WASHINGTON (Reuters) – More Americans are becoming nearsighted, but it is not clear why, US researchers reported this week.

They found more than 40 percent of Americans aged 12 to 54 developed myopia between 1999 and 2004, compared to just 25 percent in 1971 and 1972.

Their findings, published in the Archives of Ophthalmology, could be important for health policy, given the billions of dollars spent each year in caring for people with myopia.

"The cause of refractive error is not known but it is likely due to both environmental and genetic factors," the researchers wrote. " Doing close work – such as reading or using computer screens – may be a factor, they said.

"In particular, studies in Asian populations have reported epidemics of myopia in younger generations, possibly attributed to the near-work demands imposed by more intensive education," they wrote.

Myopia, known commonly as nearsightedness or shortsightedness, is caused when the lens of the eye focuses light in front of the retina, creating a blurry image for objects more than a few inches away.

It is corrected with eyeglasses, contacts lenses or surgery.

Susan Vitale of the National Eye Institute and colleagues looked at the National Health and Nutrition Examination Survey, focusing on questions answered by 4,436 people in 1971 and 1972 and by 8,339 people from 1999 to 2004.