The short answer is 'all of them'. According to emedicine.com, both divisions of ultraviolet light that we see at ground level, (UVA and UVB) can cause damage to skin tissue.
Studies are showing that even moderate sunburn greatly increases the risk
of skin cancer. The doctors advise that we all keep our kids covered.
UV-B is 85% stronger in terms of its effects but UVA
is much more prevalent at ground level, so both types are unwanted.
The only way to protect yourself is to cover up and stay out of the sun. With the added problem of holes in the ozone layer the importance of blocking UV from damaging your skin is higher than ever.
Using Outdoor suntan products such as hats and parasols and of course sunblock creams will protect you from the serious damage posed by excessive exposure to ultraviolet rays.
Dress in protective clothing such as long-sleeve shirt and longer trousers. Lightweight linnes can be very comfortable even in the warmest weather.
Limit exposure to the sun between 11 am and 1 pm. Keep children out of the sun between 10 a.m. and 4 p.m. Children's skin is more sensitive than adults so be especially careful with the kids.
At the beginning of the season limit your exposure to the sun. Progressively increase the time in the sun so that in a few weeks, you will be able to have normal sun exposure with less risk of skin cancer.
Ensure your sunglasses are rated for both UVA and UVB protection.
Wear SPF 25 and higher sunblocks.