A $249 UV-tracking necklace called Gem, made by a company called The90, alerted a wearer to rising ultraviolet exposure while she sat in the shade at an outdoor party in Los Angeles in August, an episode that led her to reconsider how sunlight affects health rather than only appearance. The wearer, Sheena Vasani, reported feeling exhausted, dizzy and nauseous after about half an hour beneath a patio umbrella with a fan nearby, symptoms consistent with heat exhaustion. Around the same time, the Gem's app warned that her UV exposure had climbed considerably.
The episode pointed to a distinction the device is designed to surface: shade does not block ultraviolet radiation, which can be reflected or scattered by objects in the environment. Vasani wrote that she had assumed the umbrella and fan protected her from both UV and overheating. She later learned that one of her medications increases her sensitivity to heat, which helped explain similar episodes over the years. The Gem did not diagnose anything, but it prompted her to investigate.
The pendant's sensor measures UVA and UVB radiation separately instead of showing a local UV forecast. UVB is primarily responsible for sunburn and helps skin produce vitamin D, while UVA contributes to premature aging even without visible burning. The titanium chain comes in gold or silver colors, battery life lasted almost exactly the advertised week, and the device recharges on an included dock. The90 plans to release a hat clip and bag tag early next year.
The mobile app presents graphs and plain-language explanations and can use an optional skin profile covering factors such as skin type, sensitivities, clothing and sunscreen to estimate metrics. Those estimates are predictions rather than direct measurements of the body's response. The app reports estimated vitamin D production, circadian light benefits, nitric oxide-related benefits, cumulative and recent UVA exposure, a Skin Aging Zone with skincare advice, a sunburn countdown, sunscreen reminders and a daily Sun Score. The company encourages users to aim for a Sun Score of 90 while noting that more sun is not necessarily better.
Readings responded quickly as Vasani moved between sunlight, shade and indoors, including near windows. She could not independently verify the sensor's accuracy. She found the Gem sometimes failed to register time outdoors until she learned the app must remain running in the background, at least on iPhone, because measurements are not stored on the sensor.
Vasani wrote that she fell short of the app's estimated sunlight-derived vitamin D target almost daily despite living in sunny Los Angeles, prompting her to research how melanin affects vitamin D production. She also found the vitamin D and sunburn countdowns fluctuated sharply, and she focused on broader patterns instead of exact times. She said the device is expensive at $249 outside a deal but noted it carries no subscription, and she questioned whether she could justify the price given how much time she spends indoors.
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