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Ultra-Violet (UV) Radiation Exposure

Ultraviolet (UV) radiation reaching the Earth’s surface is comprised of long-wave Ultraviolet Alpha (UVA) and the shorter wavelength Ultraviolet Beta (UVB). UV radiation levels are influenced by:

  • Elevation of the sun - the higher the sun in the sky, the higher the UV levels. The levels will vary with the time of day, and the time of year.
  • Latitude - areas closer to the equator are exposed to higher UV levels.
  • Cloud cover - UV levels are higher under cloudless skies.
  • Altitude - at higher altitudes the UV levels are higher.
  • Ozone - ozone in the atmosphere absorbs some of the Ultraviolet Beta Rays (UVR) that would reach Earth. Ozone depletion leads to an increase in UVB levels with little impact on UVA levels.
  • Ground reflection - grass, soil and water reflect less than 10% of UVR, snow 80%, dry sand 15% and sea foam 25%.

UV radiation cannot be seen or felt, and must therefore be measured to determine the UV levels at ground level (the ambient UVR). The UV Index is an open-ended linear scale, with higher values representing the risk level of skin damage to UV exposure and increased risk of skin cancer. Though it's not the only safeguard you need to take, sunscreen or sunblock is one of the easiest ways to protect your skin and is a good first line of defence.

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What does the UV Index mean?

The Ultraviolet (UV) Index is an internationally recognised measure of the sun’s potential to do damage to our skin and eyes.

It is based on a rather complicated calculation called the McKinlay-Diffey Erythema Action Spectrum and it basically compensates for the fact that different wavelengths of UV in the sun cause more damage than other wavelengths to our skin.

Protection measures for the different UV Index values

UV Index Description Graphic Colour on the UV Index Recommended Skin Protection Precautions
0-2 Low risk for most skin types Green Use sunblock if you have fair skin. Sunglasses are not required.
3-5 Moderate risk of harm from unprotected sun exposure Yellow Wear sunglasses and use suntan lotion, remember to wear a hat and a shirt, try to avoid midday sun exposure.
6-7 High risk of harm from unprotected sun exposure Orange Wear sunglasses and use suntan lotion having SPF 15 or higher, wear loose fitting clothing and wear a wide-brim hat, make an effort to avoid sun exposure between 11:00 and 14:00.
8-10 Very high risk of harm from unprotected sun exposure Dark Red Wear suntan lotion with SPF 15 or higher, a broad brim hat and loose fitting long sleeve tops are recommended, avoid exposure to midday sun, wear sunglasses, drink lots of water to prevent dehydration.
11+ Extreme risk of harm from unprotected sun exposure Violet / Purple Take all the above precautions, including: wearing sunglasses and using suntan lotion, cover the body with a loose fitting long-sleeve shirt and pants, wear a broad hat, if possible avoid sun exposure from 10:00 to 15:00. Essential to drink lots of fluids as dehydration can readily occur with prolonged sun exposure and excessive sweating.


What is an SPF?

All sunscreens products include an SPF, which stands for sun protection factor. The SPF number is a measurement of the amount of UVB protection — the higher the number, the greater the protection. Currently, there's no standard rating system that measures UVA protection.

SPF is not an indication of how much time you can spend in the sun. For example, if you use a sunscreen with an SPF 30 rather than one with an SPF 15, it doesn't mean you can stay in the sun twice as long. In reality, an SPF of 15 filters out about 93 percent of the UVB rays; SPF 30 filters about 97 percent of UVB rays. The beneficial effects of sunscreen decreases over time, so after a few hours the difference between the two may be even less. Don't rely on the SPF factor to decide how long you're safe in the sun and don't count on your skin to tell you when you've had too much sun, it may take up to 24 hours for a sunburn to develop fully.


Sun safety tips and measures to prevent heat stroke:

  • Don’t do any strenuous physical activity when it is very hot or if the humidity is high. If you are unable to avoid strenuous physical activities, ensure that you stay hydrated by drinking fluids such as water and energy drinks. Avoid drinks such as alcohol, soda cooldrinks, tea and coffee which are diuretic and will lead to fluid loss.
  • Wear sunglasses – choose sunglasses that offer at least 95% UV protection with a lens tint that blocks 80% of transmissible light.
  • Wear UV blocking contact lenses alongside sunglasses – UV contact lenses can help protect against the transmission of harmful UV rays to the cornea and inner eye.
  • Use sunscreen on all exposed areas. Feet should also be included if you are bare-foot or wearing sandals or flip-flops. A sun protection factor (SPF) of 15+ is recommended. Always re-apply after leaving the water as it may have washed off.
  • Wear protective clothing, including wide-rimmed hats, and use large umbrellas to keep out of the sun. Long-sleeved, loose cotton shirts are excellent for outdoor sports such as hiking and cycling.
  • Reduce spending time in the sun between 10:00 and 15:00. In extremely hot weather conditions it is advisable to stay indoors during this time of the day.
  • Don't lie in the sun purposefully seeking a tan. If you want to tan, do it gradually and sensibly, with short exposure and building up over time. Or use a safe instant tan product.
  • Pay close attention to any medications (including herbal remedies) for many of them have side-effects which include sun sensitivity.
  • Children are especially susceptible to the harmful effects of the sun, so take extra steps to protect their skin and to prevent sunburns. Babies younger than six months should be kept out of direct sunlight because their skin is even more fragile. Use sunscreen on an infant only if you're unable to keep him or her out of the sun and are unable to cover exposed skin. Use a small amount on uncovered areas, such as on hands or ears, and check for any skin reactions.

Symptoms of heat stroke can sometimes mimic those of heart attack or other conditions. Sometimes a person experiences symptoms of heat exhaustion before progressing to a heat stroke.

Signs and symptoms of heat exhaustion:

  • Nausea
  • Vomiting
  • Fatigue
  • Weakness
  • Headache
  • Muscle cramps and aches
  • Dizziness

Signs and symptoms of heat stroke:

  • High body temperature
  • The absence of sweating, with hot red or flushed dry skin
  • Rapid pulse
  • Difficulty breathing
  • Strange behaviour
  • Hallucinations
  • Confusion
  • Agitation
  • Disorientation
  • Seizure
  • Coma

A patient must be referred to a medical doctor if any of these signs are present.

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For updated weather forecasts so that you can be prepared for this type of hazard visit SA Weather Service or call the Weatherline at 082 162.

© City of Cape Town, 2014