UVA,UVB,SPF, and how to best apply sunscreens in order to get maximum protection.

A reader asked me to research a sunscreen, after the sun and the skin post, and as I was rummaging through articles and information pages, it struck me that explaining a few basic facts about sunscreens might be a good idea to do before I start posting anything else about sunscreens.


Ultraviolet A rays account for up to 95 percent of the UV radiation reaching the Earth's surface. Although they are less intense than UVB, UVA rays are 30 to 50 times more prevalent. They are present with relatively equal intensity during all daylight hours throughout the year, and can penetrate clouds and glass, which signifies the importance of wearing UVA blocking sunscreen also when your inside your house.

UVA, which penetrates the skin more deeply than UVB, has long been known to play a major part in skin aging and wrinkling (photoaging), and it is also responsible for the tan you get after a day at the beach. Studies over the past two decades also show that UVA damages skin cells called keratinocytes in the basal layer of the epidermis, where most skin cancers occur. UVA contributes to and may even initiate the development of skin cancers.


Ultraviolet B rays are the chief cause of skin reddening and sunburn, tends to damage the skin's more superficial epidermal layers. It plays a key role in the development of skin cancer and a contributory role in tanning and photoaging. Its intensity varies by season, location, and time of day. UVB rays do not significantly penetrate glass.

Sun Protection Factor (SPF):

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. It is mainly a measure of UVB protection and ranges from 1 to 45 or above.

A sunscreen with an SPF of 15 filters 92% of the UVB. Put another way, a sunscreen with an SPF of 15 will delay the onset of a sunburn in a person who would otherwise burn in 10 minutes to burn in 150 minutes. The SPF 15 sunscreen allows a person to stay out in the sun 15 times longer.

When it comes to how to apply sunscreen, most dermatologists recommend applying your sunscreen before anything else goes onto your skin in case it interferes with the chemical absorbance of the sunscreen. That goes for both chemical and physical sunscreens because even though physical sunscreens form a layer on top of the skin, pre-applied products might interfere with the layering of the sunscreen, and might even dilute the sunscreen.

Personally I tend to apply my chemical sunscreen first before I apply a moisturizer or physical sunscreen on top, as recommended.

If I’m using my physical sunscreen on the other hand, I like to put a pump of fast absorbing antioxidant containing serum underneath such as John Masters Organics - Vitamin C Anti-Aging Face Serum, or Caudalie Vinexpert Firming Serum 1 oz , as antioxidants are said to boost the effect of the sunscreen.

Another important point is that all sunscreens take a minimum of 20 min to work properly; so one should always make time for some absorbance after application.