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DEBORAH AMOS, host:

This is MORNING EDITION from NPR News. Im Deborah Amos.

STEVE INSKEEP, host:

And Im Steve Inskeep. Were going to get an eye exam today in Your Health. In a moment, well hear about the success of Lasik eye surgery and some of the complaints.

AMOS: But first, well look at the rising rates of nearsightedness and why researchers are surprised at what seems to be the cause. NPRs Joseph Shapiro explains.

JOSEPH SHAPIRO: Think of this as a kind of medical detective story. Start with this: The percentage of Americans who are nearsighted has gone way up in 30 years.

Dr. SUSAN VITALE (National Eye Institute): Thats a pretty bit increase. Its a 66 percent increase.

SHAPIRO: Thats Susan Vitale of the National Eye Institute. She did the recent study. She counted kids from age 12 to adults up to age 54. She looked at how many had myopia, nearsightedness. Back in the early 1970s, it was 25 percent. Now Vitale finds 42 percent of Americans are nearsighted.

The question is: Why? And in a mystery story, you've got suspects.

Dr. VITALE: Some of the risk factors that we know about for myopia are things like genetics, which is whether you have a family history of myopia. Things, possibly, like the amount of near-work that you do.

SHAPIRO: Near-work and genetics. Now, genetics - or heredity - is by far the main thing that determines who becomes nearsighted. But then there's near-work. That's the things you do close up with your eyes, like reading or watching television or playing video games.

Near-works been a suspect for hundreds of years. Even the scientist who came up with modern ideas for the lenses that correct nearsightedness, he blamed his own fuzzy eyesight on all the reading he did. That would be German astronomer´┐ŻJohannes Kepler.

Dr. DON MUTTI (College of Optometry, Ohio State University): Kepler wrote about it about 400 years ago, that he thought his nearsightedness was due to his intense study of astronomical tables and so forth.

SHAPIRO: Dr. Don Mutti at the Ohio State University is a kind of detective of myopia. Like other scientists researching this, he, too, suspected that after genetics, things like reading were probably a big cause.

Dr. MUTTI: It's the popular stereotype. Don't watch too much TV, or don't read, you know, under the covers with a flashlight.

SHAPIRO: For the past 20 years, Muttis followed a group from childhood to adulthood to see who develops myopia. He found something significant: Time spent outdoors during childhood was important.

Dr. MUTTI: If you have two nearsighted parents and you engage in a low level of outdoor activity, your chances of becoming myopic by the eighth grade are about 60 percent. If children engaged in over 14 hours per week of outdoor activity, their chances of becoming nearsighted were now only about 20 percent. So it was quite a dramatic reduction in the risk of becoming myopic.

SHAPIRO: At first, that seems to support the theory that near-work causes nearsightedness: The more time kids spend indoors, the more likely they're watching TV or reading a book. But then Mutti and his colleagues looked closely at the kids before they became nearsighted, and the reading and close-up things they did didn't predict who'd be nearsighted later.

Dr. MUTTI: And what we found was that near-work had no influence at all. Children really aren't doing any more or less near-work - the children who are becoming nearsighted.

SHAPIRO: So that's another mystery. Why, then, does spending time outdoors make a difference? At first, scientists thought that outdoor exercise was the key. But it turned out kids who get indoor exercise don't get the benefits of reduced myopia. Now researchers are studying whether outdoor light somehow changes the way the eye grows.

Dr. MUTTI: Light levels might have a beneficial effect on the eye. Light levels change certain aspects in retinal physiology.

SHAPIRO: The detectives of myopia are on the case.

Joseph Shapiro, NPR News.

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