Government recommendation that sunscreens with an SPF of 15 are sufficient to prevent sunburn – and the subsequent potential cancer risk – is based on standard test conditions, not on how much the public use on their skin in reality!
During the testing, manufacturers apply 2 mg/cm2 of the sunscreen to the skin but in real life people usually apply much less, only around a quarter to half of this amount (0.5 – 1 mg/cm2) and reduce the protection indicated by the labelled SPF. Therefore, a sunscreen with a high SPF such as 50 will only give an SPF of between 3 and 19.
To meet the government recommendation, an adult would need to use 35ml of sunscreen (SPF15) per application. If reapplied every two hours, as is also recommended, a standard 200ml bottle would be used up in two to three days. This is impractical and expensive.
Sunscreens carry the ‘Boots star rating system’, which indicates the ratio of UVA and UVB protection. As discussed in a previous blog, the SPF refers to the amount of UVB protection offered and the stars indicate UVA protection; more stars, more UVA protection. UVA makes up more than 95 per cent of UV radiation and contributes to ageing, UVB is the main cause of sunburn. A 5-star product at a lower SPF (SPF 15) could provide less protection than a 3-star product at a higher SPF (SPF 30), so apply higher factors to all unprotected areas. Spread sunscreen evenly rather than rub it in and re-apply regularly.