In a 2011 post, we mentioned that you should be applying an average of one ounce of sunscreen to the exposed skin on a clothed body - face, neck, ears, arms. By applying less than this amount, or by mixing sunscreens, you may be getting less than the advertised protection on the bottle. Understanding how SPF is calculated and the errors in application will help you decide if you are achieving the maximum SPF.
In the United States, SPF is tested and calculated by using real humans. We'll spare you the technical jargon and math, but in short, male and female volunteers go through a series of medical examinations and are given a skin type according to how easily they burn. Then, the test subjects are given 2 milligrams of the sunscreen moisturizer for every square centimeter of skin that's being tested, just before being subjected to synthetic ultraviolet radiation. The moisturizer is then given a sun protection number based on how well they were protected during that exposure period.
Moisture Protect SPF 20 (and its Bronze brother) were given a sun protection factor of 20, based on applying 2 milligrams of formula per square centimeter of skin.
As mentioned, the first mistake in sunscreen application is not applying enough. Since the SPF is calculated on a healthy dosage, which most people do not use, you need to apply that much to get the advertised coverage. Any less coverage means less SPF - this goes for tinted SPF moisturizers as well. Additionally, you can't apply SPF lotion once and expect to be protected all day. When it wears off, your protection wears off.
The second mistake in application is when sunscreens are mixed. Combining products with two different SPF values does not give you the addition of the SPF, but rather an average of the two. Remember:
In short, to achieve the advertised SPF, you need to apply generously and reapply often.