Calculated by the National Weather Service and published by the EPA, the UV Index provides a forecast of the expected risk of overexposure to UV radiation from the sun. It is accompanied by recommendations for sun protection and is a useful tool for planning sun-safe outdoor activities. Check the UV Index for your location.
Factors That Increase UV Intensity
The strength of UV rays reaching the ground depends on a number of factors, such as:
- Time of day: UV rays are strongest between 10 a.m. and 4 p.m.
- Season of the year: UV rays are stronger during spring and summer months.
- Distance from the equator (latitude): UV exposure goes down as you get further from the equator.
- Altitude: More UV rays reach the ground at higher elevations. Most of Colorado is already at a high altitude, resulting in less atmosphere to absorb UV radiation compared to sea level.
- Cloud cover: The effect of clouds can vary – sometimes cloud cover blocks some UV from the sun and lowers exposure, while some clouds can reflect UV and increase exposure. Regardless, UV rays can get through, even on a cloudy day.
- Reflection off surfaces: UV rays can bounce off surfaces like water, sand, snow, pavement, or grass, leading to an increase in UV exposure. Hats, sunglasses, and protective clothing are especially important when on the water (75% reflective) or in the snow (88% reflective).
The amount of melanin available for each person is genetically determined. The number of melanocytes is similar across races but their productivity varies. Dark-skinned individuals have more active melanin production while individuals with lighter skin make less melanin, putting them at a higher risk for sun damage.
The darker an individual’s natural skin color, the more protection they have from UV rays, up to 13 SPF. Although a tan is the body’s defense mechanism against UV exposure, the level of protection is limited – only about 2-4 SPF for a light-skinned individual, which is not a significant defense against skin damage. A tan should be considered a symptom of skin damage rather than a sign of protection from the sun.
Individuals with darker skin have more melanin and more natural protection from UV rays compared to light-skinned individuals, but individuals with darker skin may still get sunburned and accumulate damage resulting in photoaging. Unfortunately, dark-skinned individuals are more likely to get diagnosed with skin cancer at later stages, which can have poorer outcomes. It is important to remember that UV protection strategies apply to individuals of all skin types!
Sunburn occurs when the body’s UV response is overwhelmed by high doses. The early signs of sunburn include flushed-looking skin that is tender or painful, and giving off more heat than normal. People with darker skin tones may not notice physical signs until several hours later, and it can take 6-48 hours for the full effects of sunburn to appear. The symptoms associated with sunburn are caused by cellular damage/death, followed by the body’s immune response of redness and warmth, pain, and itching and peeling
Long-term exposure can cause early skin aging, wrinkles, loss of skin elasticity, dark patches (lentigos, sometimes called age spots or liver spots), and pre-cancerous skin changes (such as dry, scaly, rough patches called actinic keratoses).
Melanoma is the most serious form of skin cancer, and is now one of the most common cancers among adolescents and young adults ages 15-29. Learn more about melanoma and non-melanoma skin cancers.
A tan indicates damage to the skin, a response to injury from UV rays. A base tan does not protect from future damage from UV exposure, and individuals who indoor tan are more likely to report getting sunburned. Indoor tanning is also not a safe way to get vitamin D. The safest way is through food. Tanning harms the skin, and the amount of UV exposure needed to get enough vitamin D is difficult to measure, as it is different for every individual and varies with the weather, latitude, altitude, and more.
In a recent survey of CSU students, 18% reported having ever used indoor UV tanning. The rates of tanning bed use have been associated with an increase in melanoma – the deadliest form of skin cancer – among young adults. UV tanning before the age of 35 years increases the risk of melanoma by 59%, and the risk increases with the number of tanning sessions per year.
The Five S’s of Sun Protection
- Slip on sun protective clothing that covers as much of your body as possible. Long-sleeved shirts, long pants, or long skirts cover the most skin and are the most protective. Dark colors generally provide more protection than light colors, a tightly woven fabric protects better than loosely woven clothing, and dry fabric is generally more protective than wet fabric.
- Slop on SPF 30+ broad spectrum sunscreen liberally to dry skin, at least 20 minutes before sun exposure, and reapply every two hours when outdoors.
- Slap on a broad brimmed hat that shades your face, neck, and ears. A baseball cap protects the front and top of the head, but not the neck or the ears, where skin cancers commonly develop.
- Seek shade or shelter, especially between the hours of 10 a.m. and 4 p.m. when UV light is strongest. If your shadow is shorter than you are, the sun’s rays are the strongest, and it’s important to protect yourself.
- Slide on some sunglasses that block UV rays. Look for labels that say “UV absorption up to 400nm” or “Meets ANSI UV Requirements,” which mean the glasses block at least 99% of UV rays.
What is Sun Protection Factor (SPF)?
SPF measures the ability of sunscreen to block UVB rays. Only sunscreen labelled “broad spectrum” blocks UVA.
|SPF Rate||% of UVB Blocked||When to Use|
|SPF15||93%||Brief, intermittent sun exposure|
|SPF30||97%||Good all-purpose, recreational use|
SPF values are multipliers for the amount of time an individual can be in the sun before burning. For example, wearing SPF 15 will allow someone who typically burns after 10 minutes to be in the sun for 150 minutes before burning (10 minutes x 15 SPF).
How Sunscreen Works
There are two ways sunscreen can work. Some sunscreens are mineral-based (zinc oxide, titanium oxide) and are physical barriers that reflect and scatter UV rays. Most sunscreens rely on chemical ingredients (octylcrylene, avobenzone, helioplex, etc.) that work by absorbing UV rays into non-damaging wavelengths.
In order for sunscreen to be effective, it must be applied correctly:
- Apply 30 minutes before sun exposure, to allow the sunscreen to absorb into the skin.
- Reapply ever two hours – chemical sunscreen gets used up and no longer absorbs UV rays, and mineral sunscreens can rub off. Reapplying does not increase the time until burning.
- The effectiveness of sunscreen is affected by the amount used. Approximately 1oz should be used for the full body – ½ tsp for face and neck, 1 tsp for each arm, 2 tsp for torso, and 2 tsp for each leg.
- It is also important to use waterproof/water resistant sunscreen if planning to go into water or sweat, and if also using insect repellant, apply sunscreen first and wait 30 minutes to apply the repellant.
Screening: ABCDEs of Melanoma
The most important warning sign of melanoma is a new spot on the skin or a spot that’s changing in size, shape, or color. Another important sign is a spot that looks different from all of the other spots on your skin. If you have any of these warning signs, have your skin checked by a doctor. The ABCDE rule is another guide to the usual signs of melanoma:
- A is for Asymmetry: one half of a mole or birthmark does not match the other
- B is for Border: the edges are irregular, ragged, notched, or blurred
- C is for Color: the color is not the same all over and may include shades of brown or black, or sometimes with patches of pink, red, white, or blue
- D is for Diameter: the spot is larger than 6 millimeters across (about ¼ inch – the size of a pencil eraser), although melanomas can be smaller than this
- E is for Evolving: the mole is changing in size, shape, or color
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