A new study finds that using hair dye more than nine times a year increases the risk of chronic lymphocytic leukaemia (a form of cancer of the blood cells) by 60 per cent. But opinions differ on whether dyeing is that dangerous.
The study had also found that women who regularly used dyes before the Eighties were at even greater risk because older products contained toxic ingredients not found in today's hair products.
Women who used dark hair dyes were 50 per cent more likely to develop another type of blood cancer - follicular lymphoma. (Colours such as black, brown and red are thought to be more risky because it takes more chemicals to make the darker shades).
The research also suggests the risk of developing another type of cancer is increased by dark colours.
If non-permanent dark dyes are used the risk increases to 70 per cent, according to the study reported in the American Journal of Epidemiology.
The research was carried out by an international team of scientists, led by Dr Yawei Zhang of Yale School of Public Health in the U.S.
But hair dye manufacturers insist their products are among the most thoroughly tested on sale. And there is no consensus on the issue.
Hair colouring is hugely popular. In the UK, more than 60 per cent of women and around ten per cent of men colour their hair at some point in their lifetime - either at home or in a salon - according to the Cosmetic Toiletry and Perfumery Association, Daily Mail reports.
Most people dye their hair between six and eight times a year.
Permanent hair dyes - which don't wash out but leave you with 'regrowth' marks - account for around 80 per cent of the market.
The remaining 20 per cent of the market includes non-permanent dyes, such as tints and wash-out colour.
Unlike permanent dyes, which penetrate the hair shaft, non-permanent ones just coat the hair and gradually fade away.
Recently, there were concerns that the widespread use of hair dye - and its consequent presence in the water supply - had increased the risk of bladder cancer in the general population.
In a four-year study funded by the EU, scientists at Queen's University in Belfast discovered last year that dangerous elements in the dyes aren't effectively neutralised by water treatment plants - meaning everyone is effectively at risk.
And earlier this year, the World Health Organisation said some evidence suggested there was an increased risk of bladder cancer in hairdressers and barbers - because they work with these chemicals all the time.
But the study concluded that there wasn't enough evidence to prove long-term use of hair dyes increased customers' risk of developing cancer.
It is thought the bladder is at risk because the body excretes the chemicals through urine, explains Dr Chris Flower, director general of the Cosmetic Toiletry and Perfumery Association.
"When you colour your hair, around 90 per cent of the dye sticks to your head," he says. "It's inevitable that some of the dye will be absorbed by the skin. "These compounds are then broken down in the body and passed out through the urine." So should we be worried?
Shamim Khan, a consultant urologist at The London Clinic, says the evidence connecting hair dye and cancer is still inconclusive.
"We can't say there's no risk at all, but the evidence to support the link is still wishy-washy,' he adds. "It's not like the link between smoking and cancer which we know exists. "Incidentally, smoking is also the main cause of bladder cancer.
"Some studies have shown there are chemicals [in hair dye] that could induce cell changes - and thus predispose someone to develop cancer - but this would have to be over a long period of time and there are no clear trends."
Khan also points out that men are four times more likely to develop bladder cancer than women - despite the fact they use hair dye much less.
Women who regularly dye their hair "shouldn't panic about getting lymphoma", confirms Henry Scowcroft, senior science information officer at Cancer Research UK.
"Although this study did find a link between some types of hair dye and some types of lymphoma, the increase in risk was very small, and mainly applied to permanent hair dyes available before 1980.
"Furthermore, two years ago the EU Commission banned 22 different hair dye substances which didn't meet its safety standards"
John Frimage, a fellow of the Institute of Trichology, adds that modern hair dyes do not carry the risks of their earlier counterparts.
"When hair dyes were first used years ago, they tended to be progressive - they kept working on your scalp the longer you left them on. They were also harder to wash off, so, often, the chemicals would be left on the scalp and carried on reacting before being absorbed by the body.
"But modern hair dyes don't do this - they stop working after a set time, say 45 minutes."