What SPF 30 Really Means

SPF and the erythema dose

The sun protection factor (SPF) test begins with the minimal erythema dose, the smallest amount of ultraviolet radiation that reddens unprotected skin (21 CFR 201.327). The redness must show clearly defined borders when it is read 16 to 24 hours after exposure (21 CFR 201.327).

Investigators then repeat the measurement on skin covered with sunscreen at 2 mg/cm², the amount that the United States regulation sets (21 CFR 201.327). The SPF equals the minimal erythema dose of protected skin divided by that of bare skin (21 CFR 201.327). The regulation limits its volunteers to sun-reactive skin types I to III on Fitzpatrick's six-type scale (21 CFR 201.327; Fitzpatrick, 1988).

Since the SPF is a ratio of doses, its reciprocal gives the fraction of erythema-causing radiation that passes through a 2 mg/cm² layer (calculated from 21 CFR 201.327). At that thickness, SPF 15, 30 and 50 let through about 7%, 3% and 2% of the erythema-causing radiation (one divided by the SPF).

Melanin and ultraviolet protection

Light and dark skin contain a similar density of melanocytes, the pigment cells that make melanin (Tadokoro et al., 2005). Darker skin nevertheless contains about twice as much melanin in its epidermis, the outer layer of the skin, as lighter skin (Alaluf et al., 2002). Melanin can form supranuclear caps above the nuclei of epidermal cells, and these caps are thought to shield the nuclei from ultraviolet radiation (Kobayashi et al., 1998). Kobayashi and colleagues irradiated human skin explants with ultraviolet B (UVB) and found significantly fewer DNA photoproducts in capped cells than in uncapped cells (Kobayashi et al., 1998).

Kaidbey and colleagues compared ultraviolet transmission through skin samples from black and white people (Kaidbey et al., 197970018-1)). On average, about five times as much UVB and ultraviolet A (UVA) reached the upper dermis of white skin as that of black skin (Kaidbey et al., 197970018-1)).

DNA damage and tanning

Ultraviolet photons typically form cyclobutane pyrimidine dimers, lesions joining two adjacent pyrimidine bases in DNA, within picoseconds of absorption (Premi et al., 2015). Premi and colleagues found that melanocytes keep forming these dimers for more than 3 hours after UVA exposure (Premi et al., 2015). These late, or dark, dimers made up the majority of the dimers that the melanocytes formed (Premi et al., 2015).

After ultraviolet exposure, p53 in keratinocytes switches on the gene for pro-opiomelanocortin, the precursor of alpha-melanocyte-stimulating hormone, which keratinocytes secrete to drive tanning (Cui et al., 2007). Mice lacking p53 showed no tanning response, which identifies p53 as a required step in tanning (Cui et al., 2007).

Sheehan and colleagues gave 12 adults of skin types II and IV 0.65 of their minimal erythema dose on weekdays for two weeks (Sheehan et al., 2002). The resulting tan gave protection factors of about two against both erythema and thymine dimers in each skin type (Sheehan et al., 2002).

UVA grades and sunscreen labels

Sunburn comes mainly from UVB, so an SPF measured by reddening reflects protection against UVB more than protection against UVA (Salih et al., 2024). UVA reaches the dermis, and window glass and cloud cover filter it less than they filter UVB (Salih et al., 2024).

The critical wavelength is the wavelength below which lies 90% of the area under a sunscreen's absorbance curve between 290 and 400 nm (21 CFR 201.327). A higher critical wavelength therefore means that the product's absorbance extends further into the UVA range (21 CFR 201.327). A sunscreen sold in the United States may carry the words broad spectrum only when its mean critical wavelength is 370 nm or more (21 CFR 201.327).

The Japan Cosmetic Industry Association grades UVA protection with the PA system, based on persistent pigment darkening, the brown colour left after UVA exposure (JCIA). The four grades run from PA+, for a UVA protection factor of 2 to under 4, to PA++++, for 16 or more (JCIA).

Applied amount and labelled SPF

People applying sunscreen under natural conditions use about 0.39 to 1.0 mg/cm², compared with the 2 mg/cm² used in SPF testing (Petersen and Wulf, 2014). Petersen and Wulf conclude that this thinner layer gives users considerably less protection than the SPF printed on the label (Petersen and Wulf, 2014).