The UV protection factor states how long someone using sun protective textiles or wearing UV-protective clothing can stay out in the sun without suffering damage to their health (skin damage).

The UPF protection factor is comparable to the sun protection factor of sunscreen. In both cases, the basis for calculations is what is known as the intrinsic protection time of the skin, which can vary considerably depending on the individual skin type.

The skin of someone of skin type I with red or blonde hair, blue eyes and a very fair complexion has an intrinsic protection time of approximately five to ten minutes. If they are exposed to the burning sun for longer than this without protection, they risk harmful sunburn.

When protected by a textile material with a UPF of 20, for example, this individual can extend how long they are able to stay in the sun by a factor of 20, in other words, up to a maximum of 3 to 6 hours (20 x 5 min = 100 min to 20 x 10 min = 200 min), without risking damage to their skin.

Even more information: http://www.uvstandard801.de/content.asp?uv=88xx0xx0&l=02

UV protection	UPF or SPF protection factor
UV protective clothing (acc. to UV Standard 801)	20-80
Tightly woven cotton clothing (acc. to UV Standard 801)	approx. 20
Sunscreen when applied correctly (SPF)	0-30
Lightly woven cotton clothing (acc. to UV Standard 801)	approx. 10
Shade under a tree	approx. 5-15
Sun hat with a wide brim	approx. 10
Shade under a sunshade (without special UV-protection)	approx. 5

Not every type of skin reacts in the same way when exposed to the sun. Dermatologists therefore make a distinction between five skin types with different intrinsic protection times.

Skin type	Description	Sunburn	Tanning	Skin’s intrinsic protection time	Textile protection(UPF 20)
I	Skin: extremely light, pale Freckles: profuse Hair: reddish Eyes: green, blue, very occasionally brown	Always burns, painful	Never tans; white after 1-2 days, skin peels	5-10 minutes	100-200 minutes
II	Skin: slightly darker than type I Freckles: rare Hair: blonde to brown Eyes: blue, green, grey	Generally burns, painful	Rarely tans, skin peels	10-20 minutes	200-400 minutes
III	Skin: light brown Freckles: none Hair: dark blonde, brown Eyes: grey, brown	Occasionally burns, moderate	Tans well	20-30 minutes	400-600 minutes
IV	Skin: brown Freckles: none Hair: dark brown, black Eyes: dark	Rarely burns	Tans quickly and deeply	approx. 45 minutes	approx. 900 minutes
V	Skin: dark brown, black Freckles: none Hair: black Eyes: dark	Rarely burns	Tans quickly and deeply	60-90 minutes	1.200-1.800 minutes