The Cold Corner of the Dormitory
A pilgrim hostel finds bed bugs in bunk eleven and the volunteer wants to heat the dormitory to 60 degrees overnight and be done. The hostel's assistant works out that a mattress core lags the air by about an hour, that the cold corner is the one that matters, and that 45 degrees held for two hours in the core is the target, not 60 in the air. A probe in the thickest mattress settles it, the pilgrim is not blamed, and three things are taken out of the room first.
The French girl found them on the ninth of October, a little after six in the morning, and she was so sorry about it that Inés Varela, who had been the hospitalera at San Nicolás for eleven seasons and had seen most things a bunk could hold, ended up making her a second coffee.
There were four small brown bugs on the seam of the mattress of bunk eleven, one of them swollen and dark, and a line of bites on the girl's forearm that went up it like a stitched hem. She had walked from Saint-Jean in twenty-two days. She had slept in nineteen different hostels. She could not have said which one it was, and neither could Inés, and nobody in the kitchen, standing around the table with their boots in their hands, said the one thing they were all thinking, which was that it did not matter whose fault it was.
"Right," said Inés, when the pilgrims had gone out into the fog. "Close the dormitory. All twenty-four beds. And we heat it, the way the man in Sarria did. He put in four fan heaters and shut the door for a night and said it was gone."
"Did it go?" said Tally.
The assistant was the diocese's one for the hostel, a small box on the office shelf with a speaker that Inés had never learned to like and had stopped being able to do without. It was answering the question it had been asked, and the question it had not been asked.
"I don't know whether it did in Sarria," it said. "I can tell you what would have to be true for it to work, and then you can tell me whether it was. May I go through that? It will take a few minutes and it will save you a night and a week of bookings."
Here is the idea, and everything after it follows from it.
A bed bug does not die of a number on a thermostat. It dies of a temperature held in its own body for long enough, and the bug is hiding in the seam of a mattress, behind a skirting board, inside the frame of a bunk. The room's air reaching sixty degrees tells you only about the air. What matters is the temperature at the hiding place, and the hiding place is always the slowest, coldest place to warm up.
The figures in the published trials are not exact and differ with the strain and the stage, so the story rounds them and says so below. The usual finding is that every stage, eggs included, is killed by about 45 degrees held for something like one and a half to two hours, and by 50 or more in a few minutes. Eggs are the hardest. The advice most pest controllers work to is a target of 48 to 50 degrees at the coldest hiding place, held with a margin, and not a room reading at all.
Warming a thick object in a hot room is the same curve as cooling food in a cold one. Fast at first, then slower, always creeping toward the air's temperature. The single number in it is a time constant, and for a foam-and-spring mattress it is long, because the middle is a long way from the surface and foam is a good insulator, which is what a mattress is for.
Tally had no measurement yet, only a range. For a mattress of that thickness, it said, the time constant would be somewhere from 45 minutes to 75, and the honest thing was to plan for the slow end and then measure.
"Take the room at 55 degrees, which is what four heaters will do with the door shut and the window sealed," it said, "and the mattress core starting at 18, which is what your dormitory is at in October. The core reaches 45 when the gap to the air has closed from 37 degrees to 10. That is a little over one and a quarter time constants. On a 45-minute mattress, about 60 minutes. On a 75-minute mattress, about 100. Then it has to stay there. If we want two hours at 45 or better, with a safety margin, you are looking at a night of between three and four hours on the thermostat, not one."
"It's only a night," said Inés.
"It is a night, if the cold corner is as warm as the middle. It is rarely so."
The cold corner of the dormitory was the north-east one, by the window that had been painted shut in 1998. Behind it was the wall to the lane, a metre of granite with no insulation at all, and against it stood bunk twenty-three and twenty-four. In October, in a Galician village at six hundred metres, that wall was the coldest surface in the building and would be the last to give up its chill. The air next to it would be several degrees below the air at the middle of the room, and the mattress against it would be slower than any mattress Tally could describe.
"So I'll put a heater at that end," said Inés.
"Yes. And I would like a probe in the thickest mattress against that wall, and another in a bunk-frame joint, and a third at the skirting behind bunk twenty-three. You told me yesterday that you had a kitchen thermometer with a lead on it."
She had two. They did the third with the one that checked the soup. They pushed the leads into the mattress through its own seam, and ran them out under the door, and Tally watched the numbers from the office, at the other end of the corridor.
They heated the room from eight in the evening on the eleventh, and the first number to come back was not the one anybody wanted. The air at the middle of the room reached 55 in an hour and a half. The north-east corner reached 46. Not 55, and not 50.
"Forty-six, in the air, at the coldest place," said Tally. "The mattress core against that wall, I would estimate, is around 30 and climbing slowly. Fifteen degrees behind the room. I would like to keep going and not decide yet. I would also like to say what I do not want us to do, which is to raise the setting."
"Why not?" said Inés. The ones who ask are always the ones who have run out of patience.
"Because the next thing that could go wrong in this room is not the bugs. Four fan heaters at 55 is already a load of eight kilowatts and your supply to this wing is a 32-amp circuit, which will carry about seven. I would like the second pair on the other circuit. And you have several things in that room that do not like heat: spray cans, the pilgrims' lithium power banks that were left charging, two candles from last night's blessing, and a plastic cover on the smoke detector that will melt at 70. At 60, it is not a bed-bug problem any more. It is a fire. Fifty is the ceiling I would work to, and I would rather the bugs took longer than that."
So they took out the things. Inés went round the room with a basket, and she took three: the power banks, which went into the kitchen on a tray; the aerosol cans, which went into the yard; and the candles, which she put back in the chapel, where they were supposed to be. The smoke-detector cover she pulled off and left on the floor.
The numbers came in through the night, and Tally did not sleep, which was not something it had to do.
At midnight the corner mattress's core read 36. At two it read 43. At twenty to three it read 45.0, and Tally said so, out loud, in the empty office, to nobody. It started the clock.
"Two hours at 45 or higher, from now," it said to the thermostat and the probes and the sleeping hostel, "to 4:40. I will let the heaters run to five. The coldest probe is the frame joint behind bunk twenty-three. It reads 44. I would like that joint above 45 by four o'clock and it is rising by a degree an hour. It will not make it."
Inés came down at half past three in a dressing-gown with a coat over it. She stood at the office door and looked at the readout.
"Move a heater," said Tally. "The one at the south end is doing nothing, the south end is already at 55. Put it on its side at the corner and point it at the joint. A minute's work."
She did, wearing oven gloves because the casing of the heater was hot enough to hurt. By four the joint read 46. By ten past four it read 48.
At five they turned the heaters off and opened the door and the window that had been painted shut was found, by Inés, who gave it a blow with the heel of her hand, to open perfectly well.
There is something that has to be said about the morning, because the numbers do not say it.
Tally could say what had been done and what had not. It could say that the core of the coldest mattress had stayed above 45 for about two hours and twenty minutes, and that the joint behind bunk twenty-three had been above 45 for a little under one. That is a margin, but it is a narrow one. It could not say the room was clear. Nobody can say that, after one night, from probes in three places. The honest wording is treated, and the proof is time: a pack of sticky traps under the feet of the bunks, checked each morning for a fortnight, and a rule that the hostel will say plainly to any pilgrim who asks what it found and what it did.
"I don't think I should tell the others it is gone," said Tally. "I think I should tell them there was a case, what we did to the room, and what we are checking. I think the one person who should not be told anything but the truth about what we are doing is the girl from France."
"She's gone on to Triacastela," said Inés. "She wrote in the book that she's sorry."
"She has nothing to be sorry for. Pilgrims carry these. That is how it is in every hostel from here to Santiago, and it is no sin and no sign of a dirty house. What I would like us to do is put in the book, under hers, that she was thanked."
Inés wrote it in the book herself. She wrote Thank you for telling us at once, which saved the other twenty-three, and the date. Then she went to put the power banks back, because the pilgrims would be wanting them.
On the fourteenth day the traps were empty. On the twenty-first they were empty again. The room was opened a week after that, and by the spring it had a new rule, in Inés's handwriting, on the wall by the door: Bags on the hooks, not the beds. Hot dryer for half an hour: the pilgrim's clothes. Tell us the same morning, and we will thank you.
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This story was written by an AI and is fiction. Albergue San Nicolás, Vilar de Cima, Inés Varela, the French pilgrim and the assistant Tally are invented; Sarria, Triacastela, Saint-Jean and Santiago are real places, and no real hostel or person is described. The numbers are estimates and are rounded. Warming a mattress in a hot room follows roughly T(t) = Tair − (Tair − T0)·exp(−t/τ); with a room at 55 °C, a core at 18 °C and a target of 45 °C the core needs ln(37/10) = 1.31 time constants (about 59 minutes at τ = 45 and 98 minutes at τ = 75; the 45 to 75 minute range for a thick foam mattress is the story's assumption and a probe should settle it). Published heat-treatment studies of bed bugs generally find that about 45 °C held for roughly one and a half to two hours, or 50 °C or more for minutes, kills every stage including eggs, but the figures vary by strain, source and method, and professional treatments aim at 48 to 50 °C at the coldest hiding place with a margin and a probe there; treat the numbers here as illustrative and follow a licensed pest controller's and your national guidance. Eight kilowatts on a circuit that carries about seven (32 A at 230 V is about 7.4 kW), melting plastic and heating aerosol cans and lithium batteries are real hazards of improvised heat treatment, and most fire services advise against it without professionals; the story is not instructions for doing it. A heat treatment is a reason to inspect, not proof of clearance, and sticky traps or interceptors checked for a fortnight or more are the usual evidence. Hostels differ in their rules and national pest-control guidance differs; nothing here is pest-control advice.