The Gem UV tracker changed how I think about sunlight and health

The symptoms were consistent with heat exhaustion, but the app alert pointed to a separate risk: shade does not necessarily block ultraviolet radiation, which can be reflected or scattered by nearby surfaces. UV exposure and overheating are not the same thing, yet I had assumed the umbrella and fan would protect me from both. The episode also prompted me to discover that one of my medications increases my sensitivity to heat, helping explain similar experiences over the years. The Gem did not diagnose me, but it encouraged me to look into something I might otherwise have brushed aside.

Made by The90, the Gem costs $249, or $149 at the time of publication. Its pendant sensor measures UVA and UVB separately, while the app turns those readings into estimates and advice. UVA contributes to premature aging; UVB is primarily responsible for sunburn and helps the skin produce vitamin D. Users can create a profile based on factors including skin type, sensitivities, clothing and sunscreen. The app also offers estimates for vitamin D production and other potential sunlight benefits, alongside cumulative UVA readings, sunburn countdowns, sunscreen reminders and a daily Sun Score.

The readings changed quickly as I moved between sun, shade and indoors, including near windows. I could not independently verify their accuracy, and the app sometimes missed outdoor exposure. On my iPhone, tracking stopped when I fully closed the app because measurements are not stored on the sensor. Some estimates also fluctuated sharply: a vitamin D countdown shifted from several hours to under an hour, then rose above six hours within minutes. I stopped relying on exact countdowns and paid more attention to the overall pattern.

That pattern surprised me. Despite living in sunny Los Angeles, I often accumulated little UVB and fell short of the app’s estimated sunlight-derived vitamin D target. At other times, exposure raised overexposure concerns even when I thought I was protected by shade. I now pay more attention to how I feel outside, limit time outdoors during peak heat and UV hours, and try to get sunlight on days spent mostly indoors.

The pendant is lightweight, its battery lasted almost exactly the advertised week, and there is no subscription. But at $249, the device is expensive; I could not justify the price given how much time I spend indoors. Clearer guidance and tracking that continues without the app running in the background would improve it. For people who spend considerable time outside or want to be more attentive to sun protection, the Gem may offer useful perspective, though its estimates are not direct measurements of how a body responds to sunlight.