K. Radon and its sources
Sources and health implications
Radon is a radioactive gas and is ubiquitous throughout the geosphere, biosphere and atmosphere (19) and occurs in several isotopic forms, however only two of these are found in significant concentrations in human environment: radon-222, which is a member of the radioactive decay chain of uranium - 238, and radon - 220 (thoron), which is formed in the decay chain of thorium - 232. Radon - 222 and its decay products provides the major contribution to the exposure of workers and of the general population. It is colourless, odourless, and inert (boiling - point, - 61.8°C), denser than air (density, 9.73 g/1 at 0°C and 760 mmHg) and fairly soluble in water (51.0 cm3 radon/100 cm3 water at 0°C; 22.4 cm/100 cm3 at 25°C; 13.0 cm3 at 50°C) (19). Radon substances are present in all surface soils and rocks, but in concentrations which vary regionally as a function of the relative abundance of the parent uranium. Among the range of radioactive substances, radon is unique in existing in a gaseous state under normal conditions. It is therefore capable of diffusing through soils, and to a lesser extent building materials, and thus entering the internal envelope of a building. The diffusion length is conditioned by its half-life of 382 days. Although radon is a gas, its decay products are not, and they occur either as unattached ions or atoms, or attached to particles (19). It is the decay of these less stable daughter products (P°-218, Pb-214, Bi-214 and P°-214, all with half-lives of less than 30 minutes) which is the probable cause of carcinogenic radiation associated with the gas. Risks are increased when there are high levels of particulates in indoor air, for example tobacco smoke. Smoking itself is synergistic with radon exposure in increasing lung cancer.
In the majority of cases, the most important source of radon in indoor air is infiltration from the ground beneath the building. Radon may also enter in the water supply, particularly if this is drawn from wells drilled in rocks such as granite. However, certain building materials may also constitute a significant source. These include natural stones, principally those of igneous or volcanic origin, and concretes which contain aggregates of similar origin. Some examples of unusually high emission rates have been found in mill tailings used as aggregates (Grand Junction, Colorado, United States of America), aerocrete based on alum shale (Sweden), certain granites (Aberdeen area, UK), phosphate slag (Alabama, United States of America) and phosphogypsum produced as a by-product of phosphoric acid generation (10, 11). Gamma radiation is also generated by radioactive decay and in many cases may contribute more than radon to the radiation dose received by occupants.
Phosphogypsum, an aqueous slurry of gypsum (calcium sulphate), is produced as a by-product of the manufacture of phosphate-based fertilisers. Global production of this material exceeds the demand for gypsum (by 105 to 92 million tonnes per year in 1981, (63)), and thus in principle could act as a source of this material without depleting natural reserves. However, the use of phosphogypsum is restricted by three factors (63): additional costs involved in drying and purification of the slurry; contamination by heavy metals such as cadmium and lead; and the presence of radioactive isotopes particularly radium 226, which because of its chemical affinity with calcium tends to become concentrated in the by-product slurry. Phosphogypsum thus has a higher radioactive content per unit mass than the original phosphate ore. However, ores from magmatic sources have a substantially lower content of radioactive isotopes than maritime ores (the principal source of phosphates), and the derived phosphogypsum is correspondingly less problematic. Other chemical processes which give gypsum as a by-product - such as fertiliser production using nitric rather than sulphuric acid, and the desulphurisation of flue gases - also give gypsum with a lower radioactive content (see table 14).
Table 14. Radioactive content of gypsum, in terms of radium equivalence
Source: Weterings, K. (1982). The utilisation of Phosphogypsum, Proceedings of the Fertiliser Society, No. 208, London.
The main uses of gypsum are in the building industry, with products such as plaster and plasterboard accounting for 57 per cent of production and a further 23 per cent being used as retarder in cement (63). There seems to be little prospect of an economically feasible method of reducing the radioactive content of phosphogypsum (64). The suggestion has been made that the radioactive isotope content of gypsum products could be kept within reasonable limits (perhaps 370 bq/kg) by blending gypsum from natural and chemical sources (63). Suitable non-construction uses for surplus phosphogypsum could include use as fillers in paper and plastics, and in roadworks.
The International Agency for Research on Cancer (19) has established that radon and its decay products are carcinogenic to humans. Raised lung cancer rates have been reported from a number of cohort and case-control studies of underground miners exposed to radon and its decay products. The effects of radon are largely attributable to the inhalation of its decay products.
Radioactivity of various materials
A large number of materials have been tested - particularly in Europe, United States and former USSR - and results reported in the literature (64). Table 15 gives mean values from a large number of samples for radioactive content of various Finnish building materials, in terms of radium equivalence. This attributes the following weighting to different radioactive isotopes (63).
Radium equivalence = 1× Radium226 + 0.08 × Potassium40 + 1.43 × Thorium232
In addition to radiation activity levels, radon emanation rates have also been tested. The relationship is not necessarily linear, since some materials have physical characteristics such as holes that increase their surface area and thus their emission rates. In general, concrete seems to be a more efficient emitter than other materials (65). Further attempts have been made to model the relationship between radon emission rates of various materials and consequent likely radon concentration, under normal conditions, inside buildings constructed of these materials. Table 16 gives results of a simple model applied to masonry materials in use in the UK (66).
Fly-ash (pulverised fuel ash) and blast furnace slag, as industrial waste materials, have in the past been identified as building materials with a potential for radiation emission. Evidence from the literature indicates, however, that neither is normally associated with any significant increase in radiation hazard. Table 15, based on Finnish research with a small number of samples, shows blast furnace slag to have a radioactivity concentration about 50 per cent higher than normal rock derived concrete aggregates (65), other research show variable results (64) but none sufficient to give cause for concern. The available data for fly-ash is similar (64). Table 16, based on UK work, shows that, in these tests, the estimated additional radon concentration, in a room, from using fly-ash blocks was only 15 per cent higher than for normal concrete blocks: this is still less than 10 per cent of a normal indoor radon concentration. A recently published paper, based on research in Hong Kong, confirms this finding (67): it concludes that the radon emanation rate from the surface of concrete blocks with 15 per cent fly-ash replacement of normal Ordinary Portland Cement is a bit higher than from normal concrete blocks, but the difference is not significant.
Table 15. Radioactive content of Finnish building materials in terms of radium equivalence.
Source: Mustonen, R. (1984). Natural Radioactivity and Radon Exhalation from Finnish Building Materials, Health Physics 46, 1195-1203.
Table 16. Radon concentrations in a standard room calculated from radioactivity of building material
Source: Spence, R. J. S., Cambridge Architectural Research Limited (UK), Building Materials and Health (Unpublished draft report prepared for the United Nations Centre for Human Settlements (Habitat), September 1994).
These figures assume that there are no wall coverings to impede radon emanation: actual rates are likely to be lower. Since a typical radon concentration in indoor air is likely to be around 15 bq/m3 (data for temperate regions of the world) (68), this suggests that building materials can contribute up to 20 per cent of indoor radon levels. In general, however, the proportion will be much lower - particularly in houses and ground-floor flats where radon infiltration from the soil beneath the building will tend to be the predominant source.
Dose-response relationship for radioactivity
The final column of table 16 gives an estimate of the annual radiation dose to an occupant of the building attributable to the materials under consideration. This model considers only the radiation dose due to the decay of radon daughters: gamma radiation may add from 170-260 μSv to total annual effective dose equivalent (66). The relationship between radon concentration and the amount of radiation absorbed in tissue has been investigated by several international bodies: United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) has derived the reference conversion factor of 0.061 μSv/year per bq/m3 (68). It is assumed that annual radiation dose is linearly related to the risk of developing cancer. This relationship has been developed from studies of lung cancer in uranium miners: a risk of 0.02 fatal cancers per person Sv is suggested.
Factors influencing exposure
Radon emission from building materials is generally constant over time. However, it is dependent on the moisture content of the material: emission is lowest when the material is very wet or very dry (10). The position of radon emitting materials within a construction influences the rate at which radon is released into the indoor air. The greatest risk is from an exposed surface of a radon-emitting material. A second factor influencing the build-up of radon in the indoor environment is the rate of ventilation. Recent emphasis on energy efficiency, together with modern construction practice and flue-less heating systems, has tended to reduce ventilation rates. A Swedish study, for example, has shown that average concentrations of radon daughters in indoor air has risen fivefold in the past thirty years (10). Nevertheless, monitoring programs so far have failed to show unambiguously that radon levels and air-change rate are related by an inverse function (69). Neither do radon levels correlate closely with the radioactive content of the soil on which a building is situated. The general conclusion drawn from a number of monitoring studies is that, while radon concentrations in energy-efficient buildings are higher than those in the building stock as a whole, this difference is of less than one order of magnitude and is highly variable from area to area.
Acceptable exposure levels
Radon concentrations in the open air are around 5-20 bq/m3. Indoor radioactivity levels have been monitored over a very wide range, from 3.7 to 3700 bq/m3 (69). The breadth of this range causes problems for the accuracy of monitoring, and is thus a cause of uncertainty in the discussion of radiation levels. Wide variations can be found within a single structure. Table 17 shows some permitted maximum levels. WHO recommended level for remedial action in buildings is 100 bq/m3 EER animal average.
The risk of cancer attributable to indoor radiation in general has been estimated at 10-100 cases annually per million population: no more than 5 per cent of the risk of fatal illness resulting from the use of tobacco (derived from figures given in (54)). This rate of incidence could perhaps be quadrupled by restriction of ventilation, giving a substantial increase in risk only in areas where indoor radiation levels are already high.
Table 17. Permitted maximum levels of indoor radon concentration
Source: Leslie, G. B. and Lunau, F. W. (1992). Indoor Air Pollution: Problems and Priorities Cambridge University Press, Cambridge, UK, and Mustonen, R. (1984). Natural Radioactivity and Radon Exhalation from Finnish Building Materials, Health Physics 46, 1195-1203
Radon concentrate mostly in the basements of air-tight buildings. Control measures are the extensive ventilation. Some of the techniques used to mitigate indoor radon levels due to infiltration from the ground can be useful in the case of radon-emitting building materials. For example, natural ventilation or cross-ventilation can be encouraged. Mechanical ventilation systems can be adapted to give a slightly positive indoor pressure, or at least to keep negative pressure differentials to the minimum required for efficient operation of the ventilation system (70). It is important to ensure that there are no stagnant zones where effective air mixing does not occur. It is also possible to use filtration systems to remove a proportion of radon daughters from the air by filtering out the particles to which they are attached (10). The effectiveness of this strategy in reducing total radioactive dose is however uncertain. For existing buildings, however, the most effective mitigation strategy is to isolate radon-emitting materials from the indoor environment. This could be achieved with a dense layer of internal render - provided the integrity of the layers is maintained. Moisture barriers and especially air-tight barriers such as polythene sheet can be incorporated in the wall construction. Special surface coatings have also been developed to inhibit radon emission (71). For example, one organic product (similar to an acrylic-based paint) is claimed to be nearly 100 per cent effective as a radon barrier, and has an inorganic equivalent which is 78 per cent effective. It should however be borne in mind that these coatings do not impede the emission of gamma radiation from the materials.
Substitute materials (63, 66)
Major radon emitters should be banned in construction work. Some of the substitutes are as follows below:
Natural stones: Stone with relatively high levels of radon emission tend to be of volcanic or igneous origin. Sedimentary stones such as sandstone or limestone, which are likely to have much lower levels of radon emission, are possible substitutes if they are available in the region. Alternative walling constructions could use brick, adobe, timber framing or concrete block.
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