The Aachen Definition

What counts as a sauna

Adopted by the International Sauna Association, 8 May 1999

The 1999 Aachen Definition is the technical instrument the International Sauna Association uses to draw a line around what counts as a sauna at international scale. It was finalised in Aachen, Germany on 8 May 1999 by a four-nation working group, on the occasion of the XII International Sauna Congress hosted by the Deutscher Sauna-Bund for the Sauna-Bund's 50th anniversary. This page is the verbatim text, the people who drafted it, and what the document does in practice today.

The verbatim text

Both sentences below are reproduced verbatim from the ISA archive.

The document is short by design. Six technical anchors do most of the work: the building material (wood), the seating geometry (ascending stepped benches), the air-temperature window (around 80 to 105 degrees Celsius, measured at roughly 100 cm above the top bench), the heat source (a stone-filled stove), the humidity profile (low, briefly raised by water infusions), and the bathing pattern (alternating overheating and cooling). Each anchor is read in the rest of the document by what it excludes as much as what it includes.

Who drafted it

The working group was four people, one each from four ISA member associations considered authoritative on traditional sauna at the time. The four signatories are listed by the ISA archive as:

• Ernst Braunschweiler (Switzerland), representing the Swiss Sauna Association.
• Peter Jeitler (Austria), representing the Austrian Sauna Forum (Österreichisches Sauna Forum). Jeitler now serves as Vice-President of the International Sauna Association.
• Rolf-Andreas Pieper (Germany), representing the Deutscher Sauna-Bund as its then-Geschäftsführer. Pieper went on to a 38-year tenure in that role, retired from the executive seat in 2021, and now sits on the Sauna-Bund's Präsidium as Secretary while running the UVSV certification office.
• Pirkko Valtakari (Finland), representing the Finnish Sauna Society.

The host association of the 1999 congress was the Deutscher Sauna-Bund, then under president Prof. em. Dr. med. Eberhard Conradi. Conradi convened the XII International Sauna Congress in Bad Aachen on the Sauna-Bund's 50th anniversary, drawing 550 visitors from 15 countries and 28 speakers under the theme “Sauna: Wellness weltweit.” His role was institutional rather than technical: as host-association president he convened the event at which the definition was finalised, while his Geschäftsführer Pieper sat in the four-person drafting committee. The ISA's published signatory list does not include Conradi himself. Crediting him as a personal signatory is a common reading error in secondary sources; the ISA archive is unambiguous.

Why it matters

The definition does two things in practice. The first is positive. It gives the ISA and its member associations a single, written technical reference for what counts as a sauna: a wooden room with stepped benches, roughly 80 to 105 degrees Celsius at air temperature around 100 cm above the top bench, a stone-filled stove, low humidity that can be briefly raised by water infusions, with users undergoing repeated full-body heating and cooling. Outside that envelope, ISA and its member bodies say, you are looking at a different product.

The second is implicit. The definition's specifications exclude infrared cabins. An infrared cabin radiates heat directly onto the body rather than heating the air. It is not made of wood as a definitional requirement. It does not have a stone-filled stove. It is operated at lower air temperatures than the 80 to 105 °C window the document specifies. Whether or not one considers an infrared cabin a useful product, by the Aachen Definition it is not a sauna.

This is the technical instrument by which the North American Sauna Society draws its operational line. NASS's sauna-types page reads: “not a traditional Finnish sauna; maybe the best definition would be a ‘heat therapy room’.” That is the Aachen Definition applied to the American market reality, where infrared cabins outsell traditional saunas in the residential channel.

The position is consistent across the four founding member associations. The Finnish Sauna Society's working definition, attributed to former executive director Hilkka Heimonen, describes “a well-ventilated room with wooden walls, heated to 80 to 95 degrees Celsius by a wood-fired stove, with somewhere to wash nearby, and preferably a scenic lake in view.” The Aachen text is more permissive on the heat source (a stone-filled stove may be electric, not just wood-fired); otherwise the European frame is unified.

Status today

The definition has been continuously hosted on the International Sauna Association's site since its adoption and is presented as the canonical reference text. The ISA homepage names “acceptance and promotion of the Aachen Sauna Definition” as one of its four stated goals. The Deutscher Sauna-Bund's scientific-literature archive saunaliteratur.de, founded by Conradi in December 2009, anchors its sauna research under the Aachen technical envelope. The Sauna-Bund's SaunaPremium quality seal, which audits roughly 150 facilities across Germany and Austria, uses Aachen-consistent operational standards (wooden room, stone-filled stove, infusion technique, hygiene) as its baseline.

In public discourse, the most visible advocate for the definition's frame is current ISA President Risto Elomaa. Elomaa has used the platform to argue that electrical sauna stoves are “toaster saunas” rather than the genuine article and has described his standards work as countering misleading claims about infrared products, which he frames as fundamentally different from sauna by the Aachen technical standard. The tone is purist. The position is consistent. It is also unfashionable: the largest residential sauna market in North America is infrared cabins, and most consumers buying an infrared product are doing so deliberately for accessibility, lower-temperature comfort, or apartment-friendly electrical requirements.

The Aachen Definition is not a quality verdict. An infrared cabin can be well-built or poorly-built. A sauna can be hot or temperate, smoke or electric, Finnish or Japanese, traditional or modern. The Aachen Definition is a definitional choice with editorial and regulatory weight, not a consumer judgment about which sweat-bathing room to buy. If you are buying a wood-room-with-stone-filled-stove sauna, the document describes what you are buying. If you are buying an infrared cabin, the ISA does not consider it a sauna at all. Both products exist. Both have customers. The document is precise about the line it draws.