How Sunlight Effects the Eyes & Skin
Ultra-Violet B
(280-315nm) (0.25%)
This invisible form of light is the most energetic radiation that reaches the Earth's surface. It has the greatest potential to cause cellular damage to our eyes and skin. UVB causes general issues such as sunburn and photo-aging, but it is also capable of causing more intense cellular DNA damage, leading to freckles, sunspots, eye cataracts, and skin cancer.
UVB is considered "non-ionizing" radiation. UVB rays only have enough energy to excite electrons but not to completely remove them from atoms. This "electron excitation" is still harmful and capable of causing UV-absorbing materials to glow or "fluoresce" in the dark through the process of fluorescence. It also causes ink pigments and plastics to fade over time through a process called photo-degradation. UV photo-degradation is also harmful to DNA, capable of damaging the DNA inside cells over time through indirect biochemical reactions. This contrasts with "ionizing" UVC radiation, which is more energetic and able to remove electrons from atoms on contact.
The primary visual health concern with UVB is permanent injury to the cornea (the clear outer barrier of the eye) and permanent damage to the eye lens. The primary skin concern from excessive UVB is DNA damage, which can cause cancer. However, it will most likely result in freckles and sunspots on the skin surface well before causing cancer.
UVB is largely a seasonal concern; overall UV levels are eight times higher in June compared to January. UVB is only able to penetrate the atmosphere when the sun is overhead and shadows are short. A good rule of thumb is that if your shadow is taller than you, UVB levels are less than 10%. Additionally, UVB is a greater concern for people with less melanin in their skin compared to those with darker skin.
Only 0.25% of sunlight contains UVB but when compared, at equal brightness, scientist say that, "The most hazardous UV radiation has wavelengths between 240nm and 300nm",
(https://ehs.lbl.gov/uv/).
At the same time, UVB light is part of nature and vital to our health because it converts cholesterol in our blood into vitamin D upon direct skin contact via electron excitation. The key to leveraging the health benefits without significant cell damage is moderation. Guidelines suggest always shielding your eyes from bright UV light glare and avoiding skin exposure for more than five minutes without protection to prevent lasting damage.
Ultra-Violet A
(315-400nm) (4.75%)
UVA is the most abundant form of UV light, making up 95% of all UV energy on Earth's surface. This invisible form of light is the second most energetic and has the potential to cause skin and eye damage. UVA is harmful and can cause sunburn, photo-aging, wrinkles, early-stage cataracts, snow blindness, and skin cancer.
UVA rays are capable of exciting electrons when they bump into atoms. This "electron excitation" increases the electrons' energy state, causing UV-absorbing materials to glow or "fluoresce" in the dark through a process called fluorescence. UVA also causes ink pigments to fade over time through a process called photo-degradation. However, UVA is less able to degrade materials or cell DNA than UVB. A blacklight is a great example of UV fluorescence; it emits most of its light in the UVA range (around 365nm), which excites the atoms of UV-absorbing materials, such as teeth, making them glow in the dark.
Uniquely, UVA light can pass through clouds much more easily than visible light due to the structure of water. When the sky is cloudy, foggy, snowing, or overcast, UVA isn't blocked as much as visible light. For example, if overcast clouds dim visible light by 50%, UVA light may only dim by as little as 10% under the mid-day sun. This can be deceptively hazardous to the eyes since the low visible light level tricks the eyes into opening the pupils wide, allowing more UVA light to reach the back of the eye (the retina). A prime example of this hazard is the condition of snow blindness.
The primary visual health concern is that UVA can penetrate and damage the front (cornea), middle (lens), and back (retina) of the eyes. Short-term, intense UVA glare can cause severe eye damage, and long-term exposure is known to accelerate the formation of cataracts in the eye lens.
The primary skin health concern for UVA is its capability to indirectly damage the DNA of cells. Since UVA has a lower "electron excitation" energy level than UVB, it creates more DNA damage from oxidative stress than from direct photo-degradation. UVA light penetrates deeper than UVB light, exposing more skin cells to harm from UVA radiation.
UVA is also largely a seasonal concern; overall UV levels are eight times higher in June compared to January. UVA is a greater concern for people with less melanin in their skin compared to those with darker skin. UVA is also a greater concern for people that have less melanin in their skin, compared to people that are dark skinned.
Since UVA is invisible, the only way to know the UV level is to check the UV Index in weather forecasts. The UV Index is a great way to track daily UVA levels,
(https://noaa.gov/uv_index_level/).
However, UVA is part of nature, and when exposed in moderation, it can give you a glowing skin tan and provide light therapy in medicine at 300nm-320nm,
(https://ehs.lbl.gov/uv/).
Visible
(400-700nm) (45%)
This is the only type of light that our eyes can detect. This form of light is capable of causing eye damage from strong glare and can produce a small amount of "photo-aging" on the skin as well. This form of light is capable of causing eye damage from strong glare and it can produce a small amount of "photo-aging" on skin as well.
The visible light range contains all the colors of the rainbow: red, orange, yellow, green, blue, indigo, and violet. The more these colors are mixed together, the more they merge and appear white; without any visible light energy, colors will appear black. The more that the colors are mixed together, the more the colors will merge and become white in appearance; and without any visible light energy, colors will turn black.
Visible light is less energetic than UV light and is unable to excite electrons like UV light does. This lower energy means that visible light doesn't cause "electron excitation," "fluorescence," or cellular "oxidative stress" like UV light does. As a result, visible light is generally too weak to cause DNA damage. This means that visible light is too weak overall to cause DNA damage.
The blue/cool colors in the center of the rainbow have higher energy levels than the red/warm colors on the outside. The most damaging and energetic range of visible light includes the blue/cool colors, which fall within 400nm-500nm. "Lifelong exposure to the blue light portion or possibly the entire visible spectrum of ambient sunlight can increase the risk of age-related macular degeneration, a leading cause of blindness in older persons",
(https://ehs.lbl.gov/light/).
The primary visual health concern for visible light is avoiding bright glare that can cause eyestrain, discomfort, and an increased risk of age-related vision loss. Fortunately, our eyes are designed to reduce damage from bright visible light by squinting, closing the eyelids, and minimizing the pupil size (aperture), unlike other forms of light. The primary skin health concern for visible light is possible mild "photo-aging."
The health benefits of visible light include setting the brain's circadian rhythm by seeing natural mid-day blue light and, of course, preventing us from living in darkness. Most sunglasses not only block 99% of UV light, but they also block about 80% of visible light.
Infrared
(700nm-1,000,000nm) (50%)
