Outdoor time is protective for myopia, but blanket statements to that effect aren’t the best advice. Researchers recently investigated the potential protective effects of different ambient lighting levels in various locations and found that illuminance levels vary from indoor to outdoor locations, suggesting that detailed guidance is needed when recommending outdoor time as an anti-myopia strategy.

The researchers used a lux meter to measure illuminance under cloudy and sunny weather conditions, during daytime intervals (7:00, 10:00, 13:00, 16:00), in the summer and winter, with sun protection (hat and cap) and in indoor and outdoor locations. Indoor locations included a room with multiple large windows, a room with a combination light source, a room with multiple artificial lights and a room with a single artificial light.

They chose the following outdoor locations:

• open playground

• under a translucent artificial shade

• under a porch facing east

• under a porch facing south

• under a big tree

• between three buildings

• within four buildings

• under a canopy

• under a glass bowl (a simulation for a glass or bright classroom model—lux measurement taken at floor level only)

They found that the overall illuminance level in the nine outdoor locations was eight times higher than all of the indoor locations (1,175lux vs. 179lux). The “open playground” had the highest outdoor illuminance at 9,300lux, followed by the area “under a translucent artificial shade” at 8,180lux. The lowest outdoor illuminance was “within four buildings” at 11lux This location, along with “under a canopy” and “between three buildings” had illuminance levels similar to indoor locations, fewer than 1000 lux.

The researchers noted that time of day, weather conditions, season, sensor position and using sun protection didn’t alter illuminance to change from a high to low level (>1,000 to <1,000 lux). At specific times of day, however, on both sunny and cloudy days, the “room with multiple large windows” crossed the 1,000 lux threshold.

The researchers chose the times of light measurement with children’s schedules in mind—four time points closer to before and/or after school hours.

In this study, sunlight was the main light source for all of the outdoor locations. “The cardinal position of the sun can play a major role in various time points of a day,” the researchers explained. The sun’s positional impact may be higher at certain times of day in areas where artificial structures can block sun rays, such as artificial shades, under porches and between or within buildings.

They also noted that the geographic locations of the country in relation to the equator can play a role. “This study was conducted in Hyderabad, India, which is positioned north to the equatorial line and thus the extrapolation/generalizability of results from this study to other regions should be made with caution.”

While all nine outdoor locations in this study were of high illuminance, the researchers caution that not all outdoor locations will provide adequate light exposure for myopia prevention. “It’s worth highlighting that illuminance levels reported in the study didn’t vary with sun protection, time of day, weather or seasons,” they wrote. “Children should be encouraged to spend time outdoors with sun protection, even in the mornings or evenings.”

They concluded that children and parents need to be “wisely provided with more details” when officials and eye care providers make recommendations of outdoor time as an anti-myopia strategy. These strategies are “oversimplified” right now, the researchers say.

Bhandary SK, Dhakal R, Sanghavi V, et al. Ambient light level varies with different locations and environmental conditions: potential to impact myopia. PLoS One. July 8, 2021. [Epub ahead of print].