Do you need to measure radon on mezzanines?
The common question
A mezzanine is a floor situated between the ground floor and the first floor, with a lower than usual ceiling height. Because of this, it typically does not meet the minimum height requirements for residential use, but it is commonly found housing offices or other uses that do comply. Mezzanines can be open spaces within the ground-floor premises or, in many cases, have their own independent exterior entrance.
This layout raises a recurring question among owners and technicians: do you also need to measure radon on mezzanines?
The answer does not depend on the architectural name of the space. It depends on how Spanish regulations define the scope of the obligation and how they set out the measurement methodology. Let's go through it step by step.
When there is an obligation to measure radon
In Spain, the obligation to measure radon in workplaces comes from Royal Decree 1029/2022, which requires exposure assessment wherever there are workstations located on certain floors of a building.
The regulation puts it plainly:
"The estimation of the annual average radon concentration shall be mandatory in all workplaces located on basement floors or the ground floor within priority action municipalities."
The criterion is based on floor level: ground floor and basement floors. That straightforward.
What counts as "ground floor"
This is where the detail matters. To avoid ambiguous interpretations, Instruction IS-47 from the Nuclear Safety Council (CSN) explicitly defines the term in its Article 2:
"Ground floor: The floor of a building where the vertical distance between grade level and the upper face of the floor slab does not exceed 1 m."
This matters because a mezzanine is not a floor for regulatory purposes. It is an internal element located within a floor.
Inside the ground floor: how measurement is organised
Once a workplace falls within scope:ground floor or below grade:the regulations do not require measurement by architectural level. What they require is to define sampling zones.
IS-47, in Article 4(a), establishes that the interior spaces of the ground floor and basement floors must be identified. And it defines exactly what a sampling zone is:
"Sampling zone: An area of a building or facility whose characteristics relating to the entry and propagation of radon (type of walls, foundations, floor and subsoil, ventilation regime, temperature…) make the concentration of this gas throughout its volume homogeneous or very slightly variable."
This is the key point: the unit of assessment is a homogeneous interior volume, not a visual or structural subdivision of the space.
What this means for open mezzanines
When a mezzanine or loft level is open to the ground floor, without full enclosure and without independent ventilation, radon propagates and distributes throughout the entire interior volume. From the IS-47 perspective, that loft does not constitute a separate sampling zone:it forms part of the same homogeneous zone.
This does not mean the mezzanine "is" ground floor in an architectural sense. It means it is included within the ground floor assessment because it shares the same gas propagation conditions.
Representativeness and workstations
The regulations also require that measurement be representative of actual exposure. IS-47 states, in Article 4, that when defining sampling zones one must consider:
"possible spatial gradients in radon concentration and fixed work locations or those where workers spend the most time."
Furthermore, Safety Guide GS 11.4 from the CSN warns that concentration differences can appear within the same volume due to physical phenomena such as the stack effect, which can cause different radon readings in upper parts of a building.
For this reason, if an open mezzanine is regularly occupied, it is advisable that detector placement be representative of that point in the space, even though it belongs to the same sampling zone as the rest of the premises.
When a mezzanine can be a separate zone
A mezzanine can be justified as an independent sampling zone in several situations: when it is enclosed or partitioned, when it has ventilation that differs from the main premises, or when it presents conditions that make radon concentration non-homogeneous relative to the rest of the space.
A particularly clear case is that of mezzanines with an independent exterior entrance. Having their own street access means the ventilation regime and radon propagation conditions differ from those of the ground-floor premises, even though they are in the same building. In practice, these mezzanines function as independent premises and should be assessed as such.
In any of these cases, the legal definition of sampling zone itself allows, and strictly speaking requires, treating it separately.
An important nuance: occupancy
IS-47 clarifies in Article 4(d) that measurement is not required in areas with negligible occupancy:
"Zones where workers are present for no more than 50 hours per year shall be exempt from measurement."
This criterion is especially useful for mezzanines used solely for occasional storage or infrequently accessed archives.
In summary
Spanish regulations define the measurement obligation by floor level: ground floor and below grade. However, the measurement methodology is based on sampling zones, understood as interior volumes with homogeneous radon concentration.
Radon must be measured on mezzanines when they form part of the same air volume as the ground floor and there is relevant occupancy. The deciding factor is not height or the name of the space, but radon propagation, ventilation and the actual use of the area.
Regulatory references:
Royal Decree 1029/2022:Regulation on health protection against risks from exposure to ionising radiation
CSN Instruction IS-47:On protection against radon exposure in the workplace
CSN Safety Guide GS 11.4:Methodology for assessing radon exposure in the workplace
