How a SPF is Determined

How do I know that the concentration of sunscreens used for a product is enough to get a certain SPF (sun protection factor)? The short answer is: by clinical testing.

The effectiveness of sunscreen ingredients is influenced by a number of factors. First, the ability to absorb UV rays depends on the type of formula and the ingredients used in the product. Also, a certain concentration of sunscreens in a product does not guarantee a certain level of sun protection as the skin type, mode of application, and grade of UV exposure also play an important role.

Even though a certain SPF level can be expected based on the type and concentration of sunscreen added to a formula (see table below and our SPF Calculator), it is not possible to accurately predict the SPF of a product simply based on the expected SPF level. The SPF can only be determined accurately by in-vitro and/or in-vivo testing.

Sunscreen	UV Protection*	Allowed Use Level	Expected SPF Increase
Avobenzone	UVA1	USA 3%, EU 5%, Brazil 5%, Japan 10%, Australia 5%	None
Homosalate	UVB	USA 15%, EU 10%, Japan, 10% Australia 15%	No data available
Octocrylene	UVB	USA 10% if used alone and 7-10% if used with other sunscreens	No data available
OM-Cinnamate	UVB	USA 7.5 %, EU 10%, Japan 20 %, Australia 10%	1% gives a SPF of about 1.5
Oxybenzone	UVB, UVA2	USA 6 %, EU 10%, Japan 5%, Australia 10%	1% gives a SPF of about 1.5
Titanium Dioxide	UVB, UVA2	USA 25%, Japan no limit	1% gives a SPF of about 1.3
Titanium Dioxide (micronized)	UVB, UVA2		1% gives a SPF of about 2.5
Zinc Oxide	UVB, UVA2, UVA1	USA 25%, EU 25%, Japan no limit, Australia 20%	1% gives a SPF of about 1
Zinc Oxide (micronized)	UVB, UVA2, UVA1		1% gives a SPF of about 1.5
Zinc Oxide (micronized & coated)	UVB, UVA2, UVA1		1% gives a SPF of about 1.5
Zinc Oxide (micronized) plus Titanium Dioxide	UVB, UVA2, UVA1		5% each gives a SPF of about 12-19
OM-Cinnamate plus Titanium Dioxide (micronized)			5% each gives a SPF of about 12-19
OM-Cinnamate plus Octyl-Salicylate			5% each gives a SPF of about 10-18

*UVA1 = 340-400 nm, UVA2 = 320-340 nm, UVB = 290-320 nm

How SPF is Measured (UVB Protection)

Based on FDA regulations, a product claiming SPF protection must be clinically tested on at least 10 (previously 20-25) human volunteer subjects. However, because such SPF measurement requires direct administration of UV radiation to the skin, there are efforts to replace the current in-vivo method with non-invasive methods that do not require human subjects. In recent years, several in-vitro tests have now been developed.

In-Vivo Methods: Typically, the SPF is measured on human volunteer subjects by applying 2 mg/cm2 of a sunscreen formula to an area of the mid-back, allowing the sunscreen to dry for 15 minutes, and administering a series of five increasing doses of UV radiation, simulating sunlight, to skin sites treated with the sunscreen. Another series of five increasing UV radiation doses is applied within a skin area without the sunscreen. After 16 to 24 hours, the irradiated skin sites are examined to determine the SPF. The SPF is the lowest dose of UV radiation that caused mild sunburn in the sunscreen-treated area divided by the lowest dose of UV radiation that caused mild sunburn in the area without sunscreen. The label SPF of a sunscreen formula is based on the average SPF for 10 volunteers

In-Vitro Methods: Current non-invasive methods for measurement of sunscreen SPF include in vitro measurements on artificial substrates that simulate the skin surface (e.g. polymethylmethacrylate or fused silica substrates) and computerized mathematical models based on the UV radiation absorbance spectra of active ingredients. However, there is no ISO Standard, regulatory agency protocol, or currently accepted method for in vitro measurements of SPF.

How UVA Protection is Measured

Based on the FDA ruling from June 2011, testing requirements for UVA protection claims have been revised. In vivo UVA testing is no longer required; and the four star UVA labeling system and descriptors have been gone away with. A simple critical wavelength spectrometer reading has replaced the previous extensive in-vitro UVA testing.