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Sunscreens

With the damaging effect of CFCs and other gases on the ozone layer it is really important that we protect ourselves from the increased amount of ultra violet radiation that reaches the surface of the Earth.

You have to be careful not only of direct ultra violet light but also that reflected onto you. For example, snow reflects 90% of the UV light that falls on it. That is why you can get snow blindness and severe sunburns from skiing on a sunny day. Sunbathing on a beach can also give you extra UV exposure because sand can reflect up to 20% of UVB that hits it.

Many types of glass are good absorbers of UV you can't get a suntan if you are in a glass greenhouse although you may get very hot!

To stay safe in sunlight it is sensible to put on a sunscreen either a cream or a lotion. Sunscreens are designed to reduce the amount of UV hitting your body by absorbing, reflecting or scattering the incoming radiation. Clothes will also block ultra violet radiation although thin white shirts will allow some of the UV through. Most sunscreens use chemicals that have the same UV absorbing properties.

Sunscreens are labelled with a sun protection factor (SPF) number. This number only applies to the UVB (screening) action of the sunscreen. If you are a normal pale skinned person using a sunscreen with a SPF15 you can spend fifteen times longer in the sun before becoming sunburnt than you would without the sunscreen.

Remember that the SPF does not apply to UVA sunscreens so even a sunscreen with a SPF25 but with no UVA screen will less effective than a sunscreen with a lower SPF but with a UVA screen as well. When choosing a sunscreen it is best to use one that has a high SPF factor and both a UVA and UVB block. This is called a broad-spectrum sunscreen. You should also apply the sunscreen half an hour before going out in the sun and put more on regularly especially if you are going to swim, when some of the it may wash off.

The effects of UV are more damaging at altitude so wear sunscreen in the mountains even on days where there is thin cloud. A typical white T-shirt has an SPF of about 3 but if treated with colourless dyes this can be increased to about 30.

A sunblock is considered to be any sunscreen with an SPF of 15 or more. In theory, an SPF 15 sunscreen may absorb more than 92 percent of incident UV radiation. Sunblock with an SPF of 30 blocks 97% of sun- burning rays, while an SPF of 2 blocks 50%.


What can you do to stay safe in the Sun?

Stay in the shade between 11am and 3pm
Use a sunscreen with an SPF of 15 or more
Don't rely on sunscreen alone
Wear a wide-brimmed hat and tight woven clothes that cover your arms and legs
Wear sunglasses that block UV light to protect your eyes
Reapply the sunscreen every hour or so
Remember that rubbing your face will rub off some of the sunscreen
If on the beach, wear sun-protective clothing, including swimsuits and wetsuits
Remember to take extra care with children and people with fair skin
Try never to get sunburnt as sunburn causes permanent damage


Ultraviolet light stimulates the production melanin in the skin which gives it its tanned colour but if you are in the sun too long this can turn into a sunburn. Sunscreens are made of zinc oxide (which blocks both UVA and UVB) or PABA (para-aminobenzoic acid) which absorbs only UVB.



 
WORD VERSION AVAILABLE ON THE SCHOOLPHYSICS CD
 
 
 
© Keith Gibbs 2020