SPF 30 का असल मतलब क्या है?

Patients with brown skin often ask why they need sunscreen when their skin already tans rather than burns. The question is reasonable, because melanin does protect the skin, and the answer depends on how much protection constitutive melanin gives, how much a tan adds and what the SPF on a label actually measures. This page accompanies a short film for patients and sets out the evidence behind its statements.

Melanin as a cap over the nucleus

Melanocyte density is similar across skin types, so the difference in colour lies in how much melanin each melanocyte makes rather than in the number of cells (Tadokoro et al., 2005). Darker skin contains about twice as much epidermal melanin as lighter skin (Alaluf et al., 2002). Melanocytes transfer melanin to the surrounding keratinocytes, which gather it into supranuclear caps on the side of the nucleus that faces the surface. Keratinocytes with these caps show less ultraviolet damage to their DNA than keratinocytes without them (Kobayashi et al., 1998).

Constitutive melanin is a real but partial shield. In skin samples, about five times as much ultraviolet radiation reached the upper dermis of white skin as of black skin (Kaidbey et al., 197970018-1)). Brown skin therefore receives less ultraviolet radiation in the dermis, although it still receives some.

Why a tan protects late and little

Facultative pigmentation is a response to damage that has already occurred. Ultraviolet radiation forms cyclobutane pyrimidine dimers in DNA within picoseconds of exposure (Premi et al., 2015). The tanning response follows this damage, because p53 activated in damaged keratinocytes drives the production of the hormone that signals melanocytes to make more melanin (Cui et al., 2007). A tan is therefore evidence of DNA damage rather than a defence that precedes it.

The protection that a tan adds is also small. In volunteers of skin types II and IV, a tan induced over two weeks protected only about twice as well as untanned skin, and the gain was similar in both skin types (Sheehan et al., 2002). UVB is the main cause of sunburn, whereas UVA reaches the dermis and is filtered less by windows and clouds (Salih et al., 2024).

How SPF is measured

The SPF is a ratio of doses measured on volunteers. The reference test finds the minimal erythema dose, which is the smallest ultraviolet dose that produces perceptible redness with clearly defined borders 16 to 24 hours after exposure. The test is repeated on skin covered with the sunscreen at 2 mg/cm², and the SPF is the dose needed on protected skin divided by the dose needed on unprotected skin (21 CFR 201.327). The same regulation tests volunteers of skin types I to III only.

The fraction of erythemal radiation that passes through a correctly applied layer is the reciprocal of the SPF. SPF 15 therefore transmits about 7 of every 100 rays that cause erythema, SPF 30 about 3 and SPF 50 about 2. Since the SPF is a ratio of doses, it does not translate into a fixed time outdoors, and the noon sun delivers the threshold dose much faster than the morning sun.

UVA and the second label mark

The SPF reflects protection against erythema, which comes mostly from UVB, so a second mark is needed for UVA. In the United States, a product may be labelled broad spectrum only if its mean critical wavelength is 370 nm or more (21 CFR 201.327). The Japanese PA system grades the UVA protection factor, measured by persistent pigment darkening, as PA+ for 2 to under 4, PA++ for 4 to under 8, PA+++ for 8 to under 16 and PA++++ for 16 or more (JCIA).

Applied amount

Every labelled value assumes 2 mg/cm². Under natural conditions people apply about 0.39 to 1.0 mg/cm², which lowers the protection they receive considerably (Petersen and Wulf, 2014). The film closes on this gap, and the next film in the series examines how much of the labelled SPF a patient receives at the amounts people actually apply.