Handbook of Radon.
55. The influence of house occupancy, weather and building design on indoor radon levels.
It is obvious from basic building physics that radon levels in naturally ventilated houses may vary. In practice, average levels in some houses are remarkably constant, but simply choosing an adjacent room for test can make a substantial difference to the result.
Sometimes the difference between rooms can be as great as a factor of 10, and exceptionally a factor of 40 or more. This is easy enough to rationalise once typical UK houses are compared to American or Swedish designs with central air distribution systems, but see Section 56.
Some interesting questions arise. For example, over which standard year or other set of conditions should radon levels be normalised if it is desired somehow to 'classify' houses as to their radon levels?
Average radon levels can vary from room to room both over short periods of time (hours and days) and over many months.
In mid-summer, windows may be open much of the time, and a low reading may be obtained. Measurements during an unusually severe winter will probably produce a higher result if open fires are used in a moderately airtight house. Thermally massive (heavyweight) buildings with poorly insulated floors are often cooler inside than out during mid-summer conditions - a consequence of thermal mass and thermal coupling to the ground, which maintains a broadly constant temperature. Thus, the pressure gradients that induce radon entry are reversed, and with possibly a large effect upon reported radon levels - see Section 8.
Differences can also be due to how the house is used (the behaviour of its occupants) and thus the same house may exhibit different average radon concentrations when lived in by different families. This fact alone militates against any system of classifying houses as 'safe' or 'dangerous', except of course for the mass of houses that could never be induced to exhibit high indoor radon concentrations however the occupants chose to live.
A good understanding of the facts outlined in this Section could be recommended to those officials who have advocated public registers of 'affected' houses, since both property values and saleability might be much determined by unrepresentative radon measurements.
In reality, a house that registered 150 Bq/m3 in one 3-month period could register either 80 or 300 Bq/m3 the next, even when the results have been corrected to annual averages. (This correction to an annual average is also dubious for any one house.) Short term measurements in the same house might produce results between (for example) 20 and 600 Bq/m3.
Rather than attaching too much importance even to long term results (or to whether a house appears on one test to be above or below 200 Bq/m3) it would be no more than a recognition of reality to admit that domestic radon measurement is and must remain an inexact science (see Section 36 also).
KEY FACTS:
There is no such thing as a UK house at a fixed radon level, and especially not amongst houses that are moderately or severely affected. Measurement of radon levels in different rooms in the same house can produce genuinely very different answers, even if the measurement devices are reasonably accurate.
Many houses should not be classified simply as 'above 200 Bq/m3' or 'below 200 Bq/m3', and especially not on the basis of a few measurements in only a couple of rooms. In the context of broad area statistics such measurements are entirely adequate, but the possible uncertainties in respect of individual houses should be admitted.