dermatitis and, in severe cases, facial swelling.
Para-phenylenediamine (PPD) and other related agents which can trigger allergic reactions are contained in more than two-thirds of hair dyes.
The warning comes from St John's Institute of Dermatology in London.
Allergic reactions to PPD became such a serious problem that it was banned from hair dyes in Germany, France, and Sweden.
Current European Union legislation allows PPD to comprise up to 6% of the constituents of hair dyes on the consumer market.
But the researchers, based at St Thomas' Hospital, warn that no satisfactory or widely accepted alternatives to these agents are available for use in permanent hair dye.
More in clinics
A recent survey in London found a doubling in frequency of dermatitis over six years to 7.1% in a clinic for adults.
The same clinic reported that between 1965 and 1975 it saw between five and 11 patients with non-occupational PPD allergy each year.
More recently the number has consistently exceeded 40 such patients annually.
A similar trend has also been observed in other countries.
Market research also indicates that more people are dyeing their hair and are doing so at a younger age.
A survey in 1992 by the Japan Soap and Detergent Association found 13% of female high school students, 6% of women in their 20s, and 2% of men in their 20s reported using hair colouring products.
By 2001 the proportions had increased in these three groups to 41%, 85%, and 33%, respectively.
The researchers say temporary "henna" tattoos containing high concentrations of PPD might also be contributing to the problem.
Severe hair dye reactions among children have also been reported.
The researchers said a wider debate on the safety and composition of hair dyes was overdue.
"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.
"It may not be easy to reverse these trends, however, as some patients have continued to use such dyes even when advised that they are allergic to them and risk severe reactions."
Lead researcher Dr John McFadden said: "Hair dye allergy may present with mild or severe symptoms.
"For example, in mild cases there may just be tingling or discomfort of the scalp after hair dyeing.
"If severe there may be redness and swelling of the forehead ears neck and eyelids some patients have been hospitalised after an acute attack."
John Collard, a nurse adviser to the charity Allergy UK, said part of the problem was that people often did not follow the advice contained on hair dye packets to test a small area of skin for potential reactions before making full use of the product.
In a statement, the Cosmetic Toiletry and Perfumery Association said safety was a "top priority".
"Hair colorants are amongst the most thoroughly researched products on the market and their safety is supported by a wealth of scientific research as acknowledged by the European Commission and EU member states.
"Extensive studies over many years have concluded that hair colorants are safe when used as recommended in the instructions and consumers can continue to use them with confidence."