The shorter the wavelength of light/radiation the more energy it has. Another way of thinking about it the closer to the blue end of the spectrum the radiation is the more damage it will do to your skin. (Assuming a similar exposure time).
UVA radiation is responsible for sun damaging or to be more technical, photo-aging of the skin. Although it's got lower energy than UVB longterm exposure causes deeper damage to the parts of skin that keep it elastic and smooth.
UVB radiation is responsible for DNA damage as it has higher energy than UVA, and causes the majority of sunburn (or erythema), and the DNA damage it causes contributes to skin cancer.
InfraRed radiation is also responsible for deeper skin damage, but because it's lower energy it doesn't appear to damage DNA.
What about UVC?
Most of that is absorbed by the upper atmosphere by ozone.
Kids should be protected from the sun, and especially sunburn all the time. Getting sun burnt under 18 years old is a bigger risk factor for getting Melanoma than getting sun burnt during adult life, and young kids skin is more sensitive and vulnerable to sun damage than adults. Babies and toddlers can burn with just a few minutes sun exposure in midsummer midday (10am to 2pm) sun.
The best advice is to keep babies out of direct sun altogether where possible, and if they have to be out in the sun, using hats and clothing to minimise sun exposure is best, even when in the shade, and broad spectrum sunscreen on exposed skin. Most kids will tolerate "Adult" sunscreens without any problems, although zinc or titanium oxide based sunscreens are less likely to cause problems with irritation so may be more useful in kids with sensitive skins.
Kids can generally make enough Vitamin-D with a few minutes of skin sun exposure in the early morning or late afternoon.
SPF stands for Sun Protection Factor, and basically means how long you stay in the sun with a particular sun screen on per day. The important thing here is if you re-apply sunscreen after a period in the sun the "SPF Clock" doesn't start again.
Example: If you burn like me after about 10 minutes in the midday summer sun and you use and SPF 30 the most you can get is 30 x 10 minutes in the sun or 300 minutes or 5 hours.
So if I put sunscreen on at 10 am on and went out in the sun the longest I could stay out in the sun is 5 hour or until 3pm that day.
If I put more sunscreen on at midday, I can't then stay in the sun until 5pm and get the same level of protection, the most it will protect me to is still 3pm.
I will only get a maximum of 5 hours protection in a day.
There are a couple of problems with SPF.
The big one is that it's a lab number. SPF is calculated from lab measurements on a group of 10 healthy volunteers, sitting around in a lab under a sun lamp.
In the real world it is better to <strong>under estimateÂ </strong>how long you can safely stay in the sun without burning. Â Things like using a towel to dry yourself after a swim, rubbing against things, getting sand on the skin and washing that off all lead to loss of sunscreen from the skin.
We also tend to under use sun screen. Ideally we should use at least 35ml for anyone 12 years and up, to cover the exposed body. This breaks down to at least half a teaspoon on each arm, at least half a teaspoon for the face, ears, and neck, at least a teaspoon (5ml) on each leg, and the trunk front and back.
Although no mention is made in literature about bald heads, I use about a teaspoon on my head as the skin is particularly thin and sensitive.
Remember to let sunscreen soak in for at least 15 minutes before going out in the sun.
SPF numbers assume that the sunscreen is used at this application rate, and reapplication after 2 hours in the sun is recommended.
The standard for SPF is AS/NZS 2604 so make sure your sunscreen is rated against this standard for peace of mind.
Under the standard for sunscreen there are very precise meanings for label claims, advice from dermatologists is to avoid sunscreens that make some claims, and choose sunscreens that show others
To claim water resistance the sunscreen must still have an equivalent SPF after between 40 and 80 minutes in water, and it's a good idea to reapply sunscreen after being in the water anyway.
Claims to look for
Types of Sunscreen:
There are two ways in which sunscreens work. They either reflect the suns rays using a physical barrier (e.g. fine particles of Zinc or Titanium Oxide) or absorb the energy of the UV rays rendering them harmless.
For a really good commentary on sunburn check out this link