The chemistry of sunscreen

What is sunscreen protecting you from?

UVA and UVB are the two types of ultraviolet radiation that sunscreen needs to cover. UVA reaches deeper into skin, while UVB damages the outer layers and causes sunburn. Both contribute to skin cancer. Protection matters on cloudy days too, and sunscreen belongs alongside clothing and shade, not instead of them.23

UV damage involves more than visible redness. UVA produces reactive oxygen, highly reactive substances that can damage cells, and can damage DNA, the instructions inside cells. UVA also increases the activity of enzymes that break down collagen and elastin, proteins involved in skin's structure and elasticity. That helps explain the link between repeated exposure and wrinkles. UVB can cause pigmentation changes, inflammation and damage involved in skin cancer. A sunscreen formula therefore needs coverage across different wavelengths, rather than protection against just the rays that produce an obvious burn.4

How do chemical and mineral filters work?

Chemical sunscreens use organic filters that absorb UV radiation. Mineral sunscreens use inorganic metal oxides, usually zinc oxide or titanium dioxide. These also protect mainly through absorption, although they reflect visible light. Calling mineral sunscreen a purely reflective shield misses most of its UV protection mechanism.1

Combining filters helps cover gaps and improve photostability, meaning how well the formula holds up under light. Photostable filters such as octocrylene and bemotrizinol can help stabilise other absorbers. Smaller mineral particles also reduce visible white residue and make application easier. The finished formula matters more than finding one impressive ingredient name.1

What do SPF and UVA ratings actually measure?

SPF stands for sun protection factor and mainly measures protection against UVB. SPF testing compares the UV dose needed to produce a barely detectable sunburn on protected and unprotected skin, with sunscreen applied at 2 milligrams per square centimetre. SPF 30 provides about 97% UVB protection, compared with about 98% for SPF 50+. Applying less than the tested amount reduces the protection you receive.15

Look for broad spectrum protection as well as SPF. On UK sunscreens, choose at least four stars for UVA protection; the star scale runs up to five. A UVA symbol inside a circle indicates that the sunscreen meets the EU standard. A higher SPF does not let you stay in the sun longer without reapplying.25

Which formula should you choose?

Choose a water resistant formula, including when swimming is not planned: wet skin can affect how sunscreen stays in place. If your skin is sensitive to sunscreen chemicals, choose zinc oxide or titanium dioxide. Less expensive products with the same ingredients work as well as expensive ones, so the price tag is not an extra UV filter.3

Creams suit dry skin and facial application, gels suit oily skin and hairy areas, and sticks are useful around the eyes.2

For visible light protection, choose a tinted broad spectrum sunscreen with SPF 30 or higher. Iron oxide in tinted sunscreen helps protect against visible light, which can worsen dark spots, particularly in darker skin tones. A tint that matches your skin tone can also make white residue less noticeable.2

Sun protection can be a product's main function or a secondary function in makeup, because apparently even cosmetics need a second occupation.1

How do you get the protection on the label?

1. 1Check the expiry date and the period after opening shown on the container.
2. 2Apply sunscreen before going outdoors. Allow 15 to 30 minutes and follow the label; for prolonged exposure with a risk of burning, apply 30 minutes before leaving and again just before going out.
3. 3Cover exposed skin generously. Most adults need about one ounce for skin not covered by clothing; for the face, use at least one teaspoon. Include ears, neck, feet and exposed scalp.
4. 4Reapply approximately every two hours outdoors, and straight after swimming, sweating or towel drying. Reapply if sunscreen has rubbed off, even when the formula is water resistant.

What should you expect, and who needs extra care?

Sunscreen is protection against further exposure, not a way to undo permanent sun damage. Wear it whenever you go outdoors, including during winter and brief trips. Keep babies younger than six months out of direct sunlight, and speak to their healthcare provider before using sunscreen on them. For older children, mineral formulas are an option when irritation is a concern.35

Take extra care if you burn easily, have many moles, have a skin condition affected by sunlight or have a family history of skin cancer. If sunburn develops, stay out of the sun until redness has gone; seek medical help if you feel unwell or skin swells badly or blisters. Report a new mole, growth or lump, or changes in a mole's size, shape or colour, to your doctor promptly.5