I know a semipermanent will wash out! Honestly!
This is taken from Cancer Research UK: cheering, but not conclusive. And they should know.
Does hair dye cause cancer?
We don't really know the answer to this yet. Some research has indicated that there may be an increased risk for bladder cancer, while other research has suggested that there isn't. Some of the research has considered different types of dyes and others haven't. Some studies have probably been too small to show up any small increase in risk. At the moment it is impossible to completely rule out hair dye as a possible contributing cause of bladder cancer. But at the same time there isn't enough evidence to absolutely prove such a link exists.
There are different types of hair dye. There is no evidence at all that semi-permanent or wash-out hair dyes are linked with bladder cancer. The concern centres on permanent hair dyes that contain oxidative chemicals to bleach the hair, such as hydrogen peroxide.
There have been two large and well-conducted studies investigating hair dye and bladder cancer and these have produced conflicting results. The first of these, the American Cancer Society study, conducted in 1982, did not find any association between bladder cancer and hair dye use. The other, the Los Angeles study, reported in February 2001 that long-term, regular use of permanent hair dye could increase the risk of developing bladder cancer. This study estimates that risk may increase by 2 if hair dye is used monthly and by up to 3 times if permanent hair dye is used monthly for more than 15 years. The researchers found that the risk of bladder cancer was highest in smokers who regularly used hair dyes. Smoking is a recognised risk factor for bladder cancer.
The American Cancer Society study looked at other types of cancer, apart from bladder cancer. They found that using black dye for more than 10 years was associated with a very small increase in risk of death from multiple myeloma. Using black or dark brown dye for more than 10 years was associated with a very small increase in risk of death from non-Hodgkin's lymphoma. But it is important to remember that this is only one study. And these were very small differences indeed for such a large study. The researchers concluded that, if there was any risk at all, it was unlikely to be a major factor.
After the Los Angeles study was published, an EEC body called The Scientific Committee on Cosmetic Products and Non-Food Products intended for consumers (SCCNFP) asked the hair dye industry to provide evidence that its products are safe. The SCCNFP later criticised them for failing to do this and these comments were reported in the press.
More information is emerging about how hair dye might affect bladder cancer risk. Small amount of chemicals, called arylamines, are absorbed through the skin when hair dyes are used. The arylamines are made harmless and removed from the body through the urine, so they come into contact with the bladder. The most recent piece of research has suggested that our genetic make-up affects the way we deal with arylamines. We have genes that make proteins called enzymes, which make these chemicals harmless before they are removed from the body. Some people have genes that make 'fast' versions of these enzymes and others have 'slow' versions. Those with 'slow' versions may be at an increased risk of developing bladder cancer if they use hair dyes very often over a long period of time. There is more research in progress and when this is published it is likely to be widely covered in the popular press.