London:Some hair dyes, including many top selling brands, contain harmful chemicalsthat could trigger allergies in its users, warn scientists.

The culprit is a common chemical ingredient in permanenthair dyes called para-phenylenediamine or PPD, reported the online edition ofhealth magazine WebMD.

The chemical is found in over two-thirds of commercial dyes,say researchers.

Patients with severe PPD reactions commonly develop painfulrashes around the hairline or on the face, which often require treatment andcan occasionally lead to hospitalisation. Facial swelling is also common, thescientists found.

Dermatologist John P. McFadden saw a doubling of PPDreactions over the past six years in his Londonclinic.

"Dermatologists report anecdotally that the frequencyof positive reactions to PPD on patch testing is increasing," McFadden andcolleagues write in an editorial published in the Feb 3 issue of BritishMedical Journal (BMJ).

Last October, co-author Heidi Sosted of the University of Copenhagen reported on eight cases ofsevere hair dye reactions among teens aged 12-15.

Reactions were so serious that five of them had to behospitalised and one reportedly ended up in intensive care.

"Wider debate on the safety and composition of hairdyes is overdue among medical and scientific communities, the public andlegislators," the reports stated.