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UV filter contributions are roughly additive in an idealised film, but real films are uneven so a uniformity factor discounts the sum. SPF has diminishing returns: SPF 15 blocks 93.3% of UVB, SPF 30 blocks 96.7% and SPF 50 blocks 98%. Doubling SPF does not halve exposure risk in practice.
Additive contributions
Estimated SPF = 1 + (sum of filter contributions - 1) x film uniformity factor
Protection
UVB blocked % = (1 - 1/SPF) x 100
SPF and broad-spectrum claims must be substantiated by approved test methods. Calculated estimates cannot be used to support a label claim.
No. In vitro and in silico estimates guide formulation, but a claimed SPF must be established by an approved in vivo or validated in vitro method. Calculated values have no regulatory standing.
SPF is measured at 2 mg/cm2, far more than consumers typically apply. Applying half that amount gives roughly the square root of the labelled SPF, so an SPF 30 behaves like SPF 5-6.