Chemical sunscreen ingredients are systemically absorbed after one application, and some ingredients can stay in the blood for at least 3 weeks, according to an FDA study published in JAMA.

In a lab setting, 48 adults were randomized to receive one of four formulations of chemical sunscreens containing avobenzone (in most sunscreen products) , oxybenzone, octocrylene, homosalate, octisalate, or octinoxate. Sunscreen was applied once on the first day and four times on days 2 through 4. Participants had 34 blood samples collected over 21 days.

The maximum plasma concentration of avobenzone over days 1 through 21 — the primary outcome — was 7.1 ng/mL for lotion, 3.5 ng/mL for aerosol and nonaerosol spray, and 3.3 ng/mL for pump spray. For all six active ingredients, most participants had maximum plasma concentrations that were at or above the FDA’s 0.5 ng/mL threshold (for possibly waiving additional safety studies) for as long as 23 hours after a single application. For homosalate and oxybenzone, over half of participants had above-threshold plasma levels 21 days after application.

The authors say their findings reinforce the need for further studies on sunscreen safety. Editorialists conclude: “In the absence of clear data demonstrating harm, the use of chemical sunscreen may still be considered appropriate; the use of mineral-based sunscreen is a well-established safe alternative.”