Cosmetics are an indispensable part of life for many people. But what makes you feel good might sometimes be harmful. Now new research shows that some cosmetics sold throughout the U.S. and Canada contain high levels of potentially toxic chemicals.

Scientists at the University of Notre Dame tested more than 200 cosmetics, including an array of foundations, eye and eyebrow products and concealers. Nearly half, or in some cases more than half, contained elevated levels of fluorine. That is an indicator that the products contain polyfluroakyl [PAUL-ee-floor-oh-AL-kill] substances, or PFAS [PEA-fass], which have been linked to serious health conditions.

Among the tested cosmetics, high fluorine levels were found in 56% of eye products and foundations. In lip products and mascaras, high fluorine was detected in about 48% of sampled products.

Other studies have linked PFAS to kidney and testicular cancers, thyroid disease and high blood pressure. The substances were also once common in firefighters’ protective gear, treated fabrics and nonstick cookware, before being phased out.

PFAS are also known as “forever chemicals” because they don’t break down naturally. The researchers noted that because cosmetics are used around the mouth and eyes, they have the potential to enter the body through ingestion or absorption.

As in nature, the chemicals persist in the body and can accumulate in the blood. Some of the highest concentrations of fluorine were found in products marketed as “wear resistant” or “long-lasting” — including waterproof mascara and liquid lipstick.

For now, labeling rules are neither strict nor uniform — meaning there’s no one foolproof way to know what’s in cosmetics.