Safer Sun (II)

Safer Sun (II)

Part II: SPF, UVA and UVB - the ideal factor for your skin


by Dr. Huong Nguyen | Editor: Dr. Sabine Nunius | June 30, 2022

At first glance it seems quite simple: put on a tube, apply sunscreen, reapply again and again to maintain the sun protection - done! But not all sunscreens are the same.

In detail, five key figures are important:

• Skin's own protection time

• planned duration of sun exposure




UVA and UVB radiation

Ideally, a product should protect you from both UVA and UVB rays. UVB rays only penetrate the epidermis. However, in high doses they lead to the well-known sunburn. In the worst case, they can chronically damage cell nuclei and thus cause skin cancer. UVA rays have longer wavelengths than UVB radiation and penetrate into the dermis. Although they do not cause sunburn, they can lead to skin changes such as wrinkles, loss of elasticity and irregular pigmentation. Regular and sufficient protection is therefore important.


First let’s talk about the SPF. As a rule, sun protection products are classified according to SPF, i.e. sun protection factor, or SFP (Sun Protection Factor). Here we differentiate between

• low (SPF 6 to 15)

• medium (SPF 15 to 25)

• high (SPF 25 to 50)

• very high level of protection (SPF over 50)

This number indicates the protection of a sunscreen against UVB rays - i.e. the “sunburn-causing” rays, but not the protection against UVA rays! It tells you how long your skin's own protection can be extended by using sunscreen before sunburn occurs.

This can be calculated specifically using the following formula:

Skin's own protection x SPF. As an example: Your skin has a self-protection time of 20 minutes and you use a sunscreen with SPF 10. In this case, 20 x 10 = 200. This means that your skin would be protected by the product for a maximum of 200 minutes.

FACT of the day

A sun protection factor (SPF) of 2 absorbs 50 percent, SPF 20 96 percent and SPF 40 around 98 percent of UVB radiation. The sun protection does not increase steadily with the factor, but the increase curve becomes increasingly flatter. He never reaches 100 percent!

Of course, for a calculation you first need the correct self-protection time for your skin. Ideally, you should have your dermatologist determine your skin type. This table from the Federal Office for Radiation Protection will also help you for an initial orientation:

Depending on the skin type, the following self-protection times arise:


Self-protection time (min)*

Celtic type (Type I)

under 10

Nordic type (Type II)

10 to 20

Mixed type (Type III)

20 to 30

Mediterranean type (Type IV)

over 30

Dark skin types (Type V)

over 60

Black skin types (Type VI)

over 90

*) Self-protection time: maximum time spent in the summer midday sun until erythema appears (= sunburn)

2. FACt of the day

The SPF of a sunscreen only refers to the UVB protective effect. You can tell whether a sun protection product also has UVA protection by looking at the 'UVA in a circle' pictogram on the packaging. The corresponding UVA protection is then at least a third of the UVB protective effect.

After this little arithmetic for everyday life, it's now about the correct application. Just click on to the next Safer Sun story!