Why sweat is a different problem from immersion
Sweat arrives from underneath a protective film rather than over it, it is produced unevenly, and it comes with hours of friction from kit. This section is the mechanism.
Sweat is not immersion
Almost every question about sun protection in sport gets answered with guidance written for swimming. Sweat behaves differently, fails differently, and needs a different plan.
The stinging eyes problem
The single most common reason people stop using sun protection on the face during exercise, and the one problem with a genuinely mechanical solution.
Friction and transfer
Straps, collars, gloves and hands remove more protection during a long session than sweat does. Here is the map of where it happens and what survives it.
Reapplication when you cannot stop
Most sun protection advice assumes you can pause every couple of hours. Sport and work routinely make that impossible. Plan for the session you actually have.
Sweat rate and session length
Sweat output is not constant, and neither is what it does to a protective film. The way a session changes over its length is the reason duration bands appear on every page here.
The session brief
One email a month on exposure, heat and sun protection for people who train outdoors and people who work outdoors. No product recommendations, because we do not make any. Each issue carries a single clearly labelled sponsor line that has no influence on anything above it.
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