Carbon dioxide in a sauna

Carbon dioxide in a sauna is the carbon dioxide (CO2) present in the air of a sauna, principally from the respiration of bathers in a normally operating room without combustion. Its concentration may help to examine outdoor-air provision for a stated occupancy, but only when the outdoor background, generation rate, mixing, measurement position and time history are known. ASHRAE therefore treats indoor CO2 as a potentially useful tool with important limitations, not as an overall measure of Indoor air quality in a sauna.

CO2 is not an oxygen measurement and is not carbon monoxide (CO). ISO 16000-26 expressly excludes a CO measurement strategy from the scope of its carbon-dioxide sampling standard. A CO2 instrument does not thereby detect CO or establish that combustion equipment and its flue are safe. Conversely, a CO alarm is not a measure of occupancy-related ventilation. The two gases require separate instruments and interpretations.

Sources and room balance

Human metabolism produces carbon dioxide, but not at one fixed rate per person. A physiology-based method for building occupants relates generation to metabolic activity and characteristics including body size, age and sex. That method was not validated as a universal emission rate for people under sauna heat exposure. Any calculation should state its activity and population assumptions rather than silently adopt a sedentary office value.

The indoor concentration also depends on the concentration in the incoming air, room volume, elapsed time and the paths by which air enters, mixes and leaves. The difference between indoor and outdoor concentration is consequently more useful in an occupant mass balance than the indoor reading alone. Air from another internal space or entry of combustion products would change the source boundary and require separate investigation.

For a perfectly mixed single zone, a transient balance accounts for internal generation, carbon dioxide carried in with supply air and the amount carried out in exhaust air. If generation, outdoor concentration and outdoor-air flow remain constant long enough to reach steady state, the excess indoor concentration is proportional to generation divided by that flow. Each condition is part of the inference; none may be assumed merely because a meter displays a stable-looking number.

A sauna round may finish before steady state is reached. Occupants enter and leave, doors open and fans may change state. Concentration can rise while ventilation is operating whenever generation temporarily exceeds removal, while an opened door can lower or redistribute it without demonstrating a lasting solution. A peak cannot therefore be inserted uncritically into a steady-state equation to obtain a Sauna air change rate.

Concentration decay after occupants leave can support an exchange estimate when generation has ceased or is quantified, the outdoor concentration is known and the measured zone is sufficiently mixed. ISO 12569:2017 covers concentration-decay, continuous-dose and constant-concentration tracer-gas methods for building spaces considered as single zones, subject to conditions concerning concentration uniformity, the effective mixed zone and ventilation fluctuations. It is neither a sauna-specific requirement nor permission to ignore stratification and short-circuiting when assessing Sauna ventilation.

Concentration and distribution

CO2 concentration is commonly reported as parts per million by volume (ppmv). Converting it to a mass concentration such as milligrams per cubic metre requires temperature and pressure, so the two forms are not interchangeable without stated conditions. This qualification matters in a hot Sauna climate: a conversion performed for ordinary room air cannot be transferred silently to a different gas state.

Molecular mass alone does not determine where exhaled CO2 accumulates in an occupied room. Source conditions, buoyancy, supply and extract flows, recirculation and mixing govern the room-scale distribution. Experiments in mechanically exhausted Finnish electric saunas found that inlet and extract positions affected mixing and the vertical temperature and humidity distributions. A computational study of a two-metre-cube sauna model likewise found that fresh-air inlet position affected the modelled flow and temperature fields.

Neither sauna study measured a universal CO2 gradient. They instead show why Airflow in a sauna and sampling height cannot be replaced by a rule that CO2 simply “sinks”. A sensor at floor, bench, breathing-zone or extract height may encounter a different air history, and protecting a device in a cooler place changes the air path being sampled.

Water cast on stones does not chemically generate CO2. The resulting changes in heat, water vapour and air movement can nevertheless alter the distribution or an instrument's response. Löyly events, simultaneous temperature and relative-humidity measurements, door movement and fan state should therefore be logged as context rather than mistaken for carbon-dioxide sources.

Instruments and records

Many purpose-built CO2 instruments use non-dispersive infrared (NDIR) absorption, but the sensing principle alone does not establish accuracy. In tests of fifteen HVAC-grade NDIR models, Shrestha and Maxwell found wide variation in humidity, temperature and pressure sensitivity, including differences between units of the same model. Those chamber conditions represented ordinary building HVAC applications, not the hottest parts of a sauna.

Some low-cost devices report “equivalent CO2” (eCO2), an estimate derived from the response of a different gas sensor rather than a direct carbon-dioxide measurement. In a 2022 comparison, the tested equivalent-CO2 devices produced results far from the reference equipment under the study's indoor conditions. An eCO2 output should not be relabelled as an NDIR measurement.

Instrument range, calibration, compensation, response time and environmental rating all affect interpretation. A slow response may smooth a short peak or lag a ventilation change; high temperature, rapid humidity change or condensation may lie outside the specified operating envelope. Co-locating instruments before a trial can reveal offsets between units but cannot validate them beyond the tested conditions.

ISO 16000-26:2012 also warns that an inappropriate measurement strategy can misrepresent conditions. A reproducible sauna record states the objective, device, location and height, outdoor background, occupancy, ventilation and fan state, door events, heater type, water applications, sampling interval and relevant uncertainty. A synchronised time series is more informative than one photograph of a display. Multi-pollutant protocols belong to Air quality monitoring in a sauna.

Thresholds and health interpretation

The often-repeated value of 1,000 ppm is not a universal toxicological boundary. Persily traced its use to assumptions about ventilation per person and perceived human odour, and documented the erroneous attribution of a universal limit to ASHRAE Standard 62.1. ASHRAE's 2025 position states that a single concentration does not apply to every space and occupancy for assessing outdoor-air ventilation.

A 2024 review identified 43 indoor CO2 guidelines. Only 18 cited supporting evidence, which the authors considered persuasive for eight; seven of those eight addressed odour perception. The review excluded occupational guidelines at or above 5,000 ppm. Its conclusions must not be extended to occupational toxicology. Odour acceptance, ventilation performance, infection-risk management and direct toxicological exposure are different purposes and do not form one transferable traffic-light scale.

At concentrations ordinarily discussed for occupied buildings, associations between higher CO2 and symptoms or performance may reflect lower ventilation and co-varying bioeffluents or pollutants. ASHRAE characterises the evidence for direct effects at commonly observed indoor concentrations as inconsistent. Sauna heat is an additional exposure not represented by a CO2 trace, so a reading cannot diagnose headache, breathlessness or another symptom. Clinical evidence belongs to Respiratory effects of sauna.

Appropriate use and limits

Monitoring can reveal a repeatable occupancy-related pattern and test a defined ventilation hypothesis. It cannot, by itself, select a vent position, fan setting or appliance modification. Nor does a low result exclude organic compounds, particles, microorganisms or combustion hazards, because their sources and removal processes differ from those of occupant-generated CO2.

Direct peer-reviewed measurements of CO2 during sauna bathing remain scarce. Most of the evidence used here concerns general buildings, measurement methods or sauna airflow rather than bather exposure. A reported value should therefore retain its room, occupancy, background, operating state, sensor position and averaging period. The evidence supports contextual interpretation, not a universal sauna concentration, a complete air-quality score or certification of combustion safety.