During exercise the body warms, circulation increases and sweat glands activate. Sweat, largely water and electrolytes, mixes with sebum on the skin surface and can combine with dirt and environmental particles.
The core principle of post-workout care is to cleanse gently and without delay. Leaving sweat residue on the surface for long can cause discomfort for some. But don't overdo it: aggressive cleansing with harsh products strains the barrier.
The practical approach: gentle cleansing with lukewarm water after exercise, then a barrier-supported moisturiser. Avoid frequently wiping the face with hands or towels while sweaty to reduce mechanical friction. For outdoor exercise, sun protection matters especially.