Saunas and steam rooms are often treated as interchangeable heat experiences. They operate at opposite ends of the humidity range, and that single variable changes everything else.
The temperature figures are misleading on their own
A traditional dry sauna runs far hotter than a steam room, at temperatures that would be intolerable if the air carried much moisture.
A steam room operates much cooler but at complete saturation, meaning the air holds as much water vapour as it physically can.
Despite the lower reading, a steam room often feels more intense, because the body's main cooling route is disabled at that humidity.
Sweat only cools when it evaporates
The body sheds heat by producing sweat that evaporates from the skin, and evaporation carries energy away with it.
In dry sauna air, sweat evaporates almost immediately. That constant evaporation is why very high temperatures remain tolerable for a period.
In saturated air there is nowhere for the moisture to go. Sweat runs off instead of evaporating, so the cooling mechanism largely fails and heat accumulates.
What each does to the skin surface
Dry heat draws moisture out of the outer skin layer, which is why skin can feel taut after a sauna and why moisturising afterwards is routine.
Steam does the opposite, saturating the outer layer with water so that it swells and softens. This is the reason facial steaming precedes extractions.
Neither opens pores in any literal sense, since pores are not muscular structures. Softened surface material simply becomes easier to dislodge.
Why session lengths differ
Sauna traditions typically involve short sessions followed by cooling, often with cold water, then repeating the cycle several times.
Steam sessions tend to be longer at lower temperature, though the impaired cooling means the body's core temperature can still rise substantially.
Because the sensation of heat is muted relative to the physiological load, steam rooms are the easier of the two in which to stay too long.
Practical differences in the room itself
Saunas are built from wood, which tolerates the dry cycling and does not become uncomfortably hot to touch the way stone or tile would.
Steam rooms are tiled and sealed, because saturated air condenses on every surface and any porous material would degrade quickly.
Heat sessions of either kind affect hydration and blood pressure, so alcohol, dehydration and existing cardiovascular conditions all change how safe a session is for a given person.