How is SPF Determined for Makeup?
August 23, 2011
For any product, the Sun Protection Factor (SPF) is determined with a very specific series of tests and algorithms set forth by the Food and Drug Administration (FDA). Previously, sunscreen and sunblock were used interchangeably to indicate a sun protective lotion. With the action of either reflecting or absorbing ultraviolet (UV) rays that your skin is exposed to, sunscreens contain one or more of the following:
- Organic Chemical Compounds – absorb UV light
- Inorganic Particulates – titanium dioxide, zinc oxide, etc. absorb UV light
- Organic Particulates – similar to organic compounds for absorbing light, but also have properties that also scatter UV light; the ratio of absorption : reflection = 90 : 10
The Measure of UVB Protection
The clinical description of SPF:
The SPF is the amount of UV radiation required to cause sunburn on skin with the sunscreen on, as a multiple of the amount required without the sunscreen.
(Food and Drug Administration)
SPF is measured in a clinical setting (in vivo), applying the product to a volunteer’s skin, measuring the time it takes for sunburn to occur when exposed to artificial UV light. In some cases, the SPF is measured in natural sunlight (in vitro), using an instrument called a spectrometer to measure the intensity of exposure. With in vitro testing, the sunscreen’s transmittance is measured across all UVB wavelengths (290-330 nm). The analysis includes a table indicating the effectiveness of various wavelengths for causing sunburn (the erythemal action spectrum) and the intensity spectrum of sunlight. With in vitro testing, the degradation of sunscreen when exposed to natural sunlight is also measured. Using a mathematical formula, SPF is calculated from the resulting data. The formula used proves that SPF is not the simple inverse of the transmittance in the stated UVB range. Were the calculation a simple inverse, combining products is always less that the combined values; specifically, using a simple inverse would be the product of the two product SPF factors – which is not the case. The true total combined SPF is always less than the resulting product of the factors.
Measure of UVA Protection
There are two methods of measuring the UVA protection of a sunscreen:
- Persistent Pigment Darkening (PPD) – similar to the process for testing UVB exposure rates, this method measures the time for UVA to cause a persistent tanning (darkening) of the skin. A UVA PF of 10 would allow you 10 times the UVA exposure as without the product.
- Star Rating System – used in the UK and Ireland, this system rates a product, using an in vitro method, offering a ratio between UVA & UVB protection.
The FDA recommends application of sun protection at least 15 – 30 minutes prior to sun exposure, and re-applying every 15 – 30 minutes for the duration of exposure. Also, be warned that, although the SPF and UV PF may indicate that you can remain in the sun 5, 10 or even 50 times longer than without the protection, the actual safe exposure time is not calculated that easily. Because the intensity of the sun’s radiation varies throughout the day, the protection factors are not 100% accurate.
Every Mukilteo Mineral Makeup Product contains a natural, sun reflecting sunscreen – titanium dioxide. The products protect your skin and reduce the appearance of fine lines, wrinkles, age spots and blemishes. Contact the make-up specialists at LeVisage Wellness Center and Spa for your make-up consultation to determine the best products for you.