Is 20 minutes in the sun without sunscreen okay?
Twenty minutes of unprotected sun exposure can cause measurable UV damage to skin cells, even if you cannot see or feel it immediately. The amount of damage depends on your UV index at that time, your skin tone, and how much skin is exposed. Tanning or reddening after short exposure means UV damage has already occurred.
UV radiation starts affecting skin cells within the first few minutes of exposure. The visible effects like redness or tan show up later, but the cellular-level changes happen quickly. Research from the World Health Organization notes that UVA rays, responsible for premature ageing and deeper skin damage, are present throughout the day at relatively consistent levels regardless of how bright it feels outside. For Indian skin tones, tanning may happen more gradually, but UVA-driven damage accumulates regardless.
Sources:
World Health Organization, UV radiation and health, who.int/uv