Many people who use hair dye do so without any evident problems. Unfortunately, that won't be the case for everyone. Even if you've used hair-dye many times with no adverse effects, it's possible you could become sensitised to an ingredient and have a reaction the next time you colour your hair.

Dermatologist Dr Amanda Oakley says allergic reactions range in severity. They occur when the body thinks a chemical is harmful and produces an immune response against it. The result is usually an acute contact dermatitis affecting all areas touched by the dye especially the face, neck and scalp.

Some allergic reactions may not happen straightaway. Dr Oakley says reactions to PPD, for example, usually occur between 6 and 36 hours after use. Symptoms may include marked reddening, swelling and blistering. The reaction tends to be less severe on the scalp than on the more sensitive skin of the face and neck.

In rare cases, allergy to a hair-dye ingredient can cause anaphylaxis. This is an extreme allergic reaction that can be life threatening if not treated quickly. Symptoms include an immediate itchy rash, wheezing, difficulty breathing, faintness and collapse.

How can you tell?

To test whether you're allergic to a product, dye manufacturers usually recommend a patch test 48 hours before use. This involves applying a small amount of the product to your forearm or behind your ear. All the dyes we bought recommended a patch test and advised not to use the product if a reaction occurred.

But are patch tests reliable? In a report released in December 2007, the Scientific Committee on Consumer Products concluded there's a risk that patch tests can give misleading and false-negative results. The Committee cautioned that a patch test can give people "the false impression they are not allergic or not at risk of developing an allergic reaction". One reason for this is reaction time - reactions can take up to 7 days to develop, not just 48 hours.

Cancer questions

Some studies have linked hair-dye use to cancer. Recent research by the International Agency for Research on Cancer (IARC) reports that occupational exposure of hairdressers and barbers to hair-dye chemicals is "probably carcinogenic".

The jury's still out on the level of risk for home users. The IARC says current epidemiological evidence is inadequate to assess the cancer risks from personal use of hair dye.

Case-study

Anna Cottrell (pictured right) used to dye her hair regularly. But not any more.

A few years ago, Anna suffered an extreme allergic reaction to p-phenylenediamine (PPD), a common hair-dye ingredient.

Colouring her hair at home, Anna noticed a "terrific rash" on her face almost as soon as she put the dye on. She immediately jumped in the shower to wash the dye out, only to see the rash spread rapidly down her body.

Things soon got much worse. Anna felt "a vice-like grip" around her chest and started having difficulty breathing. She rushed to the doctor, arriving just before passing out. She had gone into anaphylactic shock, a rare but life-threatening allergic reaction.

She was put on oxygen and given an adrenaline shot. Anna says "I had no idea anything like this could happen" but has since heard of numerous similar cases. Anna has looked for "natural" dyes in health stores but hasn't found any without PPD.