Hair Dye Allergy

The molecules in hair dye are para-phenylenediamine (PPD), para-toluenediamine (PTDS), para-aminophenol, and hydrogen peroxide. Most reactions to hair dye and henna (kaalimehendi) are due to PPD. Ammonia, which is often blamed as the culprit, only facilitates dye penetration into the inner layers of the hair shaft (cortex). In ammonia-free hair dye, this is performed by sodium carbonate and ethanolamine. Hence, ammonia-free dyes do not prevent allergies.

People who use hair dye should not keep the dye in their hair for longer than the recommended time. It should be washed out thoroughly. 

While applying dye, wear a neoprene hand glove or a hair colouring cap with holes, and apply petroleum jelly or dimethicone cream to the skin adjacent to the hairline to avoid dye spillage on the skin. 

Always perform a patch test by applying dye with a cotton swab behind the ear or forearm and allowing it to dry. The dye can be safely used if there is no reaction within 48 to 72 hours. A negative result on the patch test does not guarantee freedom from reaction, as the patch test itself could cause sensitisation and result in rash the next time the hair is dyed.