Sun protection creams or sunscreens can broadly be classified into two categories. The first category of sunscreens would be that which contains organic compounds as its primary constituent. Certain organic compounds are intrinsic to absorbing ultra violet light; hence these find usage in sunscreens. The second category of sunscreens would be that which contains inorganic particles as its primary ingredient. Certain inorganic particles are known to scatter and reflect ultra violet rays, the most common being zinc oxide solution, often seen being used by sportsmen who need to expose themselves to direct sunlight for the whole day. There exists a third variety of sun protection creams which would largely be mix and match of the first two varieties and is not very popular.
Sun protection creams normally are assigned a number commonly known as sun protection factor or SPF. In conventional terms, SPF measures the effectiveness quotient of sunscreens under the sun. In other words, higher the SPF, higher the effectiveness of the sunscreen against sunlight. It actually measures the degree of protection that the sunscreen can offer.
Whereas it is largely accepted that a higher SPF is generally more effective than the ones with a lower rating, it is not scientifically proven whether SPF beyond the number 50 are actually more effective or not. For this, there are many advanced countries in the world that restrict assigning SPF to less than or equal to 50. Most of the sun protection creams will ideally be within SF 50 due to this reason.
The effectiveness of the sunscreens, apart from their sun protection factors, also depends upon other important factors. First, and perhaps most important, is the type of the skin that the user has. Even though sunscreen are not manufactured keeping in mind the user's skin, the skin, being the directly impacted body part, plays the most significant role in determining whether the sun protection cream will be effective or not. For this, users may need to do some trial and error with sunscreen before arriving at the right product suitable to them. Second, the quantity of sunscreen that has been applied on the exposed skin; and the number of times the sunscreen has been applied on the body. Thus, both quantities applied and number of times that quantity has been applied plays critical roles in making the sunscreen effective. It is common knowledge that frequent application does help in improving effectiveness; but the quantity again depends from person to person and varies from one skin type to the other. Third, the quantum of sunscreen the skin absorbs. This can be directly attributable to the combination skin type and the way a particular skin reacts to a particular type of sun protection cream. Fourth and final, the usefulness of the sunscreen also depends on the exact nature of activity or sport an individual engages in. For example, a cricketer can apply zinc oxide and stay with one coat of sunscreen during the entire session. On the other hand, a swimmer need to frequently re-apply as water tend to remove the sunscreen from the body.
There are however certain unanswered questions when it comes to functioning of sunscreens. It is not clearly known whether the sunscreen acts on the surface of the skin or underneath the surface of the skin. It is due to this lack of clarity that people tend to use zinc oxide based sunscreens more often than not; which in effect are inorganic compounds that reflect sunlight away from the skin. Also, as sunscreens tend to scatter away sunlight from the skin surface, the skin does not receive the daily recommended dosage of vitamin D. This may lead of deficiencies of vitamin D within and body and lead to other health related complications. It is thus always advisable to spend some time of the day under direct sunlight without application of any kind of sun protection cream.