The World Health Organization says 60,000 people around the world die every year from too much sun.

The FDA publishes the following guide to skin types:

90 the percentage of UV rays that can penetrate clouds.

25 The percentage of UV rays reflected by sand.

50 The percentage of ambient UV rays given by shade.

50 The percentage of daily UVR emitted between 11 a.m. and 2 p.m.

40 The percentage of UV rays that penetrate water to a depth of 50 cm.

Clothing:
If you can see light thru a fabric, UV is getting thru. UPF (Ultraviolite Protection Factor) measures this. A UPF of seven means 1 seventh of the UV is getting thru. UPF must be grteater than 15 to be labeled.

UPF	Protection
15-24	Good
25-39	Very good
≥40	Excellent

Some values are:

Type	UPF
Bleached cotton	4
Unbleached cotton Thin, white T-shirt	5-8
Green T-shirt	10
Tilley's sun-protection line	50+
Patagonia Sun Tech Shirt	30

Polyester provides superior UVB protection over fabrics composed of cotton, linen, and rayon, but less UVA protetion.
Washing increases UPF (From 19 to 40) in one test.
The UPF of most fabrics decreases with wetness. One explanation for this is that the presence of water in the interstices of a fabric reduces optical scattering effects and, hence, increases UV transmission of the textile. In one study linen, viscose, and polyester proved to have no significant change in UPF when wet.

Sources: "Sun smarts for runners" AMAA Journal, Spring, 2007
Arch Dermatol -- Defined UV Protection by Apparel Textiles, August 2001, Hoffmann et al. 137 (8): 1089

Sunscreen:
Most people don't use enough sunscreen -- one full ounce, or a shot glass, is required on each application

Skin problems agrevated with UVA: Polymorphic light eruption, solar urticaria, chronic actinic dermatitis, or actinic prurigo.

The Australian radiation protection and Nuclear Safety Agency (ARPANSA) has an EPF scale from 1 - 10 with 9 and 10 blocking most UVR.

HEV - High energy visible light - Recent research indicates that Blue (HEV) light might contribute to macular degeneration, or loss of vision detail.

Pterygium - An abnormal mass of tissue arising from the conjunctiva of the inner corner of the eye that obstructs vision by growing over the cornea; it arises from irritation of the pinguecula (a slightly elevated elastic tissue deposit in the conjunctiva (a transparent lubricating mucous membrane that covers the eyeball) that may extend to the cornea but does not cover it.)

Lumens:
The brightness or intensity of light is measured in lumens. For example, when you are indoors, most artificial light is around 400 to 600 lumens. If you go outside on a sunny day, the brightness ranges from about 1,000 lumens in the shade to more than 6,000 lumens on a large stretch of concrete, like a highway. A large snowfield on a bright day can reflect light at more than 12,000 lumens. Our eyes are comfortable until we get to around 3,500 lumens. Light over 10,000 lumens can result in temporary or even permanent blindness.
Light over 4,000 lumens appears as glare (white light)
See Understanding Ligt at HowStuffWorks.com

MED - minimal erythema dose

SPF (sun protection factor) Number representing the amount of sun something blocks. For example, if you normally begin to burn after half an hour in the sun, a sunscreen with an SPF of 2 should let you stay out twice as long (1 hour), SPF 4 should let you stay out four times as long, and so on.

UPF (Ultraviolite Protection Factor) Number representing the fraction of the sun's UV rayks to pass through a piece of clothing. Introduced in 2001.
A thin, white T-shirt has an average UPF of seven, which means that it allows one-seventh of the sun's UV rays to pass through.

UVC (200-290 nm) rays are absorbed by the atmospheric ozone layer and never reaches your eyes.
UVB (290-320 nm) rays are responsible for sunburn. They are at their most intense around mid-day, during mid-summer
UVA (320-400 nm) rays penetrate deeper into the skin and are implicated in premature ageing and wrinkling, as well as skin allergies and rashes. They are present all-day and all-year round.

Visible Light (Red, Orange, Yellow, Green, Blue, Violet) 500 THz - 900 THz / 400nm to 700nm.

VLT - Visable Light Transmission: See LTF.