Chemicals in sunscreen soak into your bloodstream when applied, according to tests done by the US Food and Drug Administration (FDA).
FDA held a random clinical trial which included 24 participants and four different brands of sunscreen before publishing results in The Journal of the American Medical Association (JAMA) journal.
- Consumer NZ testing shows six out of 10 sunscreens aren't up to scratch
- Don't get burnt: Which sunscreen products can you trust?
- Angry mums blast Nivea over sunscreen failure
A day after the sunscreen was applied, four common sunscreen chemicals were found in the participants' blood - all above threshold levels, the results show.
It also suggested chemicals could build up in the bloodstream as time goes on.
"The systemic absorption of sunscreen ingredients supports the need for further studies to determine the clinical significance of these findings," the study says.
Participants of the study weren't exposed to warmth or the sun, researchers say. The study also added that some sunscreen ingredients - such as zinc oxide and titanium oxide - don't drain into the bloodstream.
"[People] should absolutely still use sunscreen," University of California Dr Kanade Shinkai told Live Science.
Sun safety tips
Look for sunscreen with an SPF of at least 30+, plus water resistance and broad-spectrum protection.
Apply sunscreen at least 20 minutes before going outside.
Apply plenty about one teaspoonful (5ml) for each arm, each leg, your back, your front and your face (which includes your neck and ears). That adds up to about 35ml for a full-body application.
Ignore "once-a-day" claims. Sunscreen should be reapplied often - every two hours you're outside.
Mopping up sweat or towelling dry reduces protection: apply another coat of sunscreen immediately.
A sunscreen is only one part of your defence. Cover up with suitable clothing, a broad-brimmed hat and sunglasses. When the sun's rays are most intense (between 10am and 4pm September to April or when the UV index is greater than three), it's also a good idea to limit your time in the sun.