The National Cancer Institute (NCI) evaluated twelve studies conducted between 1977 and 2006 to determine if hair dye increases a person's risk of bladder cancer. The NCI found no connection. However, researchers have not abandoned the idea that a connection exists, and studies have been published since 2006 that do point to a correlation. For example, an article published in the "International Journal of Epidemiology" in 2009 asserts that hairdressers have a higher risk of bladder, lung, and larynx cancer due to the carcinogens in hair dye and other salon products.
As far as consumers are concerned, the American Cancer Society and Cancer Research UK both agree that current research is incredibly inconsistent so it is not possible to tell whether a link between bladder cancer and hair dye truly exists. However, since both websites offer a list of precautionary measures for consumers.