Do the chemicals used to straighten hair cause breast cancer. According to the latest research, the answer is no.

An article in Ivanhoe's Medical Breakthroughs states:

Researchers found younger women used chemical straighteners more than older women; most use them before age 20. One-third uses them at least seven times a year. However, researchers conclude they found no connections between using the relaxers and breast cancer overall or among younger women even if they used the products often and for a long time.

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However, I would still advise using caution, not in just hair straighteners, but in all cosmetics as well. Realmama.org warns:

The National Institute of Occupational Safety and Health found that of 2,983 chemicals which the cosmetic industry listed as being used in cosmetics, 884 are known to be toxic. According to a study by the Environmental Working Group, a nonprofit environmental research organization based in Washington, DC, 1 out of every 100 personal care products contains a known or probable carcinogen and 89% of ingredients in products have not been assessed for safety.

Is it really worth the risk to your health? Maybe the "natural look" is the best choice after all.