What does sun protection factor - SPF mean?
The sun protection factor, which protects from UV rays, points at the degree of protection provided in a sunscreen. Without adequate protection, sun exposure is safe only in a certain time period, which, depending on the skin type and the natural skin resistance, varies from 5 to 30 minutes.
By multiplying the sun protection factor (SPF) and the time period of the natural skin resistance (without protection), we get the total period of time of sun exposure in which we could safely enjoy it.
Let’s explain this on a simple example:
Let’s suppose that your skin naturally reddens after being exposed to sun for 20 minutes. If you choose a sunscreen with SPF 10, your skin will develop such redness after 20 (minutes) X 10, which is 200 minutes. Yet, the sunscreen should be reapplied regularly after swimming, wiping your body with a towel etc.
The assessment of UVA and UVB rays protection is made by means of laboratory measurements (in vitro), in which the SPF is calculated.