Improved knowledge related to the sun and technology advances has made sunscreen issues more complex. Which sunscreen product is most suitable depends on many factors such as how sensitive the skin is to burning and cosmetics, how dry or oily the skin is, previous sun and skin cancer history and medical history.
A sunscreen with an SPF 15 provides about 94% protection against UVB. Protection against UVB is increased to 97% with SPF 30 and increased to 98% with SPF 50+. As you can see the difference in protection when going from sunscreen with SPF 15 to one with SPF 30 or even 50+ differs only by 3–4%. Also, this protection is only provided if sunscreens are applied in quantities similar to the ones used for testing, 2 mg/cm2. This is six teaspoons of lotion for the body of one average adult person.
In reality, most people apply their sunscreen at about one third the thickness used for testing. They fail to apply it to all exposed areas of skin. And then they forget to reapply it every couple of hours or after heavy sweating or swimming.
Therefore, the actual protection may be a lot less than the tests indicate.
A sunscreen with SPF 15+ should provide adequate protection if it is being used correctly. Sunscreens with SPF 50 or more offer a safety margin since most people don’t apply sunscreens as heavily or as often as they should. Remember to reapply after half an hour, so that the ‘mountains’ as well as the ‘valleys’ are fully protected. Imagine you are painting a wall – two coats of paint provide a more even coverage than one!