The Australian Standard for sun lotion allows a maximum sun protection factor (SPF) rating of 50. An SPF 50 sun lotion will filter 98% of UV radiation while an SPF30 product filters 96.7% so the difference is not large. The difference is around the length of time that it will protect you but there are issues with this.
Everyone’s skin is different and each person’s skin can be exposed to the sun for different lengths of time before burning. Sunscreen extends the time before burning by a multiple of the SPF. Eg if you have fair skin and would normally burn in 5 minutes without any sun protection then an SPF 30 will give you 30 x 5 minutes protection. The issue is that all sunscreens will rub off over time with normal activity and perspiration and so in some cases the higher SPF can give a false sense of security. Due to normal activity all sunscreens need to be reapplied regularly.
The most important thing to remember is that no sunscreen provides total protection and will rub off over time, that’s why it is always important to reapply regularly and cover up using appropriate clothing and eye wear especially during the hottest part of the day.