The amount of UV exposure the body receives depends on many factors; it can be blocked by some things and increased by others. It is very important that the natural effects of UV exposure are taken into consideration to avoid sunsburn, both when checking UV forecasts (UV Index) and generally when in the Sun. It is very easy to become overexposed to UV rays, and severe sunburn can result. Following are some of the major factors that influence UV exposure and UV risk:
Time of day
UV exposure varies mainly with the height of the Sun in the sky. This is highest during the summer months, particularly during the 4-hour period around noon at which time the Sun’s rays travel the shortest distance to Earth. A good rule of thumb is that if the length of your shadow is shorter than your height, then sunscreen is required. Protection at midday is essential, and strong sunscreen with a high healthy SPF throughout the day is always a good idea for maintaining healthy skin.
UV risk is greatest during the summer (Northern Hemisphere: May-August; Southern Hemisphere: December-March). UV risk decreases in spring and autumn, and is at its lowest during winter.
Sunburn can occur six times faster in summer than in winter. This is because harmful UVB rays vary greatly from season to season even though UVA does not (being only about 1.5 times stronger in summer). Maintaining healthy skin by minimising UV risk means using sunscreen rather than covering the body with a tanning oil or lotion, which only serve to increase the Sun’s aging effect on the skin.
Cloud and haze
UV risk is highest with clear blue skies. Generally, cloud cover reduces UV exposure, however light or thin clouds have little protective effect and may even enhance the levels of UV rays. Because of scattering by water molecules and fine particles in the atmosphere, 50-100% of UV rays can be passing through such cloud.
Be careful of cool breezes and overcast days where UV risk can be enhanced. 40-50% of UV rays can be transmitted, and these days can be just as dangerous as bright, sunny days. Sunscreen during these times is recommended to decrease your UV exposure. Long periods in open shade or between city buildings can still expose you to enough UV rays to burn light, sensitive skin. Wearing a tanning lotion on a cloudy day can be very dangerous and sunscreen use is, as a rule, more advisable to minimise UV risk.
UV rays reflect from most surfaces, and some surfaces can even double the UV exposure. The extent to which UV rays reflect from grass, concrete and soil is around 15%, whereas sand can be as much as 17%. However, if any of these surfaces are wet this figure can double. Water reflects around 50%, and even white foam water is 25% reflection of UV rays.
But the most reflective surface is snow at around 85% reflection of UV rays, so when on snow or near water, wearing a hat is not enough! A high-factor sunscreen is essential.
UV intensity increases dramatically with altitude as the atmosphere becomes thinner and cleaner than at lower elevations. This is due to the decreasing amounts of air molecules resulting from decreased atmospheric pressure, possible elevation above lower clouds and the slight increase in proximity to the Sun.
For every 1000 meters above sea level, the UV exposure increases by up to 16%. At an elevation of 3000 meters (9843 feet), UV risk is therefore doubled. Sunscreen with a high SPF value is required at high altitudes – ideally HIMAYA’s SPF50+ or SPF60 Sports Formula.
The reflective property of the water surface means that water can reflect up to 50% of the Sun’S UV rays. This dramatically affects the amount of time you can spend on the water before you start to sunburn.
Where in the world…
Due to the shorter distance travelled by the UV rays, UV exposure is greatest at the equator and becomes weaker as you travel towards the Earth’s poles.
It has been shown that people who lived in a sunny environment for more than one year before the age of 10 exhibit FOUR TIMES the risk of developing melanoma (Gallagher et al, 2000).
The longer the time that you are in the Sun, the more UV exposure you receive. Higher than average incidents of skin cancer have been recorded among occupational groups who work outdoors (Scarlet, 2003).
Summer clothes generally allow for greater UV exposure than winter clothes. Dark, tightly woven fabrics protect the skin more than lighter and looser weaves, decreasing your UV risk.
Your age influences the extent to which UV exposure exerts its affects on skin and underlying tissue. Generally speaking, the younger your skin, the more damage UV radiation can cause. Younger people, specifically children, have a thinner epidermis, limited melanogenesis and no hydro-lipidic film covering the horny layer (the latter due to the inactivity of the apocrine and sebaceous glands).
An individual’s exposure during childhood is a strong determinant of melanoma risk (Scarlet, 2003).
Children who are encouraged by their parents to wear sunscreen are much more likely to continue to use sunscreen as adults (Autier et al, 1998; Whiteman et al, 2001), and therefore maintain a lower level solar risk.