Depending upon the length and frequency of exposure to sun light, the human body may exhibit beneficial or harmful effects. A moderate exposure to sun light particularly in winter pleases human beings. It stimulates blood circulation and increases the formation of hemoglobin.
Sun light also activates 7-dehydrocholesterol (pro Vitamin D) present in epidermis to form vitamin D which is important for absorption of calcium from intestines. Prolonged exposure or repeated exposure to sun light effect human beings adversely.
Sun light is composed of radiation of different wavelengths. Radiations of different wavelengths have different penetration power. So, radiation of wavelengths 2000-2500 Å penetrate also the network of blood vessels reaching the papillary layer.
Radiation of wavelengths 3200-3900 Å reach the derma. Radiation of wavelengths between 2800-3200 Å are both erythemogenic and pigmentogenic. Radiations of wavelengths of 3200-3900 Å are pigmentogenic but produce only a little erythema. Radiations of more than 3900 Å (3900-14000 Å) have heating action.
The first effect of sun light is the erythema (redness) of the skin followed by the formation of a tan. Intensity of erythema depends on absorption of ultra violet radiation and usually starts to develop after a latent period of 2 to 3 hours. The maximum intensity is reached within 10 to 24 hours.
The suntan depends on one’s capacity to produce pigment, melanin. This capacity is genetically predetermined.
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There are three types of tanning:
- Immediate suntan
- Delayed suntan
- True suntan
Immediate tanning response is elicit by radiations of wavelengths of 3000-6600 Å, the maximum efficacious being 3400-3600 Å. In this response, immediate darkening of un-oxidized melanin granules present in epidermal layer near the surface of the skin starts and reaches its maximum within about one hour.
In the delay tanning melanin granules present in the basal cells layer of epidermis get oxidize and migrate towards surface of the skin. This type of tanning may start one hour after the exposure and may reach maxima within about 10 hours.
The delay tanning is stimulate primarily by radiations of wavelengths of 2900-3200 Å. True tanning or melanogenesis starts about two days after exposure & reaches maxima in about 2-3 weeks. Immediate tanning begins to fade within 2-3 hours of exposure. Delayed tanning starts fading after 100-200 hours.
Symptoms of sun burn are the result of damage or destruction of cells in the prickle cell layer of epidermis. Substances released by the damage or destructed cells cause dilation of blood vessels and erythema. These substances also cause swelling of the skin and stimulate basal cells to proliferate.
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Based on the symptoms, the sun burns can be classified into four degrees.
- – Minimum erythema
- – Vivid erythema
- – Painful burn
- – Blistering burn
Sunburn does not leave scars. However, disappearance of symptoms may take varying time. A slight burn, if not exposed further to sun light may disappear within 24-36 hours. Severe burns may heal within 4-8 days and may be associate with peeling of the skin.
Chronic exposure to sun light produces more serious adverse effects and includes thickening of skin, loss of natural elasticity, appearance of wrinkles, transient dermatitis and even cancer.
Thickness of the stratum Corneum and pigmentation of skin are two natural protections against sunburn. Solar radiations increases the mitotic rate of epidermal cells which causes thickening of stratum Corneum making it more impervious to solar radiations.
Melanin granules in the basal cell layer of skin after being expose to sunlight migrates towards the stratum Corneum to be affects first in case of subsequent exposure. Painful effects of sunburn can be prevent by gradual exposure.
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- The purpose of suntan preparations is to assists the skin in tanning without painful effects and the purpose of anti-burn preparations is to minimize the harmful effects of sunburn.
- The materials which use for the above purpose are known as suntan agents and sunburn preventive agents respectively. Combinedly these are known as sunscreens.
- Sun tanning agents are those sunscreens which absorb a minimum of 85% ultra-violet radiations of the wavelengths of 2900-3200 Å, but which transmit ultra-violet radiations of wavelengths of longer than 3200 Å and produce a light transient tan.
- Sunburn preventive agents are those sunscreens which absorb more than 95% or more of ultra-violet radiations of the wavelengths of 2900-3200 Å.
- There are another type of sunburn preventive agents which scatter the sunlight. These include titanium dioxide, kaolin, talc, zinc oxide, calcium carbonate and magnesium oxide.
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An ideal sunscreen agent should have the following properties:
- It should absorb erythemogenic radiations in the range of 290-320 nm without its break down.
- They should allow full transmission of radiations in the range of 300-400 nm for tanning effect.
- Sunscreen agent should be resistant to water and perspiration and should also be non-volatile.
- And, it should have suitable solubility characteristics so that requisite amount of it is included in a cosmetic formulation.
- Preferably it should be odorless or should have only mild odor acceptable to user.
- Also, it should be non-toxic, non-irritant and non-sensitizing.
- Similarly, it should be capable of retaining its sun screening property for several hours.
- They should be stable under conditions of use. It should not stain.
However, it is almost impossible to find a sunscreen agent which will have all these properties. The one which have majority of these properties should use in cosmetic formulation.
Sunscreen agents have relatively low molecular weight, as such, many of them are easily remove from the skin on contact with eater. If it happens so, there will be necessity for further application of the product.
The higher alkyl esters of p-dimethyl amino benzoic acid are oily liquids which form continuous and adherent films which are not remove easily with water.
Some water insoluble but alkali soluble polymeric sunscreen agents have also been develop. Formulation described in a patent of National Starch & Chemical Corporation contains about 1% of alkali soluble polymeric sunscreen agents.
There are some patents which suggest use of sunscreen agents which are substantive to the skin and resistant to water and perspiration. In one of the patents, use highly substantive sulphonium salts, such as p-nitrobenzamide, propyl dodecyl methyl sulphonium bromide, has been disclosed.
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