Handbook of Radon.
10. The quantity of radon in UK houses.
Amongst the more enduring of folk tales about radon is that it is a dense gas that settles in a layer at floor level.
People sometimes ask whether they should prevent children playing or sleeping on the floor, believing that the concentration at table height will be markedly lower. In fact, radon is rapidly dispersed from cracks in the floor and other entry points into room air, and the quantities are extremely small - so small that they cannot readily be imagined. There is no layering effect because of this small concentration: the density of soil gas (if composed only of air and a little radon) is no different from that of room air at the same temperature. In any case, fresh radon presents little risk if inhaled and complete mixing of soil gas and room air will occur well before equilibrium numbers of radon daughters are formed.
How much radon in an affected home?
The concentration of chemical pollutants is often expressed as parts per million (ppm). One ppm is about a large cupful compared with the volume of the average house. Efforts are made to detect and control highly toxic chemical pollutants to parts per billion (ppb).
(A billion is here defined according to common usage as a thousand million. The English definition of a billion as a million million is now recognised as obsolete). One part per billion (ppb) is about five small drops of water compared with the volume of the average house, or 250 mm3 in 250 m3.
However, radon gas is present typically as ten parts in a billion billion (10 in 1018) in an affected house. It is difficult to conceive of such a small volume. It is about 1/100,000 the size of the ball in a ballpoint pen - as small as the tiniest speck of dust. However, even this minute volume of radon still contains nearly seventy billion atoms.
If all the radon that was present at any one time in all the 20,000 worst affected houses in the UK were to be collected together there would be insufficient to sit comfortably in the eye of a sewing needle.
If radon was a 'chemical' contaminant (like substances in tobacco smoke or pesticide residues in food), it would be difficult if not impossible to detect such a low concentration.
Despite that volumes of radon in houses are unimaginably small, radon is highly radioactive, and that is what sets it apart from other pollutants of the indoor environment.
Some useful numbers.
Avogadro's number (atoms or entities in a mole) = 6.02 x 1023. One mole of a gas will occupy around 22.5 litres under normal conditions.
Half life of radon = 3.82 days (92 hours).
Decay constant of radon = 0.0076 h-1 (sometimes called decay rate)
Basic equation for radioactive decay. dN/dt = -w N (w = decay constant)
At 2000 Bq/m3, dN/dt = 2000 x 3600, so N equals about 1 billion, or one million atoms of radon per litre. However, one litre of a gas contains in total around 2.6 x 1022 atoms or molecules. Thus the radon concentration is about 1 atom in every 2.6 x 1016. Devotees of arithmetic should now calculate the volume of a curie of radon, and estimate the activity concentration (in Bq/m3) in a house containing this quantity.