Reapplication is the most repeated instruction in sun protection. The reasons behind it are more mechanical than chemical, and understanding them explains the intervals usually recommended.
The film is physically removed
Sunscreen works as a continuous layer across the skin, and anything that disturbs that layer reduces protection at the points where it thins.
Sweat carries product away, water rinses it off, towels wipe it, and clothing and hands transfer it elsewhere throughout ordinary activity.
Loss of coverage is uneven rather than uniform, so a partially removed layer leaves gaps rather than a proportionally weaker film across the whole area.
Application amount is usually short
The protection factor printed on a bottle is determined under laboratory conditions using a defined quantity of product per area of skin.
Most people apply considerably less than that amount, so the protection achieved in practice falls below the labeled figure from the outset.
Reapplying partly corrects for this by adding a second layer, which is why the practice matters even during a period with no water exposure.
Filters degrade at different rates
Some organic filters absorb ultraviolet energy and become less effective as they do, a process that varies substantially between individual filters.
Formulators pair filters and add stabilizing ingredients specifically to slow that degradation, so modern formulations hold up better than older ones did.
Mineral filters work primarily by a different route and are chemically stable, though they are equally vulnerable to being rubbed or washed away.
Water resistance is a timed claim
A water resistance claim in the United States is tested for a defined immersion period and stated on the label as that number of minutes.
It describes how long the film persists during water exposure, not whether the product is waterproof, a term no longer permitted.
Toweling removes product regardless of the rating, which is why reapplication after drying off is standard guidance rather than a cautious extra.
Where sunscreen sits among other measures
Shade, clothing, hats and timing reduce exposure without depending on correct application, and they do not degrade over the course of a day.
Fabric provides protection that varies with weave density, color and whether it is wet, and some garments carry a rated protection factor.
Questions about skin cancer risk, changing moles or which protection suits a particular skin type belong with a dermatologist rather than with product marketing.