Reapplication, Water and Aerosol Warnings

Reapply about every two hours, add a fresh coat after swimming, and note the benzene finding in tested aerosol sunscreens.

Higher SPF products do not stay effective longer on the skin than lower SPF ones and must be reapplied about every two hours. The interval is the same for every SPF number, because it describes how long the film on the skin lasts rather than how much radiation the film blocks.

Sunscreen should be applied thoroughly and reapplied during the day, especially after being in the water. Swimming washes the film away, and towelling off removes product as well, so a fresh coat follows both. Use the same measured dose as the first coat: 2 mg per square centimetre of exposed skin.

A study found SPF 30 sunscreen reversed photoaging of the skin within 12 weeks. Photoaging is the long-term structural change in skin caused by sun exposure, and the result shows what consistent application of a tested dose can do over a three-month period.

Independent laboratory testing by Valisure found benzene contamination in 27% of the aerosol sunscreens it tested. Benzene is a contaminant rather than an intended ingredient, and the finding applies to the aerosol formats that were sampled in that testing.

The reapplication schedule and the measured dose work together: a correct first coat that is never replaced still loses effectiveness as time and water remove the film from the skin.

• Reapply about every two hours of sun exposure.
• Reapply immediately after swimming, then dry and reapply again if needed.
• Reapply after towelling off, because drying removes product.
• Keep the dose at 2 mg per square centimetre on every coat.
• Note the 27% benzene finding when choosing an aerosol sunscreen.

Further reading

• Apply Sunscreen, Step by Step
• What the SPF Number Means