Why reapply sunscreen every 2 hours, even when the label says 4 hours water-resistant?
When applied correctly, sunscreens are designed to reduce skin burn-time by the claimed SPF factor. In other words, if an unprotected skin burns in 5 minutes, it will burn in a minimum of 300 minutes, or 5 hours, when protected by an SPF50+ sunscreen.
So, why do all sunscreens recommend reapplying after 2 hours, even if the product protects for many more hours, and offers 4 hours of water resistance?
This is all to do with the way the product is tested (in controlled conditions) versus how we use the product in real life.
The only SPF test method currently approved is “in-vivo”, which means sunscreen products must be tested on humans. A specialised testing laboratory directs a simulated beam of sunlight onto the back of volunteers. The SPF is determined for at least 10 individuals then averaged to establish the maximum claimable SPF.
The Australian water-resistant standard is the world’s most stringent; to reflect Australia’s renowned outdoor lifestyle in harsh climate conditions. In Australia, 4-hour water resistant sunscreen will maintain full sun protection efficacy, even after 4 hours spent in a pool or at the beach.
However, the highly controlled conditions used to create a reliable test are very different from how we use sunscreens in everyday conditions.
In real life, such as at the beach, playing sports, or outdoors at work, sunscreen will be worn down by our active lifestyle. Actions such as towel drying, wiping off sweat, contact with surfaces (e.g. clothing or chairs), or the friction of waves or water when swimming, will each remove some sunscreen from our body.
Also, a testing laboratory will apply sunscreen under highly controlled conditions by a trained technician. Whereas in real life our application of sunscreen may be far less perfect.
So how do we ensure that people do not get sunburnt in real life conditions? We simply recommend that sunscreen is liberally and frequently applied, then reapplied to ‘touch-up’ the applied sunscreen; ensuring it is functioning effectively at all times.
It can be hard to determine precisely when to reapply sunscreen, but the commonly agreed recommendation is at least every 2 hours.
Remember, if you have been swimming and then towel dry, it is strongly recommended to reapply your sunscreen.