Let's soak up the sun ... or not? (Part 2 - The good and bad of UV rays exposure)

In my quest to understanding the beneficial and detrimental effects of sunlight, I came to know that categorically, there are three main types of the sun UV rays as a function of the wavelengths. Let's examine them one by one.

Composition of the sun UV rays

UVC with wavelength between 100 to 280nm and being the most harmful to the human skin, is fortunately completely absorbed by the earth's stratospheric zone layer and does not reach the earth's surface.^[1]

UVB, with wavelength in the region of 280-315nm, is responsible for between 1-10% of the whole UV rays (UVR) that reach the earth's surface.^[1] There is a concern that, with the depletion of the earth's stratospheric layer, we are in the risk of being exposed to a larger percentage of UVB that passes through the ozone layer.^[2] UVB penetrates as far as the outer layer of the skin's dermis.

The third type of UV rays emitted by the sun, UVA, has wavelength between 315nm and 400nm.^[1] 90%-99% of UVA reaches the earth. UVA penetrates into the deep end of the dermis layer. UVA can be further categorized into UVA1 (340-400nm) and UVA2 (315-340nm).

The detrimental effects of the UVR

The skin, when exposed directly to solar UVR, will attempt to defend itself against the damaging action caused. The skin does so by triggering sunburn, thicken the epidermis' stratum corneum, and activating the pigment cells which produce pigmentation when exposed to UVB. Further damage to the skin's cellular DNA will cause skin cancer, which occurs mostly at the skin's epidermis layer.

Image taken from the research paper
Vitamins and Photoaging: Do scientific data support their use?

Basal cell carcinoma (BCC), is the most common malignant skin cancer and the biggest risk factor has been associated to the exposure of the sun UVB rays, especially for those with fair skin.^[4] Like BCC, Squamous cell carcinoma (SCC) occurs in the epidermis layer, is not as common as BCC but certainly more dangerous.^[5] Melanoma is the most dangerous type of skin cancer that can be caused by prolonged exposure to the sun. There are many researches that associate UVR with the development of melanoma risk.^[6,7,8,9]

When exposed to UVA, the skin produces less high quality collagen and elastin fibres. This, as we know, causes premature aging. But what exactly is at work here that results in photo-aging? When UVA enters the skin layer, it is absorbed by the skin's molecules that generates harmful compounds called reactive oxygen species (ROS) and sets activator protein (AP)-1 in motion for a longer period of time.^[3] This in turn increases the production of matrix metalloproteinases (MMP) ezyme that in turn increases the breakdown of collagen. UVR also decreases the transforming growth factor (TGF) that complements the AP-1 in regulating the collagen by promoting the production of collagen. In short, an increase in collagen breakdown due to increasing AP-1 and a decrease of collagen production due to decreasing TGF will lead to skin sagging and wrinkles. The synthesis is best described in the picture on the right.

The benefits of the solar UV

The human body has the ability to synthesize vitamin D3 from absorbing the solar UVB rays. The lack of Vitamin D3 have been found to be associated with many issues such as^[10,11,12]:

* Type 1 diabetes mellitus
* Autoimmune disorders including multiple sclerosis and rheumatoid arthritis
* Rickets amongst children.
* Osteoporosis amongst adults
* Cardiovascular disease
* A certain level of exposure to UVB may even decrease the risk of non-melanoma skin cancer (NMSC).^[11]

Now that we have an inkling of the good and bad of sun rays, what should we do to ensure we stay away from the bad stuffs yet benefit from the good parts. Stay with me for the next post.

Related posts:
Part 1 - Introduction
Part 3 - Too much or too little UV rays?

References:

1 The Real Value of Novel Particulate Carriers for Sunscreen Formulation
2 The effects on human health from stratospheric ozone depletion and its interactions with climate change.
3 Overview of skin aging and photoaging
4 The role of UV radiation in the development of basal cell carcinoma
5 Fact sheet: Preventing and detecting skin cancer
6 Sun damage in ultraviolet photographs correlates with phenotypic melanoma risk factors in 12-year-old children
7 Multiple melanoma susceptibility factors function in an ultraviolet radiation response pathway in skin
8 Landmark research links melanoma to UV radiation
9 How UV Radiation Triggers Melanoma
10 Sunlight and vitamin D for bone health and prevention of autoimmune diseases, cancers, and cardiovascular disease
11 UV Radiation, Vitamin D and Epidermal Carcinogenesis
12 Solar UV Doses of Young Americans and Vitamin D 3 Production