Sunscreen is an important part of any sunprotection regime used in combination with sunprotection clothing (including sun shirts (rashies), hats and sunglasses)
The SPF refers to the Sun Protection Factor of a cream. It is a ratio of the time taken to burn with sunscreen applied compared to the time taken to burn without sunscreen. UV exposure is directly related to skin cancer risk and higher SPF creams filter (stop) more UV from reaching your skin than to lower SPF options.
Dermatologists can advise regarding lower irritant options and can arrange allergy patch testing for the components of sunscreen that may be triggering the reaction.