Investigating commercial sunscreens

Excessive exposure to the Sun's uv radiation causes sunburn, ageing and skin cancers.² The different types of uv radiation - UVA, UVB and UVC - characterised by the different wavelengths, however, have distinctive biological effects (see Table 1). Most commercial, topical sunscreens bought in Europe protect our skin from UVB and UVA radiation. The 'SPF' or sun protection factor, a number on the front of the bottles containing these products, provides information about protection, mainly from UVB radiation. Some sunscreens also carry information on the product's UVA protecting properties. For example, the Boots' zero to five 'star rating' in the UK provides information about the ability of the product to block UVA in relation to UVB radiation - a five-star rating indicates that nearly as much UVA is blocked as is UVB for that product.  

Organic sunscreens function in variety of ways, but all are designed to be photostable uv absorbers that dissipate the absorbed energy rapidly and harmlessly as heat.³

The commercial sunscreens used in this practical - Parsol 340 (1), Parsol 5000 (2) and Parsol MCX (3), from DSM Nutritional Products - achieve this through a process of photo-induced geometrical (E -Z or cis -trans) isomerisation (Scheme 1). All three display a strong absorption band that peaks in the UVB region (290-320 nm).