Four topical sunscreening preparations were tested in midday clear bright Florida sunshine. Five percent para-aminobenzoic acid (PABA) in 70% ethanol and a cream containing benzophenone derivatives oxybenzone and dioxybenzone (Solbar) were found to be good sunscreens. A new formulation using an ester of a modified PABA (Blockout) was less protective. All of the agents washed off when the subjects swam for ten minutes. Three of nine subjects complained of a burning sensation and developed an immediate bright erythema 30 minutes after application of Blockout and exposure to the sun.
Katz SI. Relative Effectiveness of Selected Sunscreens. Arch Dermatol. 1970;101(4):466-468. doi:10.1001/archderm.1970.04000040088017