What is SPF?
SPF is Sun Protection Factor. The number is determined experimentally indoors by exposing human subjects to a light spectrum meant to mimic noontime sun. Some subjects wear sunscreen and others do not. The amount of light that induces redness in sunscreen-protected skin, divided by the amount of light that induces redness in unprotected skin is the SPF. The SPF number only relates to UVB protection. UVA protection is not measured through SPF numbers, and until recently, frequently was not available in sunscreens. UVB sunburn is caused by the culprit red skin drying and has been linked to skin cancer. UVA is associated with aging of the skin, and along with UVB exposure, may increase risk or facilitate skin cancer. UVA though milder, is still not safe.
The number you see associated with SPF represents the length of time you can stay out in the sun without burning, multiplied by the corresponding number. So a person, who would normally start to burn in 10 minutes, a UV sunscreen of SPF 15, would allow you to stay out in the sun 15 times longer. That means you could theoretically have 150 minutes of sun protection.
The protectiveness of clothing can also be measured by SPF. SPF’s of various types of clothing: Nylon stockings – SPF 2, Hats – SPF 3-6, Summer-weight clothing – SPF 6.5, Sun-protective clothing – up to SPF 30. (Source: http://dermatology.about.com)
What remains important to remember is that SPF strength is less important than frequent application, and avoidance of sun between 10 am and 4 pm, when UVB rays are strongest. SPF protection should always be combined with protective clothing and sun avoidance during the hottest and brightest parts of the day.
What is PA?
PA stands for protection grade of UVA (UVA light is caused by aging factors) system and is the Japanese measurement of sun protection, which is based on the Persistent Pigment Darkening (PPD) reaction reading at 2-4 hours of sun exposure. PA is a protective level of strength to mark the three PA+, PA++ and PA+++.
PA+ provides some UVA protection with a factor of PPD 2-4. It protects sensitive skin against low to medium UV radiation.
PA++ provides moderate UVA protection with a factor of PPD 4-8. It offers better protection from UVA rays and can be used by people with normal skin exposed to medium UV radiation.
PA+++ provides good UVA protection with a factor of PPD > 8. It is the strongest grade of UVA protection now available. It is designed for normal skin under very strong or direct UV radiation.