People are getting less than half the sun protection they expect from suntan lotions, according to research.
But the problem isn't the lotion, it's our slapdash application of it, King's College London scientists say.
In theory, using sun protection factor (SPF) 15 sunscreen should be enough to stop sun damage.
But in reality, people need SPF 30 or 50 to be safe, say the study authors, who carried out experiments with volunteers in their lab.
It comes as temperatures are set to reach 33C in London on Thursday afternoon amid reports of the heat placing a strain on some health services.