You Can Track Your Sun Exposure With This New App. Here's How to Do It

CNET

23-07-2025

If you're facing down the heat wave this summer, there's a new beta app for iPhones from the founder of Twitter, Jack Dorsey, for tracking your exposure to the sun. The Sun Day app is free to testers and so far contains information like sunrise, sunset and UV index in order to assess your potential burn-limit time and, as the app description says, "track your Vitamin D from the sun."
Dorsey is currently testing UI updates and a solar noon notification, according to the app notes. In the app, you can describe the type of clothing you're wearing, such as shorts and T-shirts or swimwear, and your Fitzpatrick skin type, which classifies how quickly you'll burn.
The iOS app asks permission to connect to some Apple Health data when the app is installed.
Dorsey also recently released Bitchat, a private messaging client that uses Bluetooth as its communication platform. Although it's meant to be a secure, private app, some users have flagged Bitchat for potential security flaws that are still being tested.
How to try out Sun Day
For iOS users, using Sun Day involves downloading the TestFlight app from the App Store and then following the link to the app for Sun Day to join the beta test.
The effectiveness of an app such as Sun Day depends on giving it accurate information about your skin type and clothing, and while vitamin D levels are one way to gauge UV exposure, it's not foolproof given that some people also take vitamin D supplements.
"Jack Dorsey's new app Sun Day is exciting, not to detect vitamin D levels but really to help us understand our UV index which is so important in sun safety," said Tanya Kormeili, an LA-based dermatologist. "The app does have an interesting promise as far as I am concerned, in that using the UV index can show you the risk for the level of UV exposure."
The risks of too much sun exposure include sunburns, aging skin and skin cancer. Tracking UV levels is one way to help mitigate those risks.
People tend to think about sun exposure and protections like sunscreen most during the summer, when the sun is strongest and the days are long, and when you're heading to the beach or out gardening, golfing or otherwise getting in quality sun time. But there is always a risk of sun damage to your skin while you're outside -- year round.
"Sometimes it is hard for patients to be convinced that there can be an excessive UV risk on a cloudy day," Kormeili says. "The app would provide an objective measure of that UV index and guide you in safer sun practices."
The dermatologist suggests that Dorsey putting the app out for public consumption without medical experts endorsing it might be a missed opportunity. "I am surprised that they have not involved actual dermatologists in pointing out the true value and limitations of this app," she said.