Hair dyes are substances used to change the color of the hair temporarily. With new trends, colouring hair has become fashionable. Not only aged people colour their hair, but even younger one use dyes for hair do. Before using it, one should know as to what hair color can  do to  one’s  body and health.Chemical hair colours contain a variety of substances used to change or replace the natural hair pigment. Many of these substances, particularly PPD (P-phenylene di-amine) are known to cause allergic reactions. This may manifest in the form of localised or generalised itching and skin rashes. In severe cases, there is generalised swelling, hoarseness of voice and even death can occur. Other allergy related problems like bronchial asthma and eczema can also be aggravated. Although the hair look good temporarily after applying colour, long term use is bad for the health of hair. They become dry, brittle and loose their luster. Thus, they can break easily. Regular use of hair dye can lead to loss of hair, causing baldness.Other chemicals such as resorcinol and hydrogen per oxide, etc, are not safe for the body. Resorcinol is postulated to induce hypothyroidism. Hydrogen per oxide has been linked to neurotoxicity and has been banned in some countries. Ammonia is an irritant to eyes and nose and also produces allergic reactions and asthma.As the normal natural hair pigment is replaced or removed by hair dyes, there is a risk of graying of hair in young people. Lead acetate present in hair dyes can potentially produce many harmful effects in the body, including cancer.Although chemicals are present in small amount in hair dyes, studies have shown higher prevalence of cancer in women who use dyes regularly. It is not only the people using dyes, but also the hair stylists working with them, who are at risk of these problems. Risk of adverse effects is more with dark-coloured hair dyes. If you have some grey strands and a person wants to conceal them, it is better to use natural products such as henna and indigo. Henna if applied on the hair in form of a paste and washed after drying, leaves a reddish or orange colour. If you have grey hair and desire to cover it, check the contents of the dye before using them. Many hair colours claiming to be, "natural” contain some chemicals. One should think wisely before starting using hair dyes regularly.Dr Rachna Pande is a specialist in internal medicine at Ruhengeri Hospital