How do chemical sunscreens work?

What happens when sunlight hits chemical sunscreen?

Chemical filters are also called organic UV filters. Their molecules undergo chemical changes as they absorb UV energy. Different filters absorb different parts of the UV spectrum, so formulas combine filters to cover more wavelengths. For example, avobenzone protects across much of the UVA range, while octinoxate absorbs UVB. Some filter combinations also improve stability under sunlight, helping a formula maintain its protection.13

Mineral sunscreens contain zinc oxide, titanium dioxide or both. They absorb UV rays too, and also reflect them. Hybrid sunscreens combine chemical and mineral filters. Chemical, mineral and hybrid formulas can all protect effectively when they offer broad spectrum protection, SPF 30 or higher and water resistance.4

What should you look for on the label?

Broad spectrum means protection against both UVA and UVB. Water resistance lasts either 40 or 80 minutes during swimming or sweating, as stated on the label. The label's useful promises are broad spectrum and SPF, not how futuristic the bottle looks.5

SPF measures sunburn protection and primarily reflects protection against UVB. It is not a count of hours you can safely spend outside. UVA contributes to premature skin aging and can pass through window glass; UVB is the main cause of sunburn. In the UK, look for at least SPF 30 and at least four star UVA protection.246

Choose a format you will use consistently. Creams suit dry skin and facial use, while gels suit oily skin and hairy areas. A sunscreen you will use repeatedly is a better choice than one that spends its career decorating a shelf.4

How do you apply chemical sunscreen?

1. 1Apply sunscreen to dry skin 15 minutes before going outdoors. Use it every day you will be outside, including cloudy days.
2. 2Cover all skin not covered by clothing. Most adults need about one ounce for exposed skin, and at least one teaspoon for the face. Include your ears, neck, tops of feet and exposed scalp.
3. 3Protect your lips with an SPF 30 or higher lip balm. Avoid getting sunscreen in your eyes or mouth, and never spray sunscreen directly onto your face.
4. 4Reapply approximately every two hours outdoors and after swimming or sweating, following the bottle's directions. Reapply after towel drying or if sunscreen has rubbed off.

For prolonged sun exposure, NHS directions add a second application: apply 30 minutes before leaving, then again just before going out. Applying sunscreen too thinly reduces protection. Use shade and clothing alongside sunscreen rather than relying on a thicker layer alone.6

No sunscreen is waterproof; all eventually wash off. Keep the container out of direct sun and excessive heat, and discard expired sunscreen. If a bottle has no expiration date, discard it three years after purchase. If you do not know its age, replace it.2

Does sunscreen work?

Yes. Sunscreen used as directed with other sun protection measures helps prevent sunburn and reduces the risks of skin cancer and early skin aging. It protects during exposure rather than repairing existing damage. Some chemical filters are absorbed through the skin into the body, but the protective action takes place at the surface.2

Who should take extra care?

For sensitive skin, dermatologists recommend mineral sunscreen. Medicines including doxycycline and retinoids can increase sensitivity to sunlight, so ask your prescriber about sun precautions if you take them. Keep babies younger than six months out of direct sun and ask a doctor before applying sunscreen. Seek medical help for sunburn that causes severe swelling, blisters or feeling unwell.2467