How Does Sunscreen Work?

Sunscreen works by absorbing UV radiation before it can damage the DNA in your skin cells. The UV filters in the formula soak up that energy and release it as a tiny, harmless amount of heat. This is true of both filter types — the long-held idea that mineral filters "reflect" UV like a mirror while chemical filters "absorb" it is mostly wrong.^[1]

What it's protecting you from. Sunlight carries two kinds of UV that reach skin:

• UVB — the "burning" rays; the main cause of sunburn and a key driver of skin cancer.
• UVA — penetrates deeper; drives ageing, pigmentation, and also contributes to cancer.

A broad-spectrum sunscreen is one tested to cover both. That single word on the label matters more than the filter type.

Mineral vs chemical — same job, different molecule. When researchers measured zinc oxide and titanium dioxide directly, the minerals reflected only about 4–5% of UV and absorbed the rest.^[1] So mineral and chemical filters do the same fundamental thing — absorb UV — using inorganic vs. organic molecules. More on the difference.

Why you have to reapply. If sunscreen absorbs UV so well, why reapply every two hours? Because protection erodes in the real world:

• Filters are gradually used up and degrade as they do their job.
• Sunscreen rubs, sweats, and washes off your skin.
• Most people apply too little to begin with, so there's little margin.

Reapplying tops up the protective film. How much, and how often.

In short: sunscreen is a thin layer of UV-absorbing molecules standing between sunlight and your DNA. It's genuinely effective — but only while it's there in enough quantity, which is why *how* you use it matters as much as *which* one you pick.