SPF 30 or SPF 50? What the Research Says (and Why You Still Need It in Autumn and Winter)

What SPF actually measures

SPF (sun protection factor) tells you how well a product protects against UVB, the rays mainly responsible for burning. On paper, SPF 30 filters out roughly 97% of UVB and SPF 50 roughly 98%.

That 1% gap looks tiny. It isn't the whole story, though, because those numbers only hold if you apply the product the way it was tested.

The catch: nobody applies enough

SPF is measured in the lab using 2 mg of product per square centimetre of skin. That's a lot more than most people use.

A review of the research found that in real life people apply somewhere between 0.39 and 1.0 mg/cm², which cuts protection considerably (Petersen & Wulf, 2014). Another study found typical application was around 1 mg/cm², and simply asking people to apply two layers brought them close to the tested amount (Teramura et al., 2012).

When less product goes on, protection doesn't drop in a gentle, proportional way. Researchers testing SPF 30 and 35 products found protection fell significantly with every reduction in amount, following what looked like an exponential curve (Kim et al., 2010).

And it matters in practice. In a study of holidaymakers in Tenerife, people applying sunscreen their usual way (around 0.8 mg/cm²) still got sunburnt, while those taught to apply it properly were protected, even with a lower SPF (Young et al., 2018).

So is higher SPF better?

Given that most of us under-apply, a higher SPF gives you more of a buffer. One study measured real protection at typical application amounts and found that SPF 30 and 50 products may fall short of recommended protection levels when applied thinly, whereas very high SPFs held up better (Ou-Yang et al., 2012).

This is exactly why I don't rely on the SPF in a moisturiser. We tend to spread moisturiser thinly, so an SPF 30 moisturiser is a welcome bonus, not your main defence.

My recommendation: use a dedicated broad-spectrum SPF 50 as the last step of your morning routine, on top of your moisturiser. Use about two finger-lengths for your face and neck, and don't skip your ears, temples and the sides of your neck. They're the areas people miss most often.

Don't forget UVA

SPF only tells you about UVB. UVA is the one to watch for skin ageing. It penetrates deeper, with 20 to 30% reaching the deep dermis, where collagen lives (Battie et al., 2014).

In the UK, look for a UVA star rating (ideally 4 or 5 stars) or the UVA logo in a circle on the pack. That tells you the product protects against both.

Do you need SPF in autumn?

Yes. This is the question I get most at this time of year.

UVB, the burning ray, drops off noticeably as the days shorten. UVA behaves differently. It follows daylight, so it's around for most of the day, not just at midday (Battie et al., 2014). In the Northern Hemisphere, the ratio of UVA to UVB is actually higher in winter than in summer (Ikehata et al., 2021).

UVA also passes through glass. One lab study found ordinary smooth glass let through 74.3% of UVA (Duarte et al., 2009). If you sit by a window at work or drive a lot, your skin is still being exposed.

The strongest evidence for daily sunscreen comes from a randomised trial in Australia. People who used sunscreen every day showed 24% less skin ageing over four and a half years than those who used it only when they felt like it (Hughes et al., 2013).

The key word there is daily. Not just on sunny days, and not just in summer.

The bottom line

• Moisturiser with SPF is a nice extra, not a replacement for sunscreen.

• Choose a broad-spectrum SPF 50 with a high UVA rating.

• Apply more than you think you need, or apply two layers.

• Keep wearing it through autumn and winter.

• If you've had a skin treatment recently, be especially consistent while your skin recovers.

If you're unsure how to fit your Cutivance products around your sunscreen, drop us a message. We're always happy to help.

References

Battie C, Jitsukawa S, Bernerd F, et al. New insights in photoaging, UVA induced damage and skin types. Exp Dermatol. 2014;23 Suppl 1:7-12.

Duarte I, Rotter A, Malvestiti A, Silva M. The role of glass as a barrier against the transmission of ultraviolet radiation: an experimental study. Photodermatol Photoimmunol Photomed. 2009;25(4):181-4. PMID: 19614895.

Hughes MCB, Williams GM, Baker P, Green AC. Sunscreen and prevention of skin aging: a randomized trial. Ann Intern Med. 2013;158(11):781-90. PMID: 23732711.

Ikehata H, et al. Seasonal differences in the UVA/UVB ratio of natural sunlight influence the efficiency of the photoisomerization of (6-4) photoproducts into their Dewar valence isomers. Photochem Photobiol. 2021. PMCID: PMC8246833.

Kim SM, Oh BH, Lee YW, Choe YB, Ahn KJ. The relation between the amount of sunscreen applied and the sun protection factor in Asian skin. J Am Acad Dermatol. 2010;62(2):218-22. PMID: 19962787.

Ou-Yang H, Stanfield J, Cole C, Appa Y, Rigel D. High-SPF sunscreens (SPF ≥ 70) may provide ultraviolet protection above minimal recommended levels by adequately compensating for lower sunscreen user application amounts. J Am Acad Dermatol. 2012;67(6):1220-7. PMID: 22463921.

Petersen B, Wulf HC. Application of sunscreen: theory and reality. Photodermatol Photoimmunol Photomed. 2014;30(2-3):96-101. PMID: 24313722.

Teramura T, Mizuno M, Asano H, et al. Relationship between sun-protection factor and application thickness in high-performance sunscreen: double application of sunscreen is recommended. Clin Exp Dermatol. 2012;37(8):904-8. PMID: 23050556.

Young AR, Greenaway J, Harrison GI, et al. Sunscreen applied at ≥ 2 mg cm⁻² during a sunny holiday prevents erythema, a biomarker of ultraviolet radiation-induced DNA damage and suppression of acquired immunity. Br J Dermatol. 2019;180(3):604-14. PMID: 30307614.