Where it comes from
Radon is a naturally occurring radioactive gas. It is produced by the decay of uranium held in rock, in quantities that vary widely with geology: granite massifs and volcanic ground release far more than sedimentary basins. In Brittany, the Massif Central, the Vosges or Corsica, the potential is high. On the Beauce plain, it is low.
The gas rises through the ground and enters a building wherever that building touches the earth: slab cracks, joints between slab and walls, service penetrations, inspection chambers, crawl spaces, cellars with bare earth floors. Once inside, it is diluted by fresh air. That is why the same house can show very different values depending on the season, on how the heating is used, and on whether the ventilation is running or has been switched off.
Three practical consequences follow. The highest levels are found on the lowest occupied floor, a converted cellar or a ground floor over a crawl space. An airtight but poorly ventilated home concentrates more than a leaky one. And winter makes it worse, because the temperature difference between the inside and the ground puts the building under slight negative pressure and draws air out of the soil.
What the risk means
The World Health Organization ranks radon as the second leading cause of lung cancer, far behind tobacco but ahead of every other domestic exposure. The point to hold on to is that the two risks do not add up: they multiply. In a smoker, high radon exposure weighs far more heavily than in someone who has never smoked. That is also what makes measurement worthwhile in a household where somebody smokes.
Two reference values circulate, and they do not say the same thing.
| Value | Origin | What it means |
|---|---|---|
| 100 Bq/m3 | Reference level recommended by the WHO | A long-term objective, below which residual risk is considered low |
| 300 Bq/m3 | Reference level applied in France and in the European directive | Above it, corrective action is expected |
One becquerel per cubic metre corresponds to one disintegration per second in that volume. It is not a toxicity threshold: the relationship between exposure and risk is continuous, with no step below which nothing happens. Coming down from 400 to 200 is worth doing, even though 200 remains above the WHO recommendation.
What to measure, and when
A spot reading is worthless. Values vary by a factor of five between day and night, and by more between summer and winter. The only usable measurement is an integrated one: a passive detector, left in place for at least two months, during the heating season, in the living room or bedroom on the lowest occupied floor.
The detector costs a few tens of euros, analysis included. It needs no power and no attention. It is placed away from a window, away from a heat source, at breathing height, and then forgotten. The laboratory returns an average over the period, which is exactly the quantity needed, since risk depends on cumulative exposure rather than on any peak.
Continuous electronic monitors serve a different purpose. They do not replace the passive detector for establishing a level, but they show the dynamics: the effect of an open window, of ventilation put back into service, of a cellar door sealed. For deciding what to correct, that reading earns its price.
What brings a value down
The order of operations matters, because the first step is by far the cheapest.
Ventilate first. Mechanical ventilation in working order, air inlets left unobstructed, a cellar ventilated rather than shut: on moderate values, between 300 and 600 becquerels, that is frequently enough to fall back below the reference level. What matters is that the ventilation should not put the home under negative pressure, which would draw more gas from the ground. This is why airflows are balanced before any conclusion is drawn.
Seal next. Repairing slab cracks, treating service penetrations, closing inspection chambers, laying a membrane over a bare earth floor. A few hundred euros of materials, a day of work, and an effect measurable from the first heating season.
Extract last, when the first two steps are not enough. Sub-slab depressurisation means creating a sump beneath the slab, or ventilating the crawl space with an extractor, so that the gas leaves outside instead of crossing the floor. It is the most effective device on high values, and the most expensive: a few thousand euros depending on the configuration.
In every case, a second integrated measurement, after the work and over a fresh heating season, is the only way to confirm that the correction had its effect.
What we do
We establish the radon potential of the address from public mapping, we record the configuration of the building where it meets the ground, and we state whether an integrated measurement is warranted. When it is, we specify where to place the detector, in what period, and what will have to be done with the figures. Lif8 measures places: we are not healthcare professionals and make no medical diagnosis.
