Laboratory conditions
Sunscreen is applied at a thickness of 2 mg/cm² of skin.
How Should a Sunscreen Be Applied?
The effectiveness of a topical sunscreen depends not only on its chemical and physical characteristics, but also—and above all—on how it is applied.
As explained in the answer on Sun Protection Factor (SPF), the SPF value displayed on the packaging should be regarded as an approximate indication of the protection achieved in everyday use.
Real-life conditions
Most people typically apply only 0.5–1 mg/cm².
In practice, the actual level of protection may be reduced to one-quarter or one-half of the labelled SPF value.
Two fundamental rules should always be followed
1. Apply the sunscreen evenly to all uncovered areas of the body
“Evenly” means that the sunscreen should be carefully spread and gently massaged over the skin to ensure a uniform amount of product is applied to every exposed area.
In practice, some areas usually receive an adequate amount of sunscreen, whereas others are often neglected.
Face and head: the nose and forehead are generally well protected,
whereas the ears, nape and neck frequently receive insufficient protection.
Scalp: men with thinning hair or baldness often neglect this area,
although sunlight strikes it almost perpendicularly. Whenever possible, a hat
should also be worn.
Trunk: more attention is usually paid to the shoulders and upper
chest, while the lower back and hips are often overlooked.
Limbs: sunscreen is generally applied more generously to the
extensor surfaces than to the flexor surfaces, such as the inner arms and the backs
of the legs.
Uneven application may result in localized sunburn caused by unintentional overexposure to ultraviolet radiation.
2. Reapply the sunscreen repeatedly during sun exposure
Sunscreen should be reapplied approximately every two hours, especially between 11:00 a.m. and 4:00 p.m.
This recommendation is even more important for recreational sailors, canoeists and surfers, because swimming or falling into the water, showering, perspiration and contact with beach towels remove a substantial amount of the product from the skin.
A sunscreen can significantly reduce the harmful effects of solar radiation only when it is used correctly. Otherwise, there is a risk of feeling adequately protected while, in reality, incorrect application has markedly reduced the product's protective effectiveness.