What is a Protection Factor?

The concept of a protection factor (PF) is useful when attempting to quantify the UVR protection that products such as fabrics, sunscreen and eyewear can provide.

The PF indicates how much UVR is blocked by a material. For example, a material with a UPF rating of 20 would only allow 1/20th of the hazardous UVR falling on its surface to pass through it. This means that this material would reduce the UVR exposure by a factor of 20. Stated another way, this material would block 95% of the UVR and transmit only 5%.

Technical explanation of PF
An effective UVR dose (ED) for unprotected skin is calculated by convolving the incident solar spectral power distribution with the relative spectral effectiveness function and summing over the wavelength range 290 to 400 nm. In order to get the effective dose (EDm) for the skin when it is protected, the calculation is repeated with the spectral transmission of the protection item as an additional weighting. The PF is then defined as the ratio of ED to EDm and is given by the equation below [5]:

Commonly used protection factors
There are several types of protection factors used to rate the UVR protection provided by specific types of products. To provide adequate sun protection, materials must usually have a PF of 15 or higher. The following table shows that to provide effective protection a material must block more than 93% of UV radiation.

These figures assume that the material transmits uniformly across the UV region.

Ultraviolet protection factor (UPF) is a measure of the protection provided by clothing fabrics. The fabric is exposed to a UVR lamp that simulates the sun's intensity around noon. The amount of UVR blocked by the fabric is then calculated. Inclusion of the biological effectiveness of UVR in the calculations ensures that sufficient weighting is given to the UVB wavelengths and that the UPF can be applied readily to exposure situations involving people.

Sun protection factor (SPF) is a measure of the protection provided by sunscreens. SPF is the ratio of the UVR dose that is required to produce a perceptible reddening of skin treated with sunscreen compared to untreated skin. Use of sunscreen of at least SPF 15 is recommended for any areas of the body exposed to UVR.

Eye protection factor (EPF) is a measure of the protection provided by sunglasses and other eyewear. EPF is the ratio of UVR dose to the unprotected eye to that of the protected eye. EPF has a numerical rating scale from 1 to 10 used to classify how well a lens blocks UVR. Sunglasses with an EPF rating of 9 or 10 transmit almost no UVR.