Three-quarters of American women are interested in changing their hair color, particularly to cover gray, according to a Clairol study. But other studies show they should be wary of most traditional hair dyes and consider natural alternatives. A study from the University of Southern California published in the International Journal of Cancer, for example, identified women using permanent hair dyes at least once a month to be at the highest risk for bladder cancer.

As early as 2007, the European Union banned 22 potentially dangerous chemicals in cosmetic and body care products, including hair dyes. In the journal Materials last year, British researchers warned of the increased cancer risk from toxic chemicals called secondary amines, found in European- and U.S.-manufactured permanent hair dyes, because they remain on the hair for extended periods long after application and can penetrate skin.

Meanwhile, increasing demand by consumers for safer products has expanded the market for natural hair dyes containing henna, oils and extracts from berries and other fruits, plus vegetables. Many are now available at pharmacies, organic salons and online, including do-it-yourself recipes.