Granite, cancer, and windows

Presentación SFU
SFU slides (Spanish)
Tradución ao galego

A few months ago, I moved back to Spain to work at the Geography department of the University of Santiago de Compostela, at the NW of the Iberian Peninsula. The geological basement in this area is dominated by Variscan granites containing Uranium.

The decay chain of Uranium produces Radon-222, a radioactive noble gas that escapes the bedrock and accumulates inside the buildings, producing lung cancer.

Uranium-238 decay chain diagram (modified from Wikipedia)

The department of Geography is located in a historic building in the old town of Santiago with high levels of Radon accumulation.

Geography and History School in Santiago de Compostela (Wikipedia)

Motivated by a desire not to die of lung cancer and a certain scientific curiosity, my office mate and I decided to buy a device that would measure the radon concentration in the office, and any other places. We finally got an Airthings Wave Plus, a relatively inexpensive device that detects radon gas through alpha spectrometry.

According to Airthings, the radon sensor consists of a passive diffusion chamber, which allows air samples to flow into it. There is a photodiode located inside this chamber, which essentially counts the amount of “daughter” radon particles in the air sample.

The device produces a 24-h rolling average of the radon concentration every hour, but this was not answering the questions we were asking:

  • Are the Rn levels below 300 Bq/m³ now?
  • For how long and how often do we need to open the window to keep a safe Rn concentration?

So, we developed a methodology to use this device and our experimental data (open/close window times) to answer these questions:

The RadonSim tool can be downloaded from GitHub. It fits a simple model of radon accumulation/loss (thin blue line) according to the recorded ventilation pattern (top blue) to the data provided by the detector (thick black line):

9-day data and the fitted model.

The results show that the radon is eliminated by ventilation much faster than the accumulation rate. This means that, although radon accumulates in our office well above safe levels, it is possible to maintain a safe concentration by opening the window for a few minutes every 12 hours. So we just need to air the room one hour in the morning and work safely for the rest of the day.

Lung cancer mitigation measures in our office.

However, the behaviour of the radon is not homogeneous across the building, and our initial results can’t be extrapolated to other offices and lecture rooms. Although ventilation always helps to drastically reduce the concentration of radon in air, the distribution of the concentrations does not always follow the expected patterns:

Radon concentrations before ventilation:

Both highest and lowest concentrations correspond to rooms located on the ground floor!

Radon concentrations after 2 hours of ventilation:

The office located at the second floor shows the lowest after-ventilation radon concentrations.

Future research needs to be done to improve the ventilation testing pattern, understand the movement of the radon gas in buildings, and assess the reproducibility of the results through time.


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