Why reapply sunscreen every 2 hours even when the label says that the product offers 4 hours water resistance?
Sunscreens, when applied correctly, 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 is it that all sunscreens will recommend reapplying after 2 hours, even if the product protects for many more hours and offers 4 hours water resistance?
This is all to do with the way the product is tested (in controlled conditions) vs 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 uses a simulated beam of sunlight directed onto the back of volunteers. The SPF is determined for at least 10 individuals and averaged to establish the maximum claimable SPF.
The Australian water-resistant standard is the world’ most stringent to reflect Australia’s love of the waves and its harsh climate condition. In Australia, a 4-hour water resistant sunscreen will maintain its full sun protection efficacy even after 4 hours spent in a jacuzzi.
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 on the beach, playing sports or outdoor at work, sunscreen will be worn-out by our active lifestyle: towel drying, wiping off sweat, contact with clothing or other surfaces such as chairs, or the friction of waves or water when swimming, will each remove some sunscreens from our body.
Also, a testing laboratory will apply sunscreen under highly controlled conditions by a trained technician; in real-life our application of sunscreen may be far less perfect.
So how do we ensure that people do not get burnt under real-life conditions? We simply recommend that sunscreen is applied liberally, and frequently reapplied to ‘touch-up’ the applied sunscreen and ensure it is functioning effectively at all times.
It is hard to determine precisely when to reapply sunscreens, but the commonly agreed recommendation is at least every 2 hours.
Also if you have been swimming and then towel dry it is strongly recommended to reapply your sunscreen.