Sun protection factor (SPF)
Sun protection factor (SPF) is a number on sunscreen labels that indicates how long skin can be in the sun and maintain a low risk for sunburn. The higher the SPF number, the longer it protects a person from burning rays.
The Sun Protection Factor (SPF) rating applies only to protection from ultraviolet B (UVB) rays. Some sunscreens protect against UVA rays, but they have no SPF rating. No sunscreen gives total protection, but “broad-spectrum” sunscreens that contain ingredients such as avobenzone, benzophenones, cinnamates, salicylates, sulisobenzone, titanium dioxide, and zinc oxide usually protect from UVA and UVB rays.
What is UPF?
UPF stands for Ultraviolet Protection Factor. It is the ratio of the average effective ultraviolet radiation (UVR) irradiance transmitted and calculated through air to the average effective UVR irradiance transmitted and calculated through fabric. In other words, it is the amount of ultraviolet radiation that a fabric blocks. Fabrics are generally tested by spectrophotometer equipment and are not tested using human subjects. Hence UPF values are used since the endpoint of the test is UV transmittance, not skin reddening. Sun Protection Factor (SPF) values indicate a function of skin reddening, therefore only fabrics tested on human subjects should use a SPF value.
UV Rays: What are they?
UVA is a long wavelength that penetrates the skin very deeply, degrading the elastic fibers in the skin. This causes aging, some sunburn and adds to the effect of UVB rays.
UVB rays penetrate the top layers of the skin creating sunburn. This sunburn promotes skin aging and is the main cause of skin carcinomas.
Exposure to UVA and UVB rays has a cumulative effect on the body. The more exposure, the more potential damage is done.
Regardless of colour, skin that is repeatedly exposed to the sun tends to become tough and thick. The results can be leathery skin with wrinkles beyond the middle years of adulthood. What can help is using a sunscreen product with a high degree of protection from UV rays, listed as the concentration on the SPF (Sun Protection Factor) numbers on the products’ labels. These sunscreen products can be made with ingredients that offer protection against UVA rays and / or other ingredients to protect against UVB rays (more harmful for sun burning than UVA rays.) The best products offer ingredients for protection from both UVA and UVB rays. When sunbathing always pay particular attention to your face as it is constantly exposed to the sun. Here are two examples of the skin damage to the face that has been damaged by the sun that are highlighted using a UV camera.
The Sun Protection Factor (SPF) listed on the product label refers to the minimum amount of UVB sunlight required with that product in order for redness to appear on the skin after that product has been applied, versus the length of time bare skin or skin without the product applied would redden. To sum that up, the basic math involved in using the SPF number is like this. The Sun Protection Factor (SPF) number tells people how much longer they can last out in the sun with protection for their skin without being burned. For example, say a young high school student would normally burn after 12 minutes of being out in the sun. He applies a sunscreen, also referred to as a sun block, with an SPF of 15. This means that he should be fine for 15 times his average amount of protection time. In other words, he would be protected for up to 3 hours.
Here is the formula for calculating: 12 minutes x 15 SPF = 180 minutes (3 hours).
Note that regardless of skin type, before going out into the sun’s rays, each person should have the most suitable sunscreen product possible applied beforehand for protection against UV rays. The product should be spread fairly thick and in a uniform manner for the best possible UV protection over all areas of the skin that will be exposed to the UV rays. The table below will give an indication of the different UPF protection ranges and the percentage of UV blocked.
Sunscreens ingredients can be divided into compounds that physically block radiation or compounds that absorb radiation. The radiation blockers are very effective at reducing the exposure of the skin to both UVA and UVB radiation. Older formulations like zinc oxide are opaque and may be cosmetically unacceptable. However, a newer formulation of micronized titanium dioxide is not as opaque and provides excellent protection. The radiation absorbing ingredients are differentiated by the type of radiation they absorb - UVA absorbers and UVB absorbers.
Picking the Proper Sunscreen
The SPF measures the amount of UVB absorption, but there is no method of reporting the UVA absorption. The only way to determine if a sunscreen protects against UVA and UVB radiation is to look at the ingredients. A good broad-spectrum sunscreen should have an SPF of at least 15 and contain avobenzone, titanium dioxide, or zinc oxide.
Applying Sunscreen Properly
Most people use sunscreen improperly by not applying enough. They apply only 25% to 50% of the recommended amount. Sunscreen should be applied liberally enough to all sun-exposed areas that it forms a film when initially applied. It takes 20-30 minutes for sunscreen to be absorbed by the skin, so it should be applied at least a half an hour before going out in the sun. Sunscreen should also be the last product applied especially on the face since some sunscreens can break down in the presence of water contained in water-based foundations and moisturizers.
Most instructions on sunscreen labels recommend reapplying sunscreen "frequently", but the definition of "frequently" is vague. A common instruction is to reapply sunscreen regularly when in the sun. However, one study has shown that reapplying sunscreen 20 to 30 minutes after being in the sun is more effective than waiting 2 hours. It is possible that this time period is more effective because most people do not apply enough sunscreen initially, and this second application approximates the actual amount needed. Sunscreen should also be reapplied after swimming, excessive sweating, or towelling.
Sunscreen should be applied daily. The daily use of a low-SPF sunscreen (15) has been shown to be more effective in preventing skin damage than the intermittent use of a higher SPF sunscreen.
Sunscreen and Insect Repellents
Insect repellents reduce the sunscreen's SPF by up to one-third. When using sunscreen and insect repellent together, a higher SPF should be used and reapplied more often.