That's according to a population-based study published Monday showing that rates of myopia -- difficulty seeing distant objects -- are soaring. The trend is matched in many other countries, causing eye doctors to wonder what could be causing this decline in human vision. Some suspect both an increase in close-up work time (computer use) and a decrease in time spent out of doors. Researchers at the National Eye Institute, part of the National Institutes of Health, found that rates of myopia -- also called nearsightedness -- in people ages 12 to 54 increased from 25 percent in 1971 to 1972 to 41.6 percent in 1999 to 2004. The study included people with a range of myopia, from mild to severe. 'It's very intriguing,' said lead author Susan Vitale, a research epidemiologist with the National Eye Institute. 'It points to the need for research to look at the causes of myopia and factors that could, perhaps, prevent it.' Myopia occurs if the eyeball is too long or if the cornea, the front cover of the eye, has too much curvature. Under those circumstances, the eye can't correctly focus light entering the eye, making distant objects look blurred. The condition usually occurs in childhood. It can continue to worsen until early adulthood. The survey was based on data from 4,436 Americans from the years 1971 to 1972 and from 8,339 Americans from 1999 through 2004. Both blacks and whites were included, with rates of myopia increasing most dramatically for black people -- from 13 percent to 33.5 percent. Part of the increase in both cases is probably because of improved access to vision screening and treatment. However, better care doesn't explain everything. 'All of the evaluations show a major shift in myopia rates, and it's not 100 percent clear why this is happening,' said Dr. David S. Friedman, a professor of ophthalmology and international health at the Wilmer Eye Institute, Johns Hopkins Medical Center. Friedman was not involved in the research. Hereditary is thought to greatly influence the risk of developing myopia. But environmental factors contribute, too, said Friedman. Any activity that requires long periods of close-up work, such as reading, may change the shape of the eye. Near work, in particular, causes the eye to grow somewhat longer; looking in the distance relaxes the eye. Societal trends may be playing a part. Far more children today grow up in front of computers rather than on baseball fields; instead of tracking down balls hit from afar or seeking out each other in sprawling city parks, they're following the movements of a cursor only a foot or so from their eyes. Such early activities, Vitale said, shape visual acuity. 'Some studies suggest it's not so much the time you spend doing close-up work but the time you spend using distance vision that matters,' Vitale said. 'It's brighter outside. You are looking at more distant objects. But kids today are spending much less time outside.' The current study, published in the Archives of Ophthalmology, did not address rates in other ethnic groups. However, a study by Vitale and her colleagues published in 2008 in the Archives of Ophthalmology showed myopia rates were similar among non-Hispanic blacks (28.6 percent) and Latinos (25.1 percent) age 12 and older. Although myopia can be treated with glasses or contact lenses, it increases the risk of other eye diseases, such as glaucoma, retinal detachment and blindness. 'People think myopia is benign, but it's a major cause of blindness,' Friedman said. 'With longer, thinner eyes, the retina becomes thin and is prone to breaks and tears.' 