Ulcers of the cornea — the transparent front layer of the eye — may be twice as common in the U.S. as previously thought, likely owing to the recent rise in contact lens use, suggests a new study.

Based on more than a million people in northern California, researchers found that contact lens wearers were about 9 times more likely to develop the eye condition compared to non-wearers.

“As new contact lens innovations become available, and people hear that they can wear these contact lenses for weeks or a month without taking them off, they do just that. They don’t realize the dramatic increase in risk it causes them,” researcher Dr. David Gritz of Montefiore Medical Center in New York told Reuters Health. “Our eyes do need breaks from contact lens wear.”

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