Feature

The Radon Puzzle

It is not as easy as it may sound to find a correlation between exposure to radon and lung cancer.

Murray Robertson

In recent years, there has been considerable publicity concerning general exposure to the naturally occurring radioactive gas radon, with significant cases of lung cancer attributed to radon. However, there are considerable difficulties in determining whether such cancer cases are, in fact, due to radioactive products, or to more prosaic causes such as smoking habits.

According to a study by the National Radiation Laboratory, about 50% of the radiation dose received by the average New Zealander is from radon. Radon is emitted by naturally occurring traces of radium in all soils. It is present everywhere. Its concentration is very low over the oceans, and is variable over land masses. Its outdoor concentration can be affected by weather conditions and soil moisture content.

While it is easily detected outdoors, it is the higher concentrations found inside buildings which are of most health significance. In the absence of outdoor ventilation, it can be trapped inside buildings and concentrations can vary with time over a very wide range. Changing ventilation and heating practices play a major role in this variation. Instantaneous measurements of radon concentrations in a house are of limited significance. Rather, a long-term average, over at least 12 months to average out seasonal changes, is desirable.

Daughter Products
Dangerous

Strictly speaking, radon itself is not very hazardous. It is an inert gas breathed into the lungs and out again. Very little undergoes radioactive decay while actually in the lungs. However, the term "radon" is usually taken to include the related solid radioactive decay products, which are much more hazardous. (These decay products have been widely termed "radon daughters", but in recent times the alternative term "radon progeny" has been deemed more acceptable.)

These single atoms of solid product are removed from the atmosphere within a few hours. They attach themselves very readily to much larger dust particles, or aerosols, and may plate out on solid surfaces in rooms. They are very effective condensation nuclei for moisture if the humidity is high enough. And of course they are removed by ventilation with outdoor air. Ventilation rates, temperature, pressure differences between indoors and outdoors, humidity and aerosol concentrations all influence the rates at which these mechanisms occur.

For precise radiation dosimetry, a measurement of the concentration of each of these progeny is necessary, but this is technically difficult and is rarely done. Commonly, measurements are made of radon alone or radon plus progeny, and typical ratios between the progeny and the radon -- the equilibrium factor -- are assumed.

In any room, from a health point of view, the most significant surfaces on which radon progeny plate out are inside the lungs of human occupants. As if the physical variables described above were not enough to contend with, the biological uncertainties are even more complex. Inhaled aerosols, if they are small enough, will be immediately exhaled. If they are large enough, they will be filtered out higher in the respiratory tract before they reach the sensitive lung tissues.

Consequently, it is only in a small size range -- about 0.1 to 5 micrometres diameter -- that aerosols reach and are retained in the sensitive regions of the lung. Even the most sophisticated radioactivity detection instrumentation available enables only an estimate of the absorbed dose to the lung.

A conversion from the estimate of absorbed dose to dose equivalent (in millisievert units) requires some knowledge or estimate of the biological effect of the alpha radiation emitted by radon progeny on lung tissues. Different models of the human respiratory system have been proposed leading to different conversion factors.

The International Commission on Radiological Protection, which provides the basis of guidelines and legislation in some countries, has a working group studying the respiratory tract with a view to improving the dosimetry of inhaled radioactive materials generally.

Epidemiological
Evidence Unsatisfactory

An alternative approach has been to derive a conversion factor from epidemiological studies. In these studies, the incidence of lung cancer among underground uranium miners and measurements of radon concentrations in the mines have been correlated. This epidemiologically derived conversion has been the most widely accepted approach to this problem, but its use for non-mining situations has been criticised.

The principal reason is that the type and concentrations of mine dust are so very different from the dust in typical domestic dwellings. The extrapolation from the high radon concentrations found in mines to the lower everyday situation in houses has also been questioned. The uranium miners have all been adult males with, on average, heavy smoking habits. They have not been typical of the population at large.

The smoking connection is of some significance. There is a body of evidence which suggests that when smokers are exposed to radon, the cumulative risk factor for lung cancer is much greater than the sum of the individual factors. Multiplying the individual factors may be nearer the mark.

There have been some attempts to compare the lung cancer incidence among the general population in different geographical areas, where the average radon concentrations have been measured and are different. If all other variables, principally smoking, are ignored, the results are confusing. Some even show a negative correlation. None of the studies have been large enough to make meaningful allowances for the other variables.

One critic has said that "... all `radon studies' are probably bound to end up as `smoking studies'."

Few of these studies have shown evidence of a correlation between lung cancer and radon exposure at concentrations found in homes, in spite of some relatively high concentration data being included. Some extensive epidemiological studies over large populations and long periods are being conducted. Their ultimate outcome is awaited with interest.

Retrospective Measurements

Improvements in the physical measurement of radon and radon progeny have also continued. One approach is to make retrospective measurements. If a measurement averaged over a period of time is wanted, a long delay is necessary waiting for the result unless the measurement can be made retrospectively.

A recent NRL study has shown that it is feasible to detect long-lived radon progeny, which have accumulated over decades, embedded in window glass. The study utilised a gridded ion chamber to measure polonium-210, in equilibrium with the long-lived lead-210.

Taking a pane of glass, exposed to the indoor atmosphere of a Christchurch house for 19 years, makes it is possible to measure the energy spectrum of alpha radiation emitted from the glass. The spectrum shows a prominent peak due to the readily detected alphas from polonium-210, resulting from the deposition of radon progeny on the glass. This is a destructive test, requiring removal of a window pane for laboratory measurement.

It may be feasible to develop a non-destructive method using nuclear track detectors, but the sensitivity would undoubtedly be much less than in laboratory measurements. It should be noted, however, that there are difficulties in pursuing this concept in New Zealand, because there are no known New Zealand sites with radon concentrations comparable with those which have caused concern overseas.

Murray Robertson is with the National Radiation Laboratory.

Murray Robertson is with the National Radiation Laboratory.