Where radon comes from
Uranium sits in small quantities in almost all rock and soil, and it decays. Radium is one of the products of that decay, and radium in turn decays into radon. Because the uranium in the ground has a half life measured in billions of years, the supply does not run out on any timescale that matters to a house.
Radon is a noble gas, which means it does not react with anything. It does not bind to soil particles or dissolve into the rock around it, so it moves freely through cracks and pore spaces and drifts upward. Reaching open air, it dilutes to a level of no consequence. Reaching the underside of a heated building, it does not.
South central Pennsylvania sits on fractured limestone and shale, which is both a source of the uranium and a set of pathways for the gas to travel. That combination is why York County averages 10.75 pCi/L against 6.00 across the state.
Why radon is dangerous
Radon itself is chemically inert and mostly breathed straight back out. The harm comes from what it turns into. With a half life of 3.8 days, radon decays quickly into a series of short lived radioactive elements, informally called radon daughters or progeny, and those are solid rather than gaseous.
Those solid particles attach to dust and to the surfaces of your airways. Sitting in lung tissue, they continue to decay and emit alpha particles directly into the cells around them. That is the mechanism: not the gas you inhale, but the radioactive solids it becomes while it is inside you.
The health effect is lung cancer, and it accumulates with exposure over years. The EPA and the World Health Organization both class radon as a cause of lung cancer in the general population, with risk rising steeply for people who also smoke. A single afternoon in a basement with an elevated reading is not the concern. A decade of sleeping in one is.
What pCi/L means
Radon is measured in picocuries per litre of air, written pCi/L. A curie is a unit of radioactivity, a picocurie is a trillionth of one, and one pCi/L works out to roughly two radioactive decays per minute in every litre of air around you.
Outdoor air runs at about 0.4 pCi/L, which is the natural background you cannot escape and do not need to. The EPA action level is 4.0 pCi/L, and it is worth being clear about what that number is and is not.
It is not a safety threshold. There is no level of radon considered risk free, and the risk below 4 is lower rather than absent.
It is an action guideline. The level at which the EPA says fixing the house is warranted, set where mitigation is reliably achievable and worth what it costs.
Between 2 and 4 the EPA suggests considering reduction, which is advice rather than a recommendation to act.
Much of the rest of the world uses becquerels per cubic metre instead. One pCi/L is about 37 Bq/m3, so the EPA action level of 4 pCi/L corresponds to roughly 148 Bq/m3, which is why European guidance often quotes limits between 100 and 300.
Where radon is found in a house
Radon concentrates at the bottom of a building and thins out as you go up, because that is where it enters and because air mixing dilutes it on the way. In the York County record a basement averages 11.79 pCi/L, a first floor 5.84 and a second floor 5.43.
Anything that opens the floor to the soil beneath is a route in: cracks in the slab, the joint where the slab meets the wall, an open sump pit, gaps around pipes, and bare earth in a crawl space. How the gas actually moves through those openings is covered on the page about how radon gets into a house.
Well water can carry radon too, which then releases into the air during showering and laundry. It is a much smaller contribution than soil gas in almost every case, and it is worth testing separately only if your water comes from a private well.
Three things people get wrong
- Old houses are not automatically worse. In York County the oldest, densest part of the city, ZIP code 17401, has the lowest average of anywhere in the county at 2.83 pCi/L. What matters is the ground and the foundation, not the build date.
- A neighbour's result does not transfer. Two identical houses on one street can differ several times over, because entry depends on the particular cracks in a particular slab and how that house is heated.
- You cannot detect it without a device. No smell, no taste, no colour, no symptoms at the time of exposure. Every claim about sensing radon is wrong, and the only way to know is a test.
Common questions about radon
What is radon gas?
A radioactive gas with no colour, smell or taste, produced continuously as uranium in rock and soil decays. It disperses harmlessly outdoors and accumulates inside buildings, where it becomes a health risk.
Why is radon dangerous?
Radon decays within days into short lived radioactive solids that lodge in lung tissue and emit alpha particles there. Long term exposure raises lung cancer risk, and the risk is far higher for smokers.
What does pCi/L mean?
Picocuries per litre, a measure of radioactivity in air. One pCi/L is about two radioactive decays per minute per litre. Outdoor air sits near 0.4 and the EPA acts at 4.0.
Is 4 pCi/L a safe level?
No level is considered risk free. Four is an action guideline rather than a safety threshold, set where mitigation is reliably achievable. Between 2 and 4 the EPA suggests considering reduction.
Can you smell or see radon?
Never. Radon has no colour, odour or taste and causes no symptoms at the time of exposure, which is why a measurement is the only way to know whether a building has a problem.
Do old houses have more radon?
Not reliably. In York County the oldest and densest ZIP code averages the lowest reading in the county. Entry depends on the ground underneath and the state of the foundation rather than the age of the building.