A map is not a meter: how to check radon in your home

Radon has no colour, smell or taste, so the condition of a basement or the colour of a district on a map cannot reveal how much enters the living space. This radioactive gas from the ground can move through cracks, service penetrations and shafts in structures that touch the soil, while the building design, ventilation and everyday use strongly affect the indoor concentration.

The 2026 summary of Hungary’s National Radon Programme draws on data from 4,460 rooms in 3,150 buildings across 996 settlements. The weighted national average was 112 Bq/m³, and 90% of annual averages fell between 25 and 250 Bq/m³. These figures describe a national pattern, not an individual house. Neighbouring homes can produce different results because their ground contact, foundations, services and air change differ.

Start by defining the question

A test lasting a few days and a passive detector left in place for months answer different questions. Hungary’s public-health authority says a short measurement of at least three to five days can provide an initial indication, but radon varies from hour to hour and from season to season. Measurements lasting three, six or twelve months give a much better estimate of the annual average; a full year divided into quarterly periods is the most informative approach.

If you only want to know whether a home deserves closer attention, a short screening test can be a first step. If you are planning renovation, considering a property purchase or following up an earlier high result, choose a documented long-term measurement instead.

Where should the detector go?

Begin in the lowest regularly occupied room that touches the ground or sits above a basement. This may be a ground-floor bedroom, living room, home office or occupied lower-ground floor. In a larger or multi-storey home, test more than one room when foundations or ventilation zones differ.

Do not place the detector beside an open window, door, radiator, fan, extractor or direct sunlight. It should not sit on the floor or inside a closed cupboard. Follow the manufacturer’s or laboratory’s instructions first; a passive detector generally needs a calm interior location that represents normal room air.

Live normally during the test. Leaving windows wide open all day just for the measurement, or closing ventilation inlets that are usually open, creates an artificial result. Record start and end dates, room, floor, heating season and unusual events such as a long absence or building work.

What does 300 Bq/m³ mean?

Hungary’s reference level for the annual average radon concentration in homes and public buildings is 300 Bq/m³. European radiation-safety law likewise requires national reference levels not to exceed an annual average of 300 Bq/m³.

A reference level is not a sharp “safe versus dangerous” border. Radon-related risk rises with concentration and time spent in the space, so reasonable reduction can be useful below the reference level as well, while a higher annual average calls for investigation of the source and entry routes. Do not turn one short test into an annual average, and do not design major building work around one unusual day.

How to read the result

1. Check the duration. A few days of data is screening, not an annual classification.
2. Check the room. A basement, ground-floor bedroom and upstairs living room are not interchangeable.
3. Check the unit. Indoor radon is reported in Bq/m³.
4. Keep the report. It should identify the detector or laboratory, test period and room.
5. Confirm a high value. Use a longer test that preferably includes the heating season.
6. Do not argue with the map. A map shows probability; the detector describes the air in the room tested.

An electronic monitor can help reveal daily fluctuations and the immediate effect of ventilation, but calibration, uncertainty and placement still matter. With a passive detector, laboratory analysis and a documented exposure period provide the backbone of the result.

What follows a high annual average?

First rule out a testing error, then seek a building-specific assessment. Mitigation should match the actual pathways. Hungary’s public-health guidance lists improved ventilation, sealing soil-gas entry points, isolating ground-contact structures and sub-slab or sub-basement depressurisation among possible measures. In less common cases, a building material with high radon release may need investigation.

Ventilation can quickly reduce the concentration at a particular moment, but it is not a durable solution for every building. Sealing visible cracks alone may also be insufficient when soil gas has several routes. Before renovation, combine a measurement plan with a technical plan, then verify the work with a follow-up test.

Eight-point home checklist

• Consult the national radon map, but do not treat it as an apartment-level result.
• Identify the lowest regularly occupied room.
• Choose a documented radon detector intended for the purpose or a competent measurement service.
• Even for screening, respect the minimum three-to-five-day period and placement instructions.
• Base decisions on a three-to-twelve-month test, preferably one that approximates an annual average.
• Use the home normally during measurement.
• Around or above 300 Bq/m³ as an annual result, seek confirmation and building-technical advice.
• Always measure again after mitigation.

Managing radon is a sequence, not a gadget purchase: use the map for orientation, a correctly placed detector for real data, a long measurement for an annual picture, then targeted building work and a control measurement. That sequence turns an invisible gas into a manageable home-improvement problem.