The UV-tracking Gem made me rethink how sunlight affects my health, not just my appearance

With temperatures rising to the 90s in Los Angeles, avoiding the heat wasn’t exactly an option while I was at a relative’s outdoor party in August. So I sat beneath a patio umbrella with a powerful fan nearby, figuring I was protected from the sun and heat. Half an hour later, I felt exhausted, dizzy, nauseous, and generally unwell, symptoms consistent with heat exhaustion.

Around the same time, I received a notification from an app used by the $249 ($149 at the time of publication) UV-tracking necklace I’d been wearing, called Gem, from a company called The90. It warned that my UV exposure had climbed considerably. It turns out UV radiation wasn’t repelled by this patio umbrella, and it’s hard to ever be fully protected because radiation can be reflected or scattered by objects in the environment. While UV exposure and overheating are different things, I’d mistakenly assumed the shade and the fan would adequately protect me from both.

This led to a discovery: One of my medications increases my sensitivity to heat, which helped to explain similar episodes I’d experienced over the years. The Gem didn’t diagnose anything, but it prompted me to investigate something I might otherwise have dismissed. Since then, I’ve been more mindful about limiting time outdoors during hours of peak heat and UV, staying hydrated, and paying closer attention to how I’m feeling outside.

I’ll admit my initial interest in tracking UV was largely vanity-driven. As much as I try to challenge harmful beauty standards, I’m hardly immune to them. Having started going gray in my 20s, I’m keen on slowing down other visible signs of aging where I reasonably can. But between being Indian (I rarely get sunburned thanks in part to my skin’s higher melanin content, not that it eliminates the risk of sun damage or skin cancer) and living within walking distance of the beach in perpetually sunny LA, I hadn’t given much thought to how much UV exposure I was actually getting. The Gem has helped me better understand it by providing a range of estimates, some more useful than others.

The included UV-detecting sensor measures UVA and UVB radiation separately, rather than simply displaying my area’s UV forecast. UVB is primarily responsible for sunburn and helps your skin produce vitamin D, while UVA contributes to premature aging, even when you aren’t visibly burning. Thankfully, getting these metrics doesn’t come at the cost of wearing a bulky device. The circular pendant comes with a cute, thin necklace I’d happily wear even if it didn’t track anything. Its titanium, gold- or silver-colored chain pairs well with a variety of outfits. Battery life lasted almost exactly a week (the advertised lifespan), and the Gem recharges using an included charging dock. The90 plans to release a hat clip and bag tag early next year, which seem ideal for those who prefer not to wear it around their neck.

The mobile app is central to the experience. It has easy-to-read graphs and jargon-light explanations. I enjoyed using it. While not required, you can create a skin profile so that it can estimate a range of metrics based on the sensor’s UVA and UVB readings. The profile is based upon factors like skin type, sensitivities, clothing, and sunscreen. That means its estimates aren’t based on UV exposure alone, though they’re still predictions rather than direct measurements of how your body is responding to sunlight.

There’s a lot here, including your estimated vitamin D production, potential circadian light benefits, estimated nitric oxide-related benefits, cumulative UVA exposure, and recent accumulated UVA exposure. Alongside cumulative UVA exposure and weekly trends, its Skin Aging Zone assesses potential aging-related risk and offers personalized skincare advice. It also provides a personalized sunburn countdown, sunscreen reminders, and a daily Sun Score.

The Sun Score is a rating that incorporates various metrics to balance sunlight’s potential benefits against the risks of overexposure. The company encourages users to aim for a daily Sun Score of 90, though more sun isn’t necessarily better. Too little exposure means missing potential benefits, while excessive UV can damage your skin. I appreciated the context, but the individual estimated measurements proved more revealing than the Sun Score.

The UV readings in the app responded quickly as I moved between sunlight, shade, and indoors, even registering changes near windows. I couldn’t independently verify the sensor’s accuracy, but the readings behaved as expected as I shifted between environments. However, there were times when I noticed the Gem sometimes failed to register that I had been outside. Eventually, I discovered the app needs to remain running in the background (at least on iPhone; I haven’t tested on Android), as measurements aren’t stored on the sensor, and fully closing the app interrupts tracking.

My experience at the outdoor party challenged my assumptions about how much protection I was getting from the shade. Just as surprising to me was learning how little UVB I accumulated during a typical week, despite living somewhere so sunny. I fell short of the app’s estimated sunlight-derived vitamin D target almost daily, prompting me to research how melanin, while offering some natural UV protection, impacts vitamin D production from sunlight. Until then, I’d mostly thought about sun exposure in terms of protecting my skin from damage. Seeing how little UVB I was accumulating made me consider what I might be missing out on, too.

The vitamin D countdown wasn’t as helpful as I had hoped. During one test, its estimate of how much additional sunlight I needed dropped from several hours to under an hour, then climbed past six hours just minutes later. The fluctuations appeared related to changing UVB intensity, though I couldn’t verify that. I noticed similar fluctuations in the app’s estimated time to sunburn. With both countdowns changing so dramatically, I stopped paying much attention to the exact times and focused instead on the broader picture: whether it felt like I was getting too much or too little sun.

That was perhaps the biggest surprise of testing the Gem. After my experience using it at the outdoor party in August, I’d expected it just would encourage me to spend less time in the sun each day. In reality, my exposure often falls at one of two extremes: either too little to meet the app’s estimated targets or enough to raise concerns about overexposure, even when I thought I was protected by the shade.

I now try to be more mindful about my UV exposure, even when I’m not using the Gem. I try to limit time spent outdoors during peak UV hours, and I try to get some sunlight on days spent mostly indoors. I credit the Gem with helping me better understand how my time outside could affect my well-being in ways I’d never considered, though the usefulness of your findings may vary.

At $249 outside of a deal, it’s expensive, though it’s nice that it doesn’t have a subscription. I personally can’t justify the price given how much time I spend indoors, but I see the appeal for people who spend considerable time outside, or those who want to be more mindful of sun protection. For the price, I wish the app’s guidance was clearer at times, and I wish it could track UV without the app being open in the background, but the Gem ultimately got me thinking less about how my body looks and more about how I’m treating it.

Photography by Sheena Vasani / The Verge