Radon (Rn)

In December 1984 an engineer named Stanley Watras walked into the Limerick Generating Station in Pennsylvania and set off the newly installed radiation monitors. This was awkward, because Limerick had no fuel in it yet. There was no plausible source of contamination inside the plant, and the alarms were triggering on his way in.

The contamination was on him, and it had come from his own house in Boyertown. When it was measured, the indoor air held around 2,700 picocuries per litre — hundreds of times the level now considered a reason to act, and by some estimates a radiation dose equivalent to smoking many packs of cigarettes a day. The Watras house sits on the Reading Prong, a belt of uranium-bearing metamorphic rock running through Pennsylvania, New Jersey and New York. The episode is why the United States has a residential radon programme at all; the Indoor Radon Abatement Act followed in 1988.

It is not the radon that gets you

Radon is a noble gas, and it behaves like one in the body. Breathed in, most of it is breathed straight back out again without reacting with anything. If radon were the whole story it would be close to harmless.

The damage is done by what it turns into. Radon-222 alpha-decays to polonium-218, which decays through lead-214 and bismuth-214 to polonium-214, and all of those are solids. Formed in mid-air as electrically charged atoms, they attach themselves within seconds to dust and aerosol particles — and then get inhaled and stick to the lining of the bronchial airways. Polonium-218 and polonium-214 both emit alpha particles, at close range, into the cells of the bronchial epithelium, repeatedly, for as long as the exposure continues. The dose is delivered by a handful of short-lived metals that were briefly a gas.

Three point eight days is exactly the wrong number

Radon-222's half-life is the reason it is a problem, and a slightly different value in either direction would make it a non-problem.

Radium-226 decaying inside a mineral grain produces a radon atom that has to escape the grain, diffuse through soil pore space, and find its way into a building — a journey of hours to days. At 3.8 days, plenty of radon survives the trip. Its sibling radon-220, produced from thorium and known as thoron, lasts 56 seconds and mostly decays before it gets anywhere, which is why it contributes far less indoor dose despite thorium being more abundant than uranium.

Push the other way and the problem also disappears. A much longer-lived radon would disperse into the open atmosphere and dilute to nothing before decaying. Outdoor radon is in fact harmless for exactly that reason. What makes indoor air different is that a building is a low-pressure box sitting on the ground: warm air rising inside draws soil gas up through cracks in the slab, sump openings, and gaps around service pipes, concentrating something that would otherwise blow away.

Schneeberg lung, four centuries early

The disease was described long before the cause. Georgius Agricola, writing about the silver mines of the Erzgebirge in 1556, recorded a wasting affliction of the lungs that killed miners young. In 1879 Härting and Hesse examined miners at Schneeberg and identified the illness as lung cancer, at a rate far above anything in the surrounding population. Radioactivity had not been discovered.

Joachimsthal, on the Bohemian side of the same mountains, is the source of the pitchblende the Curies worked with — so the ore that produced the first radium came from mines whose lethality had been documented for three hundred years. Radon was proposed as the cause in the 1920s. Confirmation came from post-war epidemiology of uranium miners on the Colorado Plateau, in Czechoslovakia and in Ontario, and those miner cohorts remain the empirical backbone of every residential risk estimate in use.

The numbers, and what they mean

Radon is the second leading cause of lung cancer after smoking, and the leading cause among people who have never smoked. The United States Environmental Protection Agency attributes roughly 21,000 lung cancer deaths a year to it domestically; the World Health Organization puts radon's share of lung cancer worldwide somewhere between 3% and 14%, depending on a country's housing stock and geology.

Radon and tobacco are not additive. They multiply: for a smoker, a given radon exposure carries something like an order of magnitude more absolute risk than the same exposure carries for a never-smoker, because the two damage the same tissue by different mechanisms.

Guidance levels reflect this. The EPA's action level is 4 picocuries per litre, about 148 becquerels per cubic metre. The WHO recommends a reference level of 100 Bq/m³ and a ceiling of 300. The United Kingdom acts at 200 Bq/m³. None of these is a safety threshold — the dose-response curve has no observed floor — they are policy lines drawn where mitigation becomes practical.

Radon has no smell, no colour and no taste, and no symptom appears until decades later. A measurement is the only way anyone knows. Health agencies in every affected country therefore advise testing every home, and note that neighbouring houses on the same geology routinely differ by a factor of ten because construction and ventilation matter as much as the rock does.

Emanation, niton, radon

The naming took a quarter of a century and the credit is genuinely divided. Ernest Rutherford and Robert Owens observed a radioactive "emanation" from thorium in 1899, and the Curies observed one from radium in the same year. Friedrich Ernst Dorn characterised the radium emanation in 1900 and is the name most often attached to the discovery, including on the data card above — though Rutherford has an equally strong claim, and what Dorn had was arguably a gas that others had already noticed.

William Ramsay and Robert Whytlaw-Gray settled the matter in 1910 by collecting enough of it to weigh. They measured a density around eight times that of air, confirming it as the heaviest gas then known, and named it niton, from the Latin for shining, because solid radon glows. The name radon was standardised in 1923.

Not actually inert

The noble-gas label oversimplifies. Radon's outermost electrons are far enough from the nucleus and well enough shielded that it is the most readily oxidised of the stable-enough noble gases, and radon difluoride is a real compound. Studying its chemistry is close to impossible for a practical reason rather than a chemical one: any sample destroys itself and its container's contents through its own radiation faster than most experiments can be run.

The same short half-life means every radon atom on Earth was made recently, from radium in rock. Radon also seeps from soil in response to changes in ground stress, which is why fluctuations in soil-gas radon have been studied for decades as a possible earthquake precursor — an idea that produces suggestive individual cases and no reliable predictive record.