Does not redden skin or cause sunburn

Penetration ability (UV-A's longer wavelength has a greater penetration ability than UV-B)

-   Penetrates earth's ozone layer, clouds, smog.  UV-A is not absorbed by ozone

-   Penetrates to dermis in skin.   i.e. deeper than UV-B, which only reaches surface level (epidermis)

-   70-90% UV-A penetrates window glass

UV-A rays are pretty constant.  During all daylight hours throughout the entire year;

Tanning rays.   UV-A oxidizes pre-existing melanin from melanocytes (skin's melanin-producing cells), which in turn creates the tan color in the skin;

UV-A can break down vitamin D in your skin.   Formed after outdoor UV-B exposure.

Many suntan lotions do not absorb or block UV-A rays

UV-A creates ROS (Reactive Oxygen Species) causing indirect DNA damage

-    Unlike UV-B (which alerts us to its DNA damage by causing sunburn) UV-A's indirect damage is pain-free and has no other warning signals.   Thus, you are oblivious to the ROS (E.g. hydroxyl radical and singlet oxygen) traveling through the body to areas where UV rays don't reach, and affecting even inner organs. E.g. 92% of malignant melanoma is caused by indirect DNA damage;

-    UV-A can cause oxidative stress leading to cancerous mutations when naturally protective mechanisms are overwhelmed:

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.   UV-B's direct damage to cell DNA/RNA. Melanin is transferred into melanosomes, organelles of the skin'sepidermal 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.

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 include:

•   Antioxidants.   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-A can cause photo-aging.  By damaging collagen and elastin fibers and destroying vitamin A in skin; Actinic (or solar) elastosisis 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.

Primary cause of sunburn and direct DNA damage. UV-B has both beneficial and harmful health effects / Partially absorbed by ozone layer

UV-B only affects skin's epidermis (surface)layer

UV-B helps skin produce vitamin D3.  This vitamin is a potent defense against melanoma (Melanoma cells convert vitamin D3 to CALCITRIOL, which causes growth inhibition and apoptotic cell death in vitro and in vivo). 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: requires sufficient dietary fat, trans fats are inhibitory, as are certain medical conditions, including parathyroid gland or kidney Diseases, and others. For more information:

Vitamin D -“The Sunshine Vitamin”

Extended UV-B exposure can cause DIRECT DNA damage → Sunburn, Melanin production, and mainly basal and squamous cell carcinomas

UV-B photon is directly absorbed by DNA (DNA has a much lower absorption for UV-A).    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 (thyminebase 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 indirect damage.

-    Increases MELANIN production (commonly known as a suntan).  Direct DNA damage by UV-B stimulates melanocytes (certain skin cells) to produce melanin, a brown protein pigment metabolized from tyrosine, that protects against overexposure to UV 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 also stimulates production of Melanocyte Stimulating Hormone (MSH), important in weight loss and energy production.

UV-B ray intensity is affected by various factors

-   Latitude.  Most ofthe 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