Don's Home Health Sun Protection
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The World Health Organization says 60,000 people around the world die every year from too much sun.
See SPF below.

Sun Intensity:
Sun intensity is a factor of time of year, time of day, latitude (equator, north pole or somewhere in between), altitude, other environmental factors (e.g. snow & water reflection)
See: Solar Concepts at USC.
The gases and particles in the earth's atmosphere scatters and absorbs some solar energy, to the more air the sun goes thru before reaching you affects it's intensity. The amount of air the sun goes thru is affected by it's angle and your altitude.

Because the air is thinner at high elevations (25% thinner at 8,000 ft than at sea level) and the sun passes thru less atmosphere it is more intense.
Intensity increases at a rate of 6% per 1000 ft. above sea level for the same latitude. For example, at 8000 ft. the sun is 48% stronger than at sea level.
Low humidity at altitude also dries the skin making it more prone to burning and long-term skin damage.
See Rocky Mountain Sunscreen High Altitude Testing (pdf)
Latitude and time of year and time of day:
As the earth spins and moves around the sun the angle of the sun in the sky changes. Because the earth's axis is tilted 23.5° the time of year affects the sun's position.
In the spring and fall equinoxes (about Mar. 20 and Sept 22) the sun is directly overhead at noon at the equator. At the summer solstice (june 21) it is directly overhead at noon in the Bahamas. At the winter solstice (Dec 21) it is directly overhead in northern Argentina.
NOAA Solar Position Calculator
Compute the solar position and intensity from time and place

The peak sun intensity hours, when UV light is strongest, are between 10 a.m. and 3 p.m.

New FDA rules for 2012
In the summer of 2011 the FDA released new regulations for sunscreens that will require the products to be tested for protection against two types of sun exposure that cause sunburn, premature aging of the skin and skin cancer.

Under the new rules:

  • The agency will ban claims of "waterproof" or "sweatproof," requiring a manufacturer to instead say how long the product will protect the skin after exposure to water.
  • SPF (Sun Protection Factor) values will be capped at 50. Anything over that offers only marginally more protection, scientists say. See SPF below.
  • Manufacturers must phase out a four-star system used by some companies to rate UVA protection.
  • Sunscreens may be labeled "broad-spectrum" if they provide protection against UVA and UVB radiation.
  • A sunscreen may claim to be "water resistant"; however, the product must specify if it offers 40 minutes or 80 minutes of protection while swimming or sweating, based on standard testing. It must offer at least 40 minutes.
  • Sunscreens cannot claim to provide sun protection for more than two hours without reapplication
  • For the first time, the FDA will have an industry-wide test procedure to measure a product's UVA protection relative to its UVB protection.
Regardless if your sunscreen is waterproof, water-resistant, or "sport" sunscreen, you need to reapply at least once every two hours, and more if you're swimming or sweating profusely.
Reapplying regularly is more important than a sky-high SPF. SPF 30 will protect you from most harmful UV rays. Broad-spectrum sunscreen is important to protect you against UVA and UVB rays, both of which are cancer-causing.

The FDA publishes the following guide to skin types:

Type Sun History Example
I Always burns easily, never tans, extremely sun sensitive skin Red-headed, freckles, Irish/Scots/Welsh
II Always burns easily, tans minimally, very sun-sensitive skin Fair-skinned, fair-haired, blue or green-eyed, Caucasians
III Sometimes burns, tans gradually to light brown, sun-sensitive skin Average skin
IV Burns minimally, always tans to moderate brown, minimally sun-sensitive Mediterranean-type Caucasians
V Rarely burns, tans well, sun-insensitive skin Middle Eastern, some Hispanics, some African-Americans
VI Never burns, deeply pigmented, sun-insensitive skin African-Americans
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 (19 in).

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.

A broad broad brimmed hat is very important to protect your neck and ears as well as your face.

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

EPF: Eye Protection Factor - In 2004 EPF was proposed to be to non-prescription sunglasses what the Sun Protection Factor (SPF) is to sunscreen. It is an index from 1-100 proposed by Dr. Gary Hall, M.D. Phoenix, AZ. The EPF rating is based on frame coverage, ultra-violet (UV) protection, blue light protection and infrared protection, or the ability to shield the eyes from heat. The final EPF rating is a result of averaging the scores of these four factors known by the acronym FUBI. As of 2011 it had not seen widespread usage.

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.)

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 Light at

MED - minimal erythema dose

SPF (sun protection factor) Number representing the amount of sun something blocks.
* SPF15 blocks 93% of UVB rays
* SPF30 blocks 97% of UVB rays
* SPF50 blocks 98% of UVB rays
Sunscreens with really high SPFs, like SPF75 or SPF100, do not offer significantly greater protection than and SPF30 and are thought to mislead people into thinking they have more protection than they actually do. Using enough to begin with and reapplying regularly are more important than a sky-high SPF.
SPF only measures UVB. A sunscreen with broad spectrum (UVA & UVB) protection is important.

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.

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.
UVB (290-320 nm) rays are responsible for sunburn. They are at their most intense around mid-day, during mid-summer
UVC (200-290 nm) rays are absorbed by the atmospheric ozone layer and never reaches your eyes.
UVR - Ultraviolet radiation,

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

VLT - Visable Light Transmission: See LTF.

Sun Protection Factor: SPF Sunscreen Info by Badger
Solar Concepts at USC
Expert Advice: Understanding Sun-Protection Clothing from REI
Dressed to protect
Solar Radiation

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last updated 9 July 2011