1.  In the upper atmosphere, the name of the gas in the layer that reduces the amount of ultraviolet

      radiation reaching the Earth's surface is  ozone. 

      Exposure to ultraviolet radiation can cause suntan, sunburn and skin cancer.

      Environmental pollution from CFCs is depleting the ozone layer.

      The higher the risk factor of a sun block the longer the skin can be exposed without burning.

  2.  Hannah can stay in the Sun for 30 minutes without burning. 

       The longest time she can safely stay in the Sun if she uses a sunscreen with a sun

       protection factor number 15 is: 15 x 1/2 = 7.5 hours.

  3.  How Science Works

       Scientists say that high factor sun block can reduce damage from ultraviolet, this is based on data.