Chemical relaxers used to straighten hair may increase the risk of developing multiple cancers by as much as 166 percent, a new study has found. 

Experts have long been concerned about the presence of the carcinogen formaldehyde in chemical hair-straightening and relaxing treatments, such as Keratin treatments and Brazilian blowouts.

And in a bid to see the long term health consequences of this chemical, researchers from Emory University in Georgia analyzed data from a long-term health study involving more than 50,000 US women.

The Sister Study enrolled participants who were between 35 and 74 years old, had no personal history of breast cancer, but had at least one sister who had been diagnosed with the disease. 

Enrollment took place between 2003 and 2009, and participants were followed through September 2021.

For the final analysis, researchers included data from 46,287 women who were followed for an average of 13.1 years. 

The study focused on cancers with at least 100 reported cases during the follow-up period and examined whether participants had used hair straighteners or relaxers in the 12 months prior to joining the study.

Use of these products was linked to significantly higher cancer risks. Compared to non-users, women who used hair straighteners or relaxers had a 166 percent higher risk of pancreatic cancer, which is considered one of the deadliest forms of the disease because it lacks a specific screening test, making early detection difficult.

Women who used hair straighteners or relaxers also had a 71 percent higher risk of thyroid cancer and a 62 percent higher risk of non-Hodgkin's lymphoma.

The risk increased further among frequent users - defined as using the products more than four times per year. 

In this group, the risk of pancreatic cancer more than doubled, while thyroid cancer and non-Hodgkin's lymphoma also showed elevated, though not statistically significant, risks.

The researchers also noted a possible link between straightener use and kidney cancer, though the association was described as 'positive but imprecise'.

Formaldehyde, which is used as a preservative in hair products, can escape from them as a gas over time, a process known as 'off-gassing' - especially when they come into contact with heat.

If these fumes are inhaled repeatedly, they can lead to anything from minor side effects like eye and respiratory irritation to major health issues like an increased risk of head and neck cancers.

These risks are heightened if an area like a bathroom or bedroom is poorly ventilated. 

The American Cancer Society notes that in animal and human studies, formaldehyde has been linked to various forms of cancer including in the nose, the upper throat, the stomach and it has also been shown to increase the risk of leukemia. 

Demographic data in the recent study showed that hair straightener and relaxer use varied significantly by race. 

The products were used by 66 percent of Black women, 25 percent of Hispanic/Latina women, and just 1.3 percent of non-Hispanic White women in the study.

'These novel findings expand the scope of the adverse health outcomes that have been associated with these products,' the researchers wrote. 

They emphasized the need for further investigation to confirm the results and to identify specific ingredients in hair straighteners and relaxers that may be contributing to cancer risk.

The study adds to a growing body of evidence raising concerns about the long-term health effects of chemical hair treatments, particularly among populations with higher usage rates.

The European Union and at least 10 US states have banned or proposed to ban formaldehyde and formaldehyde releasers in personal care products. 

In 2023, the Food and Drug Administration proposed a national ban on formaldehyde and formaldehyde releasers in hair straighteners, but it has yet to be enacted. 

Despite this numerous states have started banning formaldehyde in all cosmetics and personal care products, with Washington being the latest to enact a new ruling.

Under the Toxic-Free Cosmetics Act, all formaldehyde-releasing chemicals intentionally added to cosmetics and personal care products will be banned starting January 1, 2027. 

In-state retailers will have through that year, however, to sell the existing stock of products that contain these chemicals. 

The findings from the new study Emory University were published in the Journal of the National Cancer Institute.