Squint at this: The percentage of people in the U.S. who are nearsighted is growing, according to a new meta-analysis by the American Academy of Ophthalmology.

Researchers looked at 145 studies that included more than 2 million people, and analyzed the global prevalence of myopia, or near-sightedness. 

In 2000, 28 percent of people in North America were myopic. By 2010, that number grew to 34.5 percent.

And by 2030, the researchers estimate that nearly 1 in 2 North Americans will be myopic.

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Myopia, which is the leading cause of vision problems, occurs when the eyeball grows too long.

This causes light rays to focus in front of the retina, instead of directly on the retina, according to the National Eye Institute. As a result, faraway object appear blurry.

The researchers don’t fully understand why myopia is becoming more common, but they think a combination of different lifestyle factors may be to blame. 

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For instance, people who spend a lot of time inside or do lots of close-up activities—think reading a book or scrolling through Twitter on your phone—are 2 to 3 times more likely to be nearsighted, says study author Kovin Naidoo, Ph.D., O.D., CEO of the Brien Holden Vision Institute in Australia.

When you spend too much time indoors, your eye can’t relax as fully as it does in the natural daylight.

This prevents the distant objects from focusing clearly on the retina, which can trigger myopia development, he says.

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And when you’re constantly looking at things close to your face, your eye muscles have to work harder, says K. David Epley, M.D., a clinical spokesperson for the American Academy of Ophthalmology.

As a result, your eyeball elongates over time—contributing to myopia.

Unfortunately, once you’re myopic, you can’t reverse it. But keeping your eyes healthy can help prevent it from getting worse. 

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Eating a diet high in antioxidants—found in foods like green, leafy vegetables—and in fatty acids, like those found in fish, are critical for maintaining good eye health, says Dr. Epley.

On a computer all day? Incorporate the 20-20-20 rule: Every 20 minutes, take a 20 second break to look at something 20 feet away. 

If you start noticing abnormally blurry vision, headaches, sore eyes, or any vision changes at all, make an appointment with your eye doctor. 

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These symptoms can indicate that you’re developing myopia or your vision is getting worse, meaning you might need a stronger prescription.