Sunscreen works by creating a protective film on the skin containing ultraviolet filters. These filters reduce the amount of ultraviolet radiation that reaches and damages the skin. Different filters interact with UV radiation in different ways, but absorption is an important protective mechanism in both organic and mineral sunscreens.
Sunscreen does not make your skin completely resistant to sunlight. Its effectiveness depends on the UV filters it contains, how evenly and adequately it is applied, whether it provides broad-spectrum protection and what happens to the sunscreen layer while you are outside.
Ultraviolet Radiation and the Skin
Sunlight contains ultraviolet radiation. The two main types relevant to sunscreen are UVA and UVB.
- UVA: penetrates more deeply into the skin and contributes to premature skin ageing and skin damage. UVA also contributes to skin cancer risk.
- UVB: is strongly associated with sunburn and also contributes to skin damage and skin cancer.
Both matter. Effective sun protection should therefore not focus only on preventing visible sunburn. A sunscreen described as broad spectrum is designed to provide protection across both UVA and UVB radiation.
The intensity of environmental UV also changes with location, time and atmospheric conditions. Our guide to the UV Index explains how those changing UV levels are communicated.
How Sunscreen Creates a Protective Film
The active ingredients in sunscreen are called UV filters. When sunscreen is spread across the skin, these filters become part of a protective film that reduces the amount of ultraviolet radiation reaching the skin underneath.
Organic UV filters primarily absorb ultraviolet radiation. The energy from that radiation excites molecules within the filter. As those molecules return to a lower-energy state, the absorbed energy is released in less harmful forms.
Mineral filters such as zinc oxide and titanium dioxide also interact with ultraviolet radiation. They absorb a substantial proportion of UV while also reflecting and scattering some radiation.
This makes the familiar explanation that chemical sunscreen absorbs UV while mineral sunscreen simply reflects it too simplistic. Absorption is an important protective mechanism in both types of sunscreen.
Organic, Mineral and Hybrid Sunscreens
The distinction between sunscreen types comes mainly from the UV filters used in the formulation.
So-called chemical sunscreens use organic UV filters. Different filters absorb different parts of the ultraviolet spectrum, so formulations can combine several filters to achieve broader protection.
Mineral sunscreens use inorganic filters, most commonly zinc oxide, titanium dioxide or both. Particle size, composition and formulation influence how these filters interact with ultraviolet radiation.
Some formulations combine different types of filters. From a practical perspective, the more important questions are whether a sunscreen provides appropriate broad-spectrum protection, whether it is applied correctly and whether the formulation is suitable for the conditions in which it will be used.
SPF and Broad-Spectrum Protection
SPF stands for Sun Protection Factor. It is a standardised measure related primarily to how well a sunscreen protects against the ultraviolet radiation responsible for sunburn, particularly UVB.
A higher SPF provides greater protection under standardised test conditions, but SPF should not be treated as a timer. SPF 30 does not mean that every person can remain in the sun exactly 30 times longer without consequences.
When correctly applied under standardised conditions, SPF 30 allows roughly 1/30 of the relevant UV radiation through, while SPF 50 allows roughly 1/50 through. No sunscreen blocks 100 percent of ultraviolet radiation.
The SPF number also does not tell the complete UVA story. Broad-spectrum protection is important because both UVA and UVB contribute to skin damage.
Real-world UV exposure also changes with location, time of day, cloud conditions, altitude and reflective surfaces. The sunscreen layer itself can become uneven or be reduced during use.
Application Is Part of the Protection
The rating on a sunscreen is based on standardised testing. Real-world protection depends heavily on the layer that actually ends up on your skin.
Sunscreen needs to be distributed adequately and reasonably evenly across exposed areas. Applying too little, missing areas or creating an uneven layer can result in substantially less protection than the labelled SPF suggests.
Areas such as the ears, neck, tops of the feet and backs of the hands can be easy to miss. The appropriate amount and application method should follow the instructions provided with the specific sunscreen.
Application guidance can differ between products and regulatory systems. Following the directions for the sunscreen you are using is more reliable than assuming every formulation should be applied in exactly the same way.
Why Sunscreen Needs to Be Reapplied
The sunscreen layer does not remain perfectly intact indefinitely. Water, sweat, towelling, rubbing, clothing, equipment and normal movement can remove or redistribute sunscreen across the skin.
Reapplication helps restore a more complete protective film. Public-health guidance commonly recommends regular reapplication while outdoors and reapplication after activities such as swimming, heavy sweating or towelling, while the directions for the specific sunscreen should always be followed.
Water-resistant sunscreen should not be interpreted as permanently waterproof. Water resistance describes performance under defined test conditions and for a limited period rather than permanent protection in water.
Swimming, sweating, rubbing and towelling can therefore make reapplication particularly important even when a product is labelled water resistant.
Sunscreen Around Water
The coastal environment makes these limitations particularly relevant. Surfing, kitesurfing, swimming, sailing and long days at the beach can involve extended periods outdoors with limited access to shade.
Water and other bright surfaces can contribute to reflected ultraviolet exposure. At the same time, swimming, sweating, towelling and contact with equipment can gradually disrupt or reduce the sunscreen layer.
Water and wind can also cool the skin, making strong UV conditions feel less intense. This combination helps explain why UV exposure can be easy to underestimate around water.
For time in and around water, water resistance, comfortable application and the practical ability to reapply sunscreen become particularly relevant.
Sunscreen and Protective Clothing
Sunscreen does not need to carry the entire burden of sun protection. Clothing and sunscreen protect the skin in different ways and can complement each other.
Sunscreen is particularly useful for skin that remains exposed. Clothing covers other areas without needing to be reapplied in the same way. Garments designed and tested for UV protection can carry a UPF rating indicating how much ultraviolet radiation passes through the material.
The difference between these protection systems is explained in SPF vs UPF, while What Does UPF 50+ Mean? explains how a garment's UPF rating should be interpreted.
For long periods around water, combining appropriate clothing coverage with sunscreen on exposed skin can reduce reliance on a single protective measure. The Water Peak's Performance-Fit Coastech™ Rashguard and Regular-Fit Visibility Water Shirt both use UPF 50+ fabric for covered areas of the upper body.
Protection Is More Than the SPF Number
Sunscreen works by placing UV filters across the skin that reduce the amount of ultraviolet radiation reaching the tissue underneath. Organic filters primarily absorb UV radiation, while mineral filters absorb UV and can also reflect and scatter part of it.
The labelled SPF is only one part of how well that protection works in practice. Broad-spectrum coverage, adequate application, water resistance, reapplication and the conditions in which the sunscreen is used all influence real-world protection.
Sunscreen also does not eliminate all ultraviolet exposure. Around the coast, it is most useful as part of a broader approach that can include appropriate clothing, shade where available and limiting unnecessary exposure during periods of intense UV.