Why is short-sightedness on the rise, and can time outdoors help?

Short-sightedness, or myopia, is rising too fast to be explained by genes, so researchers look to how children live, and the best evidence so far points to too little time outdoors in daylight. Myopia is when distant things look blurry while near things stay clear. It usually happens because the eyeball has grown too long, so light from far away comes to a focus before it reaches the retina.

The numbers are striking. Myopia is the most common eye disorder, estimated to affect 1.5 billion people, 22 percent of the world. Among children, it affects about 1 percent of rural Nepalese, 4 percent of South Africans, 12 percent of people in the US, and 37 percent in some large Chinese cities. And rates have increased since the 1950s.

Genes matter: population studies suggest they account for 60 to 90 percent of the variance in refraction. But genes can't change that fast in a few decades, so the rise points to the environment. Insufficient light exposure, low physical activity, near work and years of education are all risk factors. Near work is associated with myopia progression, but how big its effect is, and whether it is the cause, is still undetermined.

The best-supported remedy is the simplest. A 2004 to 2015 Singapore and Sydney study of children of Chinese descent found that time spent outdoors was a factor. Reviews of the evidence describe outdoor time as correlated with a lower rate of myopia, with good evidence of a causal link, and credit it with cutting the rate of onset by about 50 percent. The figure usually recommended for children is two hours a day.

In Taiwan, government policies that send schoolchildren outdoors for a minimum amount of time have driven the prevalence down. Nobody is sure exactly why daylight helps. There is only preliminary evidence for the role of a chemical in the retina called dopamine.