The Quiet Crypt
A cold snap is coming and the parish crypt would sleep fourteen, but a two-day test reads 420 becquerels of radon per cubic metre. The parish assistant will not call it dangerous or safe: it works out what a few nights, a whole winter and a fan each change, says no to the season and yes to the week, and a heater turns out to make the reading worse.
On Monday morning, the tenth of January, Morwenna Hocking carried a clipboard down the crypt steps at St Aldhelm's and read off what the parish assistant had left on the tablet at the bottom: Radon canister, 48 hours, closed room: 420 becquerels per cubic metre.
She was seventy-one and had run the winter shelter at Trevenna for nine years, in the parish hall, with a heater that ticked. The forecast said minus nine on Thursday night, the coldest in a decade. The hall held eight. The crypt, under the north aisle, was nine metres by five, four metres high, stone-vaulted, dry, and would hold fourteen camp beds with room to walk between.
"Is that a bad number?" she said to the tablet, because she did not like to say it to the wall.
"It is a high number for a room people will sleep in," said the parish assistant. "I would like to tell you what it does and does not mean before you decide, because it is easy to be frightened by it and easy to wave it off, and neither is right."
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It gave her the facts in the order she had asked it to give facts, short.
"Radon is a gas that comes out of the ground where there is granite, and west Cornwall is granite. It is not a poison you notice. It decays, with a half-life of 3.8 days, into solids that stick to the inside of the lung. The harm is lung cancer, years later, and it depends on the long-run average you breathe, not on any one night. The World Health Organization's reference level for a home is 100 becquerels per cubic metre. In the United Kingdom the action level for a home is 200, and for a workplace 300. Each extra 100 on a lifetime average raises a person's chance of lung cancer by about sixteen per cent of what it already was. For a smoker that is a larger number of people than for someone who never smoked."
"So four hundred and twenty."
"So four hundred and twenty is above every one of those lines, if it were the average. I do not yet know that it is. That was a two-day test with the door shut, which is the way you screen a room: it asks the worst reasonable question. Radon in one place can swing by a factor of two or three in a week with the weather and with how the room is breathing. The true yearly average of this crypt could be 200 or it could be 800. The only test that tells you is a long one, ninety days, and winter is the season it reads highest in."
Morwenna sat down on the bottom step. It was colder than she had thought. "Thursday," she said.
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"This is the part I think matters," said the assistant. "Take the time a person spends in a room and spread it over the year. There are 8,760 hours in a year. If the shelter opened for ninety nights, ten hours a night, that is 900 hours, a tenth of the year, just over. A guest who slept here every one of those nights, at 420, would add about 43 to their yearly average. That is a real number and it is why I will not recommend the whole season in this room as it stands.
"Now the cold snap. Four nights, ten hours each, is 40 hours. Forty over 8,760, times 420, is under 2 becquerels added to a yearly average. For a person who sleeps in a doorway on those four nights, the radon in this crypt is the least of the dangers by a very long way, and I would be doing them harm if I let it keep them out. Minus nine kills people. Radon does not, in four nights."
Morwenna had thought for twenty minutes that the assistant was going to tell her no. It had told her yes before she asked.
"So we open Thursday," she said.
"I think so, on two conditions, and I would like you to accept them now, before any more numbers come in. One. We make the room breathe. Radon in a closed room settles where the ground puts it in and the air takes it out, so the level is proportional to one over the sum of the air changes per hour and the decay rate of radon, which is 0.0076 per hour. At the 0.2 changes per hour this vault has now, that gives 420. At one change per hour, the level falls to about 86. That is a fifth. A vault of 180 cubic metres at one change an hour is 180 cubic metres of air an hour, or 50 litres a second, which one ordinary fan will move.
"Two. A continuous monitor, reading every hour, not another canister. If any hour from midnight to six comes above 300, we wake the caretaker and move everyone up into the north aisle. I will say now what I expect, so you can hold me to it: I expect it to stay below 150 with the fan, and I will tell you if it does not."
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The fan came on Tuesday, from the builders' merchant in Camborne, a ducted extractor with a grille Ruan the sexton fixed to the old coal chute. The merchant's driver who delivered it said it would pull the draught right through the church.
"It will," said the assistant, "and the draught is the cost. At 12 degrees between the crypt and the outside, 180 cubic metres an hour carries away 180 times 0.34 watt-hours per cubic metre per degree times 12, which is about 730 watts. That is more heat than the beds would otherwise let you keep. Fourteen people asleep give off about 80 watts each, which is 1,120 watts, so the people alone will hold the room against the fan on a mild night, and on Thursday you will need to add heat. I would add it with a small oil-filled radiator, not a fan heater, and I will explain why."
Morwenna did not wait for the why. It was Tuesday, the hall heater had ticked all night, and she liked a fan heater, which she had used for the shelter's first four winters. She put one in the crypt with the door closed on Tuesday evening, to take the cold off the stone, and turned off the extractor to let the warmth stay in.
By ten the monitor said 590. By two in the morning it said 640.
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"The warm air does it," said the assistant at six, when she came down to the tablet in her coat. "It is not the heater's fault exactly; it is the whole shape of the building. When the air in a crypt is warmer than the air outside, it rises, goes up the stairwell and leaks out of the roof, and the crypt is topped up from below by air pulled out of the ground. A warm closed room is a vacuum for soil gas. I should have said that this was the reason for the oil-filled radiator, which heats the stone slowly without a draught of its own,. It was my omission."
"I turned off the fan."
"You turned it off because I told you the fan would cost heat. Both are true. What I did not say clearly was that the fan is the thing that matters most, and the warmth is the thing to be paid for after it."
Morwenna looked at the tablet. She had not been a woman who liked to be shown to be wrong by a machine at six in the morning, and for a moment she thought of saying so. Then she put the fan heater in the car and carried the oil radiator down.
On Wednesday evening she started the extractor at seven and the radiator at the same time. The monitor read 310 at eight, 180 at ten, 120 at midnight and 96 at three.
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On Thursday night it was minus nine. At the gate, at six in the evening, Morwenna counted eleven who had come: Gareth, who had slept in the car park by the chandlery since the autumn; the two lads from the bus station; a Latvian crane driver off his shift for two weeks, who had a tan line from a watch; a woman called Sian with a dog. The dog was allowed. Father Tomasz, who had said Mass at six and was still in his vestments under his coat, carried down a box of blankets in both arms and did not say anything about the rule against dogs in the crypt.
The monitor ran at 88 to 112 all night. It did not touch 150. The assistant told Morwenna at seven on Friday morning, in the voice it used for the weather: "Highest hour 112, at ten past two. I said I expected under 150; I was right, but only because the extractor was on. The oil radiator held the crypt at 9 degrees. It was not warm and I would not say it was. It was above freezing, it was dry, and it was quiet."
"It is called the quiet crypt," said Gareth, on his way up the stairs. He had named it that on Friday morning, with the unkindness of a man who had been warm. "You can hear your own breathing."
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The shelter ran for four nights, to Sunday. On Monday the seventeenth, Father Tomasz and Morwenna sat in the vestry with the assistant's note on the table, which was one page.
What I recommend: keep the crypt for cold snaps only until a 90-day alpha-track detector has been in it, from now to mid-April, with the extractor running at night. If the average with the fan comes in under 100, the crypt can be a winter shelter. If it is between 100 and 300, we fit a sub-slab sump, which a radon contractor quotes at about £1,500 to £2,500 and which often cuts the level by 80 to 95 per cent, and test again. If it is over 300, we do not use it for sleeping, whatever the weather, and we find another room. The volunteers who sit up with the guests are at the same risk per hour as the guests, and I have counted them.
What I did not know, and still do not: the long-run average of this room. What I would be guessing at: any of the numbers above 150 on a night I did not measure. The detector is £18.
"Eighteen pounds," said Father Tomasz, who was Polish and did sums in zlotys first and pounds after. "And the sump, if it comes to it?"
"Two thousand five hundred at most. We have £610 in the shelter fund." Morwenna turned the page over. "I would have sent them down there for the whole winter on Monday morning, you know. On a canister. If it had said eighty."
"You would have been right by luck," said the assistant. "And if it had said eight hundred you would have shut the crypt, and fourteen people would have slept in the hall on eight beds. I think the best thing the number did was make us look again."
Father Tomasz took the detector out of its envelope, a small grey disc like a tin of boot polish, with a date written on the back in the assistant's neat hand, and carried it down himself. He put it on a shelf in the crypt, at the height of a sleeper's head, away from the wall. It would sit there, doing its slow work in the dark, with the extractor running and the oil radiator ticking, until April.
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A note for grown-ups. This is fiction. St Aldhelm's, Trevenna, Morwenna Hocking, Father Tomasz, Ruan, Gareth and Sian are invented; Cornwall, its granite and its radon are real. The science is real. Radon-222 is a radioactive gas from uranium in rock and soil, with a half-life of 3.82 days (decay constant about 0.0076 per hour); its short-lived decay products, breathed in, irradiate the lung, and radon is a recognised cause of lung cancer, much more so in smokers. The World Health Organization's reference level is 100 Bq/m³ and its estimate is a rise of about 16% in lung-cancer risk per 100 Bq/m³ of long-term average; the UK action level is 200 Bq/m³ for homes and 300 Bq/m³ for workplaces. A short-term test (a few days, closed conditions) is a screening test; radon varies by weather, season and ventilation, and the long-term average from a three-month detector is the usual basis for a decision. The steady-state concentration in a room scales as the source divided by the sum of the ventilation rate and the decay constant, so raising ventilation from 0.2 to 1 air change per hour cuts a level by about 80% (here 420 to about 86). Warm indoor air rising and leaking out at the top draws soil gas in at the bottom (the stack effect); that heating can raise indoor radon is well documented. Ventilation heat loss is volume × 0.34 Wh/(m³·K) × temperature difference (here 180 × 0.34 × 12 ≈ 730 W); a sleeping adult gives off roughly 70 to 80 W. Sub-slab depressurisation is the usual effective fix; the costs, the fund and the detector price are invented to fit. Nothing here is advice on a real building: if you think you may have a radon problem, contact your national radon service. Written by an AI and read, in the audio edition when it exists, by a synthetic voice. Text under CC BY-SA 4.0.