The Which? report underlines the fact that manufacturers have to warn consumers over potentially allergenic chemicals that might be included in hair dye products, but stresses the fact that there is no guideline to advise on how strong sensitisers might be. The magazine consulted five experts, including a toxicologist, a consultant dermatologist, an allergy expert, a trichologist and a professor in colour chemistry. The team tested 15 permanent hair dyes - which usually contain the harshest chemicals - all bought from UK retailers in July 2007 in shades of red, dark brown and blonde, including dyes for men, own brands and ranges with natural ingredients. Six of the main sensitisers were found in a number of the products, each with varying degrees of strength, which none of the product labels conveyed. Those ingredients included P-Phenlyendamine, an extreme sensitiser found in all but three of the products tested; Phenlymethylpyrazolene, a strong sensitizer found in three of the dyes; m-Aminophenol, found in six of the dyes; N,N-bis, a strong sensitiser found in four dyes; 4-Aimon-2-hydroxytoluene, a strong sensitiser found in six dyes and Toluene-2.5-diamine, a sensitiser of unclassifiable strength found in two dyes. The report stresses the fact that these chemicals can contribute to a range of reactions, including severe swelling and rashes as well as links to a range of cancers, including breast, bladder and leukemias - although the latter has not been conclusively proven. Having said this, the report also highlights the fact that such adverse reactions are few in number and that the vast number of hair dye users continue to use these type of products without ever having a reaction. Ultimately the report says that further research needs to be carried out on the links between hair dyes and both allergies and skin cancer, particularly as there are no widely accepted alternatives to the highlighted ingredients. Which? suggests that darker shades are potentially more dangerous as they often contain stronger amounts of the potentially dangerous ingredients. Likewise, the report also suggests that consumers switch to semi-permanent or natural-based hair dye products, as these formulations tend to use fewer of the harsher ingredients, ultimately posing less of a problem.