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The electromagnetic spectrum emitted from the sun includes:
|UVC||200-290||Not present at Earth's surface as removed by ozone layer. Germicidal band used in labs for sterilization. Very damaging to superficial skin.|
|UVB||290-320||Sunburning rays. Main cause of sun damage and skin cancer.|
|UVA||320-400||Long-wave black light tanning rays. Can penetrate into dermis. Contributes to erythema, skin aging and skin cancer.|
|Visible light||400-760||Does not significantly damage skin.
Most UVR is absorbed by the ozone layer or reflected back into space; in New Zealand, ozone (O3) is maximum in spring and minimum in early autumn. According to the National Institute of Water and Atmospheric Research (NIWA) ozone declined by 8% over New Zealand between 1980 and 1990. This has resulted in a summer-time increase in UV of approximately 9% per decade and is thought related to chlorine in the stratosphere.
Ultraviolet (UV) levels at ground level are greater when the sun is higher in the sky, as it passes through thinner atmosphere than when the sun is at a greater angle:
About 30% of the daily amount of UVB is received between 12.30–2.30 pm and 78% is received 10.30 am –4.30 pm.
There is also greater total UV:
Variation is greater for UVB than for UVA.
Only 10% of the sunburning UVR is transmitted through standard window glass.
A: The difference in UVR on a clear day compared with a cloudy day, by time of day.
B: The difference in UVR in different seasons, by time of day.
UVR incident on the skin may be:
The total quantity of UVR transmitted through skin depends on:
UVR causes biological effects when it is absorbed. Target biomolecules are:
There are several repair mechanisms to UVR damage.
The acute response of the skin to exposure to UVR has rapid onset (minutes to days) and short duration (hours to weeks).
Erythema mainly results from UVB i.e. wavelengths <320nm. It arises from dilation of the superficial blood vessels. Solar-induced erythema is delayed by a few hours after exposure.
The mechanism that results in erythema is unknown. Possibilities include:
Skin phototypes 1 to 6 are used to classify the effect of sun exposure on an individual's skin.
|Type 1||Very fair. Burns easily, doesn't tan|
|Type 2||Fair. Burns easily, tans lightly|
|Type 3||Olive. Burns somewhat, tans readily|
|Type 4||Light brown. Burns rarely, tans well|
|Type 5||Dark brown. Doesn't burn, tans deeply|
In skin types 4-6, immediate tanning may arise within minutes of exposure to long wave ultraviolet radiation (UVA) or visible light and is due to oxidation of pre-formed melanin in basal keratinocytes.
Delayed tanning is noticeable two days after exposure and most intense a week afterwards. It is due to melanogenesis and distribution of melanin to keratinocytes throughout the epidermis. UVA and even visible radiation may cause melanogenesis, but UVB is the most effective in initiating it.
Further exposure to UVR causes less damage:
UVB converts precursors in the skin into vitamin-D. The liver then the kidneys change vitamin-D into calcitriol (1,25 dihydroxycholecalciferol), which is required for calcium homeostasis.
UVR results in profound alterations of both local and systemic immune responses.
Sun damage (photoaging) includes:
Skin cancers are induced by:
A 5% increase in erythemally effective global UVR may result in a 15% increase in skin cancer in a susceptible population after about 60 years.
The best protection from sun damage is to stay indoors. If outside, then protection from directly incident light can be obtained by seeking shade: trees, verandas, and umbrellas.
As much skin as possible should be covered with densely woven darkly coloured clothing.
Apply sunscreen to all uncovered skin liberally. Reapply after half an hour to ensure an even coating soaks into the skin, and then follow the instructions on the container. These generally advise reapplication 2-hourly and after bathing.
Sun protective factors (SPF) are calculated from in-vivo testing, and are the ratio of the time for the skin to burn when exposed to solar simulating UVR with the sunscreen compared without the sunscreen. Thus white skin that normally burns in 10 minutes will burn in 150 minutes (2 1/2 hours) with SPF 15 sunscreen however often it is reapplied. In general, sunscreens are much less effective than predicted by their SPF number as they are applied significantly less thickly than is tested and are rubbed, sweated or washed off.
Desirable properties of the sunscreen include:
What are the required criteria for a sunscreen to comply with the Australian and New Zealand Standard?
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