For models comparing indoor radon and (i)
eRa226 derived from airborne eU data and (ii) soil gas radon data, some of the geological groupings
have significant slopes. For these groupings there is reasonable agreement in slope and intercept
between the three regression analysis methods (LS, TS and WTLS). Relationships between radon in
dwellings and radium in the ground or radon in soil differ depending on the characteristics of the
underlying geological units, with more permeable units having steeper slopes and higher indoor
radon concentrations for a given radium or soil gas radon concentration in the ground. The
regression models comparing indoor radon with soil gas radon have intercepts close to 5 Bq m-3
whilst the intercepts for those comparing indoor radon with eRa226 from airborne eU vary from
2
about 20 Bq m-3
for a moderately permeable geological unit to about 40 Bq m-3
for highly permeable
limestone, implying unrealistically high contributions to indoor radon from sources other than the
ground. An intercept value of 5 Bq m-3
is assumed as an appropriate mean value for the UK for
sources of indoor radon other than radon from the ground, based on examination of UK data.
Comparison with published data used to derive an average indoor radon : soil 226Ra ratio shows that
whereas the published data are generally clustered with no obvious correlation, the data from this
study have substantially different relationships depending largely on the permeability of the
underlying geology. Models for the relatively impermeable geological units plot parallel to the
average indoor radon : soil 226Ra model but with lower indoor radon : soil 226Ra ratios, whilst the
models for the permeable geological units plot parallel to the average indoor radon : soil 226Ra model
but with higher than average indoor radon : soil 226Ra ratios.