Assessing radon risk in your home

Corentium Home Radon Detector

Measuring radon levels in indoor spaces needs a device such as the one illustrated which performs continuous monitoring. This is because levels can and do fluctuate considerably. While the short-term figure goes up and down from day to day, and to a lesser extent from week to week (the display shows alternating 1-day and 7-day figures), the long-term average tends to be quite steady and undulates only gently with the seasons. This is the figure I use to assess radon risk in accordance with official guidelines.

What is radon and where does it come from?

Radon is a chemical element that exists as a colorless odorless radioactive gas. It is not detectable by human senses, but its presence can be measured by radiation monitoring devices designed to detect it. Radon is formed wherever uranium-containing mineral deposits are found. Uranium-238 atoms undergo progressive radioactive decay to atoms of a lower mass number, eventually becoming radium-226. Up to that point, all the decay products are solids and simply coexist with uranium in the mineral. But radium-226 decays to radon-222, which is a gas. Like all gases, it diffuses into spaces available to it and so can leave the uranium deposit and come up from below and into your home.

How does radon accumulate in air spaces?

Radon seeps into air spaces through conduits, joints and cracks; it is also exhaled by diffusion through surfaces. Having infiltrated an air space, some of the radon will escape, either through back-diffusion or infiltration into adjacent spaces. At the same time radon undergoes decay, emitting a nuclear fragment called an alpha particle. A dynamic is thus set up in the air space between ingress, escape and decay, with accumulation occurring when the rate of ingress is greater than the combined rates of escape and decay. This will more easily be the case in relatively undisturbed and unventilated spaces such as a cellar or storeroom.

Radon infiltrating homes consists largely of the isotope radon-222. It decays into polonium-218 and polonium-214, both of which are alpha particle emitters adding significantly to the radiation dose if radon is inhaled.

Are alpha particles harmful?

Exposure of the external surface of the body to alpha particles is not considered hazardous due to their lack of penetrating power, but inside the body they can be very harmful. If radon is inhaled into the lungs the alpha particles emitted there can cause serious damage to sensitive living tissue. This is why it’s important to test radon levels in your home and if necessary take action or seek advice on mitigation.
Fact source: US Environmental Protection Agency (https://www.epa.gov/radiation/radiation-basics)

What does Bq/m3 mean?

Bq/m3 stands for ‘becquerels per cubic meter’ which means the number of radioactive decays per second in a cubic meter of air. The unit is named for French dude Henri Becquerel (1852-1908) who was among the first to study radioactivity. In the radon detector an average of 8 Bq/m3 means that in a cubic meter of air, 8 radon atoms decay every second, each emitting an alpha particle.

What does pCi/L mean?

pCi/L stands for ‘picoCuries per liter’ and is an alternative way of measuring radioactive decay rate per unit volume. One pCi/L is equivalent to 37 Bq/m3. The unit is named for Polish born Maria Sklodowska-Curie aka Marie Curie (1867-1934) who co-discovered radium. She was the first woman to win a Nobel Prize, the first person to win a Nobel Prize twice, and the only person to win a Nobel Prize in two scientific fields.

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More about alpha particles
(for the technically minded)

An alpha particle, which is identical with the nucleus of a helium atom (2 protons, 2 neutrons) is emitted from the nucleus of the radon-222 atom when it decays. The range of alpha particles is only a few centimeters in air but they are ejected at high speed – 12000 km/sec or 7500 miles/sec.

This raises two questions:
1) What determines the actual moment when the radon atom decays?
2) Where does the energy come from to emit an alpha particle at such speed?

The answer to the first question involves an aspect of probability theory called the Poisson distribution and is described in a post on this blog which can be seen here. The answer to the second question involves the famous Einstein equation E = mc² where E is energy, m is mass and c is the velocity of light. When radon-222 decays, the daughter nucleus and the alpha particle are found to have a collective mass which is a tiny fraction less than the mass of the parent nucleus. This minuscule amount of mass – just 1 x 10^-31 kg – is converted into kinetic energy in accordance with Einstein’s mass-energy relation, and is sufficient to give the alpha particle its large velocity and a kinetic energy of about 5.49 MeV (8.8 x 10^-13 joules).

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P Mander July 2023