Personal care · lesson 2 of 10
Sunscreen: the evidence and how to use it
The one skincare product with randomized-trial evidence for cancer and aging — if you apply enough.
8 min read · reviewed October 2026
A skincare product with a real trial
In 1992, 1,621 adults in Nambour, Queensland, were randomized either to apply an SPF 15+ broad-spectrum sunscreen to their head, neck, arms and hands every morning, or to keep using sunscreen however they liked. The trial ran for 4.5 years, and researchers kept following participants long afterward.
- Squamous-cell carcinoma was about 39% less common in the daily group. Basal-cell carcinoma was not significantly reduced (Green et al., Lancet 1999).
- Melanoma: ten years after the trial ended, the daily group had half as many new melanomas (11 vs 22, a borderline result statistically) and far fewer invasive ones (3 vs 11) (Green et al., Journal of Clinical Oncology 2011).
- Skin aging: daily users showed no detectable increase in skin aging over 4.5 years, and were 24% less likely to show increased aging than the comparison group (Hughes et al., Annals of Internal Medicine 2013).
Daily beats occasional
Nambour didn't compare sunscreen with no sunscreen — it compared a daily morning habit with 'whenever you think of it'. The habit was the intervention. Put it next to brushing your teeth, not in your beach bag.
Reading a sunscreen label
Measures protection against UVB (the burning rays), tested at 2 mg of product per cm² of skin. SPF 15 blocks about 93% of UVB, SPF 30 about 97%, SPF 50 about 98%. Higher numbers add little on paper, but they give you a margin when you under-apply — which almost everyone does.
In the US, 'broad spectrum' means the product passed a UVA test (a critical wavelength of at least 370 nm). Without it, a high SPF can still let a lot of UVA through.
In the EU, 'UVA' in a circle means UVA protection is at least one-third of the SPF. Many Asian labels use PA+ to PA++++ — more plus signs, more UVA protection.
US labels may claim 40 or 80 minutes of water resistance. 'Waterproof', 'sweatproof' and 'sunblock' aren't permitted claims. Reapply after that time, or after towelling off.
US sunscreens must stay at full strength for at least 3 years or carry an expiry date. Heat degrades them, so don't store a bottle in a hot car.
About how many teaspoons of sunscreen does it take to cover an adult's whole body at the thickness SPF is tested at?
Somewhere between 1 and 15 tsp.
Reveal the answer →
~6 tsp (about 30 mL, a shot glass)
Roughly 30 mL — about 6 teaspoons — for an adult body in swimwear. Studies find people typically apply somewhere between a quarter and a half of the tested amount, and at a quarter of the dose the real protection can fall to about a quarter of the SPF on the label. For the face and neck, plan on roughly a quarter to half a teaspoon.
How to actually use it
- Use broad-spectrum SPF 30 or higher whenever the UV index is 3 or above — in many places, that's most days of the year.
- Apply generously to all exposed skin, including ears, the back of the neck, the backs of your hands and any scalp part.
- Reapply at least every 2 hours outdoors, and after swimming, heavy sweating or towelling.
- Treat sunscreen as one layer: shade around midday, a wide-brimmed hat, UV-blocking sunglasses and clothing do a lot of the work.
- SPF in makeup or tinted moisturizer only counts if you apply a full dose — which you almost never do with makeup.
- Keep babies under 6 months out of direct sun rather than relying on sunscreen, and ask a pediatrician before using it on them.
Mineral vs chemical filters — and the absorption debate
Mineral filters (zinc oxide, titanium dioxide) and organic or 'chemical' filters both work. The best sunscreen is the one you'll wear every day in the right amount. Mineral filters are the natural pick for sensitive skin, or if you want the filters with the most regulatory certainty.
How it works →
Despite the common claim that minerals 'reflect' UV, both types protect mainly by absorbing it — lab measurements put reflection from zinc oxide and titanium dioxide at only about 4–5%. Zinc oxide covers UVB and UVA well; titanium dioxide is weaker in UVA. Organic filters are combined for coverage: avobenzone, for example, is a strong UVA filter that needs stabilizing. Mineral formulas can leave a white cast; tinted versions with iron oxides also block some visible light, which matters for people with melasma.
What the studies show →
In FDA-run randomized studies (Matta et al., JAMA 2019 and 2020), volunteers applied sunscreen at the full tested amount to most of their body, and six common organic filters — avobenzone, oxybenzone, octocrylene, homosalate, octisalate and octinoxate — reached blood levels above the threshold at which FDA asks for further safety testing. That threshold is a trigger for more study, not a finding of harm, and the authors said people should keep using sunscreen. In 2019 FDA proposed that only zinc oxide and titanium dioxide had enough data to be 'generally recognized as safe and effective', and asked for more data on most organic filters rather than declaring them unsafe. In June 2026 FDA added bemotrizinol, a broad-spectrum filter long used in Europe and Asia, to the US sunscreen monograph (at up to 6%) — the first new US sunscreen ingredient in more than two decades. Hawaii has banned sales of sunscreens with oxybenzone and octinoxate since 2021 over coral-reef concerns; that reef evidence comes largely from lab studies, and 'reef safe' is an unregulated label.
Browse the body-care shelf for broad-spectrum SPF 30+ sunscreens. Check the filters, the water-resistance claim, and — most important — whether it's a texture you'll wear every day. Worried about vitamin D? A 2019 review in the British Journal of Dermatology found little evidence that real-world sunscreen use lowers vitamin D levels; if you're concerned, test your level rather than skip protection.