Ensuring that children spend at least 2 hours outdoors each day is currently the only myopia‑prevention method backed by randomized trials.

There is no monetary cost. However, it requires setting aside time each day — often at the expense of homework or online classes. The real challenge is maintaining this routine daily, not just occasionally.

In this trial, first‑grade students from 12 Guangzhou schools were randomly assigned to intervention or control groups (952 students in the intervention group, 951 in the control group; mean age 6.6 years). The intervention involved adding a 40‑minute outdoor class each day and encouraging parents to take their children outdoors after school. Over three years, the new myopia rate was 30.4 % in the intervention group (259 of 853 eligible participants) versus 39.5 % in the control group (287 of 726 eligible participants). The difference was –9.1 percentage points (95 % CI –14.1 to –4.1, P<0.001), indicating a true reduction ranging from roughly 4 to 14 percentage points. The average increase in refractive error was –1.42 D in the intervention group versus –1.59 D in the control group, a difference of 0.17 D (P=0.04); the intervention thus slowed progression slightly. D is the unit used to measure refractive error. No significant difference was observed in axial eye length between groups (P=0.07). The “Ten Core Principles for Myopia Prevention” also call for at least 2 hours of daytime outdoor activity per day or 14 hours per week; they note that even on cloudy days this activity provides protection and advise avoiding peak midday sun.

He M, et al. (2015). Effect of Time Spent Outdoors at School on the Development of Myopia Among Children in China: A Randomized Clinical Trial. JAMA. https://doi.org/10.1001/jama.2015.10803；国家卫生健康委办公厅 (2023). 防控儿童青少年近视核心知识十条（国卫办妇幼函〔2023〕278 号）. https://www.gov.cn/zhengce/zhengceku/202307/content_6894284.htm

The trial lasted only three years and involved children starting at age 6. A 9.1‑percentage‑point difference means it can help prevent myopia in some children, but it does not guarantee protection for everyone. The same guidelines also recommend maintaining a distance of “one foot, one fist, one inch” while reading or writing, taking a 20‑second break to look into the distance after 20 minutes of close‑up work, limiting non‑educational screen time to 15 minutes at a time (no more than one hour per day), and recognizing that simply standing outside during recess counts toward the daily outdoor requirement — it is not the same as attending a formal PE class.