A gas that comes out of rock
Uranium is in almost all soil and rock — a few parts per million, more in granite, shale and phosphate. It decays through a chain of fourteen steps to lead, and one of those steps produces radon 222.
Every other member of the chain is a solid, locked in the mineral grain where it formed. Radon is a noble gas. It escapes.
It moves through pores and fractures in the ground, and where the ground meets a building it finds a way in. A heated house is slightly lower in pressure than the soil beneath it, so it draws air up through cracks in the slab, gaps around service pipes, and unsealed sump pits.
⚠️ Outdoors this is harmless — radon disperses to a few becquerels per cubic metre. Indoors it accumulates, and in poorly ventilated basements over uranium-rich ground it can reach hundreds or thousands.
Why an inert gas is dangerous
Radon does not react with anything and does not stay in the body. Breathe it in and most of it comes back out.
The harm comes from what it turns into. With a half-life of 3.8 days, some of the radon inside the lung decays while it is there — into polonium 218, then lead, bismuth and polonium 214.
Those are solid metals. They attach to dust and aerosol particles in the air, lodge in the lining of the airways, and emit alpha particles at close range into tissue that cannot shed them.
⚠️ Alpha particles travel only a few centimetres in air and cannot penetrate skin, which makes them harmless outside the body and the worst kind of radiation inside it. The whole energy is deposited in a very short track through the cells it passes.
What the evidence rests on
The first evidence came from uranium miners, who worked in poorly ventilated shafts with extremely high radon levels and developed lung cancer at rates far above the general population.
Extrapolating from miners to houses was contested for years — much lower doses, different exposure patterns. It was settled by pooling residential studies: thirteen European studies, seven North American, two Chinese, tens of thousands of cases between them.
They agreed. The risk rises roughly linearly with exposure, with no threshold below which it disappears.
⚠️ Radon and smoking multiply rather than add. A smoker in a high-radon house faces a risk far greater than the sum of the two separately, which is why most radon-attributed deaths occur in smokers and why the two public health messages belong together.
The WHO estimates 84,000 deaths a year, and recommends action above 100 becquerels per cubic metre.
A fan, and a pipe
The fix is mechanical and cheap.
Sub-slab depressurisation puts a pipe through the floor slab into the gravel below and a fan on the end of it, running continuously. The fan holds the space under the slab at slightly lower pressure than the house, so soil gas goes up the pipe and out above the roof instead of into the rooms.
It reduces indoor radon by 80 to 99 % and costs less than most household appliances.
⚠️ Sealing cracks alone helps much less than people expect. The pressure difference that drives the gas is still there, and it will find another path. Sealing is worth doing alongside a fan, not instead of one.
Testing is the part that gets skipped. A detector left in the lowest occupied room for three months costs very little, and radon varies so much between neighbouring houses that a neighbour’s result says nothing about yours.
Where it is worst
Radon follows geology, not geography.
Granite regions are the classic case — Cornwall in England, Brittany in France, parts of the Czech Republic and Sweden. Black shales and phosphate deposits are others. Two houses on the same street can differ tenfold depending on what is under them and how they are built.
Iowa has the highest average residential levels in the United States, not because of granite but because glacial till there is rich in uranium-bearing rock.
⚠️ Building tightly for energy efficiency makes it worse. Less air exchange means whatever comes in stays longer, and radon mitigation has become part of building regulations in several countries for that reason.