A Spa Visit Runs About 11 Pounds of CO2. Therme and Surrey Published the Math.

For years a bathhouse could bury its footprint inside an annual, facility-wide sustainability report, if it reported at all. That cover just got thinner. Therme Group and the University of Surrey have published a peer-reviewed method that puts a hard number on a single visit: roughly 11 pounds of CO2 equivalent at a large destination resort, roughly 7.7 at a small sports facility. It is a benchmark no operator asked for, and one they will be asked to answer to.

The tool is SPA-DEC, the Spa and Wellness Dynamic Emissions Calculator. It reads a building’s metered energy and water, adds normalized figures for waste and staff travel, covers all three scopes of the Greenhouse Gas Protocol, and divides the total across the people who came through the door. The number updates as the building runs.

What is actually metered, and what is not

The mechanics matter more than the announcement, because they decide whether an operator can do anything with the output. Energy and water come off live meters. Waste and staff travel are normalized rather than measured, which is the usual compromise and the usual soft spot. Everything is then allocated per visit, which makes throughput part of the denominator: fill the building and the per-visit figure drops even if the plant burns exactly the same fuel. That is how energy intensity works in every hospitality metric. A busy Saturday lowers the per-visit figure without lowering total emissions.

What neither announcement states is the resolution of the metering, and that is the difference between a number for a report and a number you can run a building with. Read at the main incomer, you learn the site total moved. Sub-metered by zone, you learn the pool hall is carrying the sauna suite. The paper sits behind an access wall we have not cleared, so the per-visit and renewables figures here come from the Therme and Surrey releases, and we have not reviewed the boundary choices or the uncertainty ranges ourselves. Anyone citing these numbers in a board deck should read the paper first.

The renewables split is the operator’s lesson

The per-visit headline will travel. The number to sit with is the other one: on-site generation cut the footprint 33 percent at the large Bucharest site and 14 percent at the small Surrey one, across 2024.

That delta undercuts the assumption that a small urban bathhouse is inherently the greener operation. A destination resort has roof and ground area for generation, a capital budget that can carry a payback period, and a flat round-the-clock base load that makes on-site supply and heat recovery pencil out. An independent social sauna club retrofitted into a leased city-center shell usually has none of those: no roof rights, no capital line for an array it cannot take with it at the end of a ten-year lease, and continuous-duty electric heaters pulling grid power at commercial rates it has no way to offset.

So scale is turning into a decarbonization advantage in this category, and the small operator’s lever is not generation. It is recovery and control: heat-recovery ventilation on the exhaust, demand management on heater duty cycles, and scheduling that stops empty rooms running hot. Those are equipment and commissioning decisions, made at buildout or not at all.

What this means for a US operator

We can find no North American bathhouse publishing a per-visit carbon figure. That silence has a shelf life. A peer-reviewed method in a Nature journal is what moves a metric out of marketing and into procurement, and the people who ask for it first will not be regulators. They will be the hotel group reporting under its own disclosure regime for the spa it leases space to, the insurer pricing a wellness asset on energy intensity, and the city attaching a baseline to an incentive.

Run the arithmetic on a US social sauna club cycling 250 visits a day, six days a week. That is about 78,000 visits a year, and at the large-facility figure it puts roughly 430 short tons of CO2 equivalent through the building annually. Those are our assumptions, not Therme’s or Surrey’s, and a club with a different fuel mix and hot-water load will land somewhere else. The point is the order of magnitude, and that a facility running un-recovered exhaust on peak-rate grid power will look heavy on paper the first time somebody asks.

The practical move this quarter is not to adopt SPA-DEC. It is to find out whether you could produce a comparable figure at all. Is your energy and water metered separately from your landlord’s, or are you on a shared bill? Do you count visits reliably enough to be a denominator? Could you answer a hotel group that asked for your footprint per guest? Anyone building or refitting right now should be pricing heat-recovery ventilation and zone-level sub-metering as line items rather than upgrades, because retrofitting either into a finished room costs several times what it costs on the drawing.

Who gets to set the benchmark

One more thing, because nobody else in this trade is going to say it. Therme calls SPA-DEC a world-first, and we have not independently checked that against every prior attempt. What is not in dispute is the shape of it: a standardized carbon metric for the category, authored by an operator, validated substantially on that operator’s own flagship, and published under that operator’s name.

That is not a scandal. Peer review in a Nature journal is a real bar, the academic partner is credible, and the Innovate UK funding is public. Industry-authored standards are normal, and most of the codes governing sauna heaters were written with heavy manufacturer input. But the reference case for the category now runs through Therme Bucharest, a building almost nobody else operates, and first movers set defaults. Whoever publishes the method decides what sits inside the boundary, what gets normalized, and what a good number looks like.

The questions to ask before adopting it: does the allocation hold for a dry Finnish sauna club with no pools and a completely different heat-to-water ratio? Against whose baseline are staff travel and waste normalized? Is SPA-DEC available to operators outside Therme, on what terms, and who audits an operator’s inputs? Neither announcement answers those.