Sunburn is a kind of radiation damage done by the sun. It's by far the most common form of radiation damage. While most people know that radiation is dangerous, they voluntarily expose themselves to the harmful ultraviolet (UV) light of the sun on a regular basis.
Radiation can provoke cancer, and the popularity of sunbathing has brought a steady climb in new cases of skin cancer and actinic keratosis, a precursor to skin cancer. Actinic keratosis and all types of skin cancer, particularly non-melanoma types (basal and squamous cell cancer), are directly linked to sun exposure. Exposure in early life is especially relevant. Many people get the bulk of their sun exposure during childhood, and it has been shown that even one childhood sunburn increases the risk of developing skin cancer later in life. However, sunburn is not required to damage skin. A tan is also clear evidence of UV skin damage.
Sun exposure and sunburn in particular also contribute to photoaging, or the appearance of aging and wrinkled skin. Most of the wrinkles and pigmentation problems seen in the elderly are a direct result of cumulative sun exposure.
Skin cancer is the most common form of cancer in Canada. If it weren't for skin cancer, sunburn would be a minor health problem, with only the most extreme cases requiring hospital treatment. As it is, any sun or other UV exposure, including a gentle tan, increases the risk of skin cancer.
The Sun, like any star, emits vast quantities of energy across a range of wavelengths. The light that's visible to us is only a small fraction of this energy. The human eye can only perceive electromagnetic radiation with wavelengths between 400 and 700 nanometres (nm), which is called visible light. Violet light, at 400 nm wavelength, and red light, at 700 nm, are the upper and lower boundaries of our ability to see radiation.
The smaller the wavelength, the higher the frequency, resulting in greater power and damage. Long-wavelength, low-frequency emissions that are beyond the visible spectrum, such as radio waves, are felt to be harmless, although longer wavelengths generally have greater penetrating power (for instance, radio waves pass through things light can't pass through). This is the same as with sound waves: you can hear the low, rumbling and thumping parts of the sound from your neighbour's stereo, and not the high notes, but it's the high, shrill notes that can hurt your ears.
Much of the energy emitted by the Sun is shorter-wave, more powerful radiation, most of it in the form of ultraviolet (UV) light. UV light's place on the electromagnetic spectrum is immediately above the most energetic form of visible light, which is violet.
UV is classified into 3 degrees of energy: UVA, UVB, and UVC. UVC has the shortest wavelength and the most energy, but it doesn't reach the Earth's surface because it's stopped by ozone in the Earth's atmosphere. UVA has the longest wavelength, the least energy, and the most penetrating power of the 3 types. UVB light is between the UVA and UVC in wavelength and energy.
Of all the UV light that reaches the Earth, about 95% of it is UVA. Because it has the least energy, it's less likely to burn skin than UVB, but because there's so much UVA, it plays a part in most sunburns.
Just as UVA penetrates the atmosphere better than more potent UVB and UVC, it also penetrates deeper into the skin. On a microscopic scale, UVA light is more likely than UVB to penetrate the upper skin layers and be absorbed by the basal skin layer. Although it's 1,000 times less potent than UVB, UVA exposure is believed by many experts to be more relevant to wrinkling and aging of the skin, and possibly to skin cancer.
When UV radiation strikes the body, the skin cells react immediately. Specialized cells produce melanin, the body's defence against UV radiation. People with naturally dark skin have more melanin and more melanin-producing cells. When light-skinned people are damaged by UV radiation, these cells produce more of this dark substance, creating a tan. Since they have fewer pigment-producing cells, the total amount of pigment (melanin) is low and thus the protection against further UV light is poor. So further exposure leads to sunburn and sun damage even if there is some tanning.
With higher levels of exposure, there's also an inflammatory reaction. Histamine, the chemical involved in most allergic reactions, is released in the skin, along with other inflammatory substances. Blood flow is increased and the skin turns red and warm to the touch. This occurs during sun exposure, but rapidly fades. It then comes back 2 to 6 hours later, with pain, as a sunburn. Typically, it's at its worst about 12 to 24 hours after sun exposure, and the soreness lasts about 3 days. After 4 to 7 days, the outer layers of skin peel off and the redness fades.
Any sunburn is a real burn caused by real heat, delivered over time in tiny packets too small to notice. With extremely high doses, sunburn can result in second-degree burns, with severe blistering of the skin, dehydration, fever, and nausea.
Actinic keratosis (AK) is a rough, scaly skin lesion that appears on sun-exposed areas of the skin. AKs are caused by exposure to UV light. They are not cancerous, but may lead to skin cancer if not treated. AKs are usually pink and scaly.