Steam

Steam is water in the gaseous phase. In everyday language, however, the same word commonly covers the white cloud seen above hot water. That cloud is not the gas alone: it contains fine liquid droplets that scatter light, while invisible water vapour may surround and pass between them. Technical writing about a sauna therefore needs to distinguish gaseous water, suspended droplets, liquid water and a room or event informally labelled “steam”.

Two sauna-related contexts are especially important. Water applied to hot stones may produce a short-lived vapour-rich löyly event. A steam bath, by contrast, uses purpose-built equipment to sustain a humid bathing environment. Both involve phase change and moist air, but they are not alternative names for one operating system. Neither the visibility of a plume nor the word steam by itself specifies the room's temperature, humidity or heat transfer.

Water states and terminology

Evaporation transfers water molecules from a liquid surface into the gas and can occur below the liquid's boiling temperature. During boiling, vapour bubbles form and grow within the liquid when the local pressure and thermodynamic state permit. The saturation temperature changes with pressure, so steam is not inherently at 100 °C. Condensation is the reverse phase change from gas to liquid.

A visible plume can arise when invisible vapour enters cooler air and some of it condenses into an aerosol of small droplets. Water application can also entrain liquid droplets by splashing or incomplete vaporisation; it is therefore too strong to claim that every visible droplet first passed locally through a gas-only state. Conversely, clear air can contain appreciable water vapour. Visibility depends on droplet formation, size, concentration and illumination, and is not a calibrated measure of the vapour present.

Sauna air is normally a mixture of dry-air gases and water vapour, with droplets sometimes suspended in it. It is not a room of pure steam. Even when moist air reaches 100 per cent relative humidity, water vapour contributes a partial pressure within the gas mixture rather than necessarily constituting the whole gas. Pure-water steam tables can describe the water substance at a defined state, but cannot on their own describe a room-air mixture.

Phase change and energy

Liquid water must receive energy to warm and vaporise. In a stone-heated room that energy is drawn chiefly from the stones and heater; in a generator-fed room it is supplied by the generating equipment. Pouring water does not create heat. Vaporisation absorbs latent energy, while subsequent condensation transfers latent energy to the phase boundary and its surroundings. The relevant enthalpy differences vary with state, so a single memorised latent-heat value is not exact for every calculation.

These transfers explain why sensation, surface temperature and room-air temperature need not change in parallel. In one experiment, Nore, Kraniotis and Brückner added one, two or three litres of water in a modified spruce-lined test room. Relative humidity and measured spruce-surface temperatures responded much more markedly than room-air temperature. The door and a ceiling duct were sealed and most of the heater was covered with aluminium foil for thermography. Those alterations and unusually large water additions make the study useful for illustrating transient processes, not for prescribing an ordinary pour or predicting another room.

Water introduced at the heater does not all have to become uniformly mixed vapour. Some can remain on or pass through the stone bed, leave as splash or droplets, condense on cooler boundaries, or be absorbed temporarily by hygroscopic materials. The resulting air and material responses are parts of one mass-and-energy balance, developed more fully in Löyly physics.

Vapour in moist air

Several humidity quantities describe different aspects of the same air state. Relative humidity compares the water-vapour pressure with saturation pressure at the current temperature. Absolute humidity is vapour mass per volume, whereas humidity ratio expresses vapour mass relative to dry-air mass. Dew point is the temperature at which a specified vapour state would reach saturation on cooling under defined conditions. The quantities are related, but their numbers and units are not interchangeable.

Temperature must accompany an RH result because saturation pressure depends strongly on temperature. Equal RH values at different air temperatures can represent different vapour contents. A sharp RH rise after water application also does not state how many grams vaporised, how much remained airborne, or what fraction reached a bather. A visible cloud supplies still less quantitative information.

Local conditions matter. A cooler surface may receive condensation when adjacent air is at a vapour state whose dew point exceeds the surface temperature, but transport through the boundary layer and time affect the amount formed. This condition is treated in Condensation in a sauna; it should not be inferred from a room-average RH percentage alone.

Intermittent water-on-stones events

When water reaches sufficiently hot sauna stones, part of it may vaporise rapidly and form a warm, moisture-rich plume. The Finnish word löyly denotes more than the material vapour: it also refers to the heat-and-moisture event and its experienced quality. Translating it simply as steam therefore loses cultural and physical distinctions.

The plume entrains room air and follows the circulation rather than filling the enclosure instantaneously. VTT experiments in mechanically extracted test rooms examined humidity distribution during water casting and found that air mixing and vent placement affected how the response extended through the room. The particular mechanically ventilated arrangements do not establish one universal vertical profile or vent prescription. Ventilation may mix and remove moisture, while room surfaces can temporarily store it.

A 2026 human experiment recorded temperature and RH with six paired measurement points: three horizontal locations at each of two heights. Fifty healthy, recreationally active adults completed four ten-minute sauna rounds and chose when to add water after the first two minutes. Conditions varied with position and time. Because humidity was not independently assigned and water quantity was not the analysed exposure, the study does not define a steam dose or ideal distribution.

Continuously supplied rooms

A Steam generator supplies vapour to a room more continuously than an intermittent application to stones. Generator capacity, outlet arrangement, water treatment, pipework and controls belong to that equipment topic. The supplied room still contains humid air rather than pure gaseous water.

EN 18164:2026 distinguishes sauna, warm-air, steam and soft-steam rooms within its European public-facility scope. The expression Steam sauna is nevertheless used inconsistently outside such classifications: it may denote a steam bath, a humidified warm-air cabin or a hybrid operating mode. An adequate description states the equipment and operating pattern instead of relying on the label.

Continuous humidification also changes the demands placed on finishes, joints, drainage and waterproofing. Those building provisions are installation- and jurisdiction-specific; they cannot be obtained by copying details from a comparatively dry enclosure or by choosing a single humidity number.

Heat exchange and physiological evidence

Water vapour can alter human heat exchange in two distinct ways. Higher ambient vapour pressure reduces the vapour-pressure gradient available for sweat evaporation. If skin is cooler than the local dew point, vapour may also condense at the skin and transfer latent energy there. Convection and thermal radiation continue at the same time, so vapour alone does not determine total heat flow or perceived heat. A short, non-peer-reviewed mechanistic conference analysis describes this coupled sensible and latent transfer, but its simplified assumptions do not provide a universal human heat balance.

A stable-isotope experiment supplied direct evidence that some water dripping from participants was condensate derived from labelled thrown water rather than sweat. Mean estimates ranged from about 30 to 54 per cent, depending on which of two experiments and which mass-balance calculation was used. The percentages describe collected dripping liquid in those protocols, not all skin moisture or every sauna.

In the 2026 study, statistical models found temperature and RH independently associated with immediate heart-rate and core-temperature changes. These were observational associations within the sessions: participants controlled water use, four heaters differed, and no vapour dose was randomised. Harvia provided the sauna infrastructure and publication costs, and Toyota partly funded the measurement phase. The study supports careful description of humidity, not a causal claim that additional steam improves health or a threshold at which it becomes safe.

Measurement and limits

A technical report should state what steam denotes, then give temperature, the chosen humidity quantity, sensor position, time resolution and the water-application or generator protocol. During a brief pulse, instruments with different response times can record different peaks. The public scope of ISO 7726:2025 covers instrument characteristics and methods for measuring environmental physical quantities. It does not approve a particular sauna sensor or prescribe a sauna-specific measurement position.

No retained evidence establishes a universal amount of steam that is ideal, authentic or safe. Operational assessment also involves the heater or outlet, water application, exposure time, room geometry, ventilation, wet surfaces and applicable rules. A Canadian rapid review, for example, treats heat, burns, wet-floor hazards, ventilation and moisture management as interacting concerns rather than consequences of one humidity value. It is a Canada-centred, single-reviewer synthesis and does not set a general steam dose. Detailed maintenance and mould control likewise belong to their own operational and building contexts. Within Sauna technology, steam is best treated as a phase or explicitly defined shorthand, not as a complete specification.