UV-A vs UV-B radiation on the skin
UV-A (320-400 nm)
95% of UV radiation that reaches us on earth is UV-A
Unlike UV-B, UV-A is not absorbed by ozone. UV-A penetrates earth’s ozone layer, clouds, smog, and 70-90% of that UV-A can penetrate through home and auto glass.
UV-A rays are pretty constant. During all daylight hours throughout the entire year.
UV-A radiation causes INDIRECT damage
UV-B penetrates to the epidermis (skins surface layer), but UV-A penetrates further and can damage skin components. UV-A’s longer wavelength enables it to enter the skin’s dermis, and although UV-A radiation is less energetic than UV-B, excessive amounts can still promote damage to protein skin components by INDIRECT means. UV-A can create reactive oxygen species (ROS), e.g. hydroxyl radical and singlet oxygen, which can cause oxidative damage to (steals electrons from) DNA, lipids and proteins. Out of the sun, humans are not usually exposed to UV wavelengths > 294 nm, and human cells have inherent systems to protect against oxidant damage from exposure to higher UV wavelengths of sunlight, including antioxidant systems glutathione-glutathione peroxidase and thioredoxin and DNA repair mechanisms, but these systems can be overwhelmed when called upon to excess.
Unlike UV-B, which alerts us to its DNA damage by causing sunburn, UV-A’s damage is pain-free and has no other warning signals, making us oblivious to the ROS traveling through our bodies to areas where UV rays don’t reach, affecting even inner organs.
- UV-A rays are tanning rays. UV-A oxidizes pre-existing melanin from melanocytes (skin’s melanin-producing cells), which in turn immediately creates the tan color in the skin; tanning beds use UV-A (some also use UV-B). Excessive amounts of UV-A can cause sunburn. Only broad-spectrum suntan lotions absorb or block UV-A rays.
- Can cause some skin cancers. UV-A can cause oxidative stress leading to cancerous mutations when naturally protective mechanisms are overwhelmed. E.g. 92% of malignant melanoma is caused by indirect DNA damage;
- UV-A can break down vitamin D in your skin. previously formed from exposure to UV-B rays.
- UV-A is known for causing premature aging. UV-A can cause photo-aging by damaging collagen and elastin fibers and destroying vitamin A in skin. Actinic (or solar) elastosis is an accumulation of abnormal elastin (elastic tissue) in the skin’s dermis layer (and in the conjunctiva of the eye), as a result of prolonged / excessive sun exposure.
Excited Chromophore* (UV-A + Chromophore ) + 3O2 → Chromophore + 1O2 (singlet oxygen)
1O2 + Intact DNA → 3O2 + Damaged DNA
Naturally protective mechanisms against oxidative stress caused by UV-A
The skin pigment melanin is a natural “sunscreen”. Melanin, the primary color determinant of skin, hair and eyes, is produced by melanocytes (cells found in the basal layer of the epidermis, eyes, hair, also inner ear, and brain) by UV-B radiation. Melanin absorbs UV-A and UV-B.
Melanin is transferred into melanosomes, organelles of the skin’s epidermal cells, where by efficiently absorbing the sun’s UV rays they protect the nuclear DNA/RNA from possible mutations caused by UV.
Melanin is composed of a chromophore (and attached protein) and when UV-A rays excite a chromophore that is unable to quickly convert UV-photon energy into harmless heat, ROS are produced.
Antioxidants:
- Adequate vitamin D levels (dietary or formed in the skin by UV-B rays).
- The adequate presence in the skin of other antioxidants. E.g. vitamin A /retinol, vitamin C, resveratrol and other polyphenols, vitamin B3, vitamin E, CoQ10, and astaxanthin can neutralize ROS.
Essential fats (omega-3 and omega-6)
UV-B (280-320 nm) - Vitamin D-making Rays
5% of the UV rays reaching earth (most UV-B absorbed by ozone layer, water vapor, oxygen and carbon dioxide);
UV-B penetrates only as far as the outer layer of the skin, called the epidermis.
UV-B photons can be DIRECTLY absorbed by:
- DNA: UV-B penetrates only as far as the outer layer of the skin, called the epidermis and UV-A penetrates to the dermis.
- Chromosomes: present in skin cells.
- Melanin (Chromophores)
- Essential fatty acids (present in skin cell membranes). Omega-3 fatty acids mitigate skin damage caused by UV-B
Beneficial effects of UV-B
UV-B helps skin produce vitamin D3 – a potent defense against melanoma in epidermis. Melanoma cells convert vitamin D3 to its active form CALCITRIOL, which triggers growth inhibition and apoptotic cell death. (Conversely, research shows that increased UV-A exposures together with inadequately maintained cutaneous vitamin D3 levels will promote melanoma)
- The body’s production of ACTIVE VITAMIN D (CALCITRIOL) from D3 is affected by certain factors: sufficient dietary fat is required; trans fats are inhibitory, as are certain medical conditions, e.g. parathyroid gland or kidney diseases.
Vitamin D – “The Sunshine Vitamin”
UV-B stimulates production of Melanocyte Stimulating Hormone (MSH). Important in weight loss and energy production.
UV-B rays can cause damage
Delayed tanning is the primary cause of sunburn and DIRECT DNA damage causing skin cancer / premature aging.
- Extended UV-B exposure can cause DIRECT DNA damage leading to sunburn, premature aging and mainly basal and squamous cell carcinomas.
UV-B causes DIRECT photodamage when absorbed by DNA nucleobases (the 5 fundamental units of the genetic code – adenine (A), cytosine (C), guanine (G), thymine (T), urasil (U). A, C, G and T are in DNA). DNA is able to efficiently transform >99.9% of the photons into harmless heat (via a photochemical process called internal conversion), but the remaining <0.1% create an excited state that can cause a disruption in the DNA strand (thymine base pairs next to each other in genetic sequences bond together into thymine dimers), which reproductive enzymes cannot copy. Only UV-B can cause DIRECT DNA damage.
- Leads to SUNBURN. Damage from the untransformed <0.1% UV-B is able to cause sunburn, a painful warning signal that DNA damage is occurring to cells being DIRECTLY illuminated by UV-B rays, which can lead to basal cell carcinoma and squamous cell carcinoma on the skin if damage is not immediately repaired. No such warning is generated from UV-A’s INDIRECT damage.
- Increases MELANIN production (commonly known as a suntan). UV-B stimulates melanocytes (certain skin cells) to produce melanin, a brown protein pigment metabolized from tyrosine, that protects against overexposure to UV-B radiation by absorbing UV energy and dissipating it as harmless heat and also colors the skin. The tan resulting from increasing melanin production takes about two days to develop but is longer-lasting and less harmful than the one obtained from UV-A.
- Responsible for only 8% of melanoma cancers
UV-B ray intensity is affected by various factors
- Latitude. Most of the US is between 30° and 45°latitude, which for several months a year, has insufficient UV-B sunlight to produce optimal D level:
30° (N and S). Insufficient UV-B 2-6 months of the year, even at midday; only Florida and S. Texas are below 30°N in mainland US
40°. Insufficient UV-B during 6-8 months of the year; includes Oregon, Idaho, Wyoming, Nebraska, Iowa, S. Dakota, N. Illinois, Wisconsin, N. Indiana, Michigan, Ohio, Pennsylvania, New York and the New England States;
45° (far N or S). Even summer sun is too weak. Includes most of Washington, Montana, N. Dakota, and northernmost parts of Minnesota, Wisconsin and Maine;
- Altitude. UV-B is stronger at higher altitudes;
- Glass. Only ~5% of UV-B goes through glass;
- Ozone layer, clouds, smog or fog. Full cloud cover decreases UV-B ~50%; stratospheric ozone depletion increases;
- Time of day/Sun’s position in sky – More UV-B rays reach earth at midday than in the morning or evening. When the sun is high in the sky around noon, its rays have a shorter distance to travel through the Earth’s ozone layer to reach the surface of the Earth, which reduces absorption by the ozone layer and so increases amount of penetration. Conversely, when the sun’s rays are at an oblique angle early and late in the day, they have a further distance to travel through the ozone layer, and so have a more reduced intensity than when rays hit directly. When the sun goes down toward the horizon, UV-B is filtered out much more than UV-A
