Is radon dangerous in an apartment building?

Upper floors usually have low levels; the risk mainly concerns the lowest and ground-contact spaces.

What is the radon action level?

300 Bq/m³ in an existing home (STUK); a new one is designed below 200 Bq/m³, and the WHO recommends below 100.

Radon in a flat: measurement, limits and risk areas

By Lumi · Updated: Jun 16, 2026, 12:00 AM

Radon is an odourless indoor-air risk, highest on the lowest floors. How to spot the risk, measure it, and read STUK's limits.

Important: On the lowest and ground-contact floors radon can exceed the limit without symptoms — measure before buying.

Radon is a radioactive gas that rises from the ground into indoor air. It's odourless and invisible, but long exposure is a health risk. In Finland levels vary by area.

Where radon comes from

Radon travels from the soil into a building's lowest spaces. The risk is highest in ground-contact and lowest floors; upper apartments typically have low levels.

Limits (STUK)

STUK reference values: in an existing home the action level is 300 Bq/m³, and a new building is designed so radon stays under 200 Bq/m³. The WHO recommends below 100 Bq/m³.

The median varies by municipality — around 100 Bq/m³ in the capital region, clearly higher in some areas.

How to measure and act

Radon is measured with a detector over at least two months (three recommended) between September and May. If the value exceeds the limit, mitigation (sealing or a radon sump) is usually effective. Lumi shows the municipal median by address.

Frequently asked questions

Is radon dangerous in an apartment building?: Upper floors usually have low levels; the risk mainly concerns the lowest and ground-contact spaces.
What is the radon action level?: 300 Bq/m³ in an existing home (STUK); a new one is designed below 200 Bq/m³, and the WHO recommends below 100.
How is radon measured?: With a radon detector over at least two months (three recommended), September–May.

Sources

STUK — radon