B.K. Sapra

A model to predict radon exhalation from walls to indoor air based on the exhalation from building material samples.

In recognition of the fact that building materials are an important source of indoor radon, second only to soil, surface radon exhalation fluxes have been extensively measured from the samples of these materials. Based on this flux data, several researchers have attempted to predict the inhalation dose attributable to radon emitted from walls and ceilings made up of these materials. However, an important aspect not considered in this methodology is the enhancement of the radon flux from the wall or the ceiling constructed using the same building material. This enhancement occurs mainly because of the change in the radon diffusion process from the former to the latter configuration. To predict the true radon flux from the wall based on the flux data of building material samples, we now propose a semi-empirical model involving radon diffusion length and the physical dimensions of the samples as well as wall thickness as other input parameters. This model has been established by statistically fitting the ratio of the solution to radon diffusion equations for the cases of three-dimensional cuboidal shaped building materials (such as brick, concrete block) and one dimensional wall system to a simple mathematical function. The model predictions have been validated against the measurements made at a new construction site. This model provides an alternative tool (substitute to conventional 1-D model) to estimate radon flux from a wall without relying on ²²⁶Ra content, radon emanation factor and bulk density of the samples. Moreover, it may be very useful in the context of developing building codes for radon regulation in new buildings.

Deposition-based passive monitors for assigning radon, thoron inhalation doses for epidemiological studies

The International Commission on Radiological Protection dose limits for radiation protection have been based on linearly extrapolating the high-dose risk coefficients obtained from the Japanese A bomb survivor data to low doses. The validity of these extrapolations has been questioned from time to time. To overcome this, epidemiological studies have been undertaken across the world on populations chronically exposed to low-radiation levels. In the past decade, the results of these studies have yielded widely differing, and sometimes, contradictory, conclusions. While recent residential radon studies have shown statistically significant radon risks at low doses, high-level natural radiation (HLNR) studies in China and India have not shown any low-dose risks. Similar is the case of a congenital malformation study conducted among the HLNR area populations in Kerala, India. It is thus necessary to make efforts at overcoming the uncertainties in epidemiological studies. In the context of HLNR studies, assigning radon and thoron doses has largely been an area of considerable uncertainty. Conventionally, dosimetry is carried out using radon concentration measurements, and doses have been assigned using assumed equilibrium factors for the progeny species. Gas-based dose assignment is somewhat inadequate due to variations in equilibrium factors and possibly due to significant thoron. In this context, passive, deposition-based progeny dosimetry appears to be a promising alternative method to assess inhalation doses directly. It has been deployed in various parts of India, including HBRAs and countries in Europe. This presentation discusses the method, the results obtained and their relevance to dose assignment in Indian epidemiological studies.

Variation of the aerosol charge neutralization coefficient in the entire particle size range

The rate at which the mean charge on aerosol particles relaxes to its steady-state value under bipolar charging is characterized by the neutralization rate constant, p (s-l). It is an important parameter for fixing the nt product in charge neutralizers as well as in the theory of charging-induced diffusion. Here we compute the neutralization coefficient, P/n (where n is the mean ion density), as a function of particle size through the use of ion-particle combination coefficients provided by the recent theories. The results indicate that p/n decreases from a continuum limit value of 3.1 x 10m6 cm3 s-l, to a free molecular limit value of 1.4 x 10m6cm3 s-l, The changeover occurs rapidly in the transitional regime (lo-100 nm). This clearly indicates that the nr product required to attain steady state is higher for nano particles than for larger ones. The paper also presents the variations of the mean square variance of charge, the coefficients of charging-induced drift and diffusion, as a function of particle size. Copyright 0 1996 Elsevier Science Ltd

Image forces on a collection of charged particles near conducting surfaces

Abstract In a recent study, Vauge (2002) showed that particles in a charged aerosol system confined to a conducting cavity do not experience image forces provided the aerosol is treated as a continuous distribution in space. However, in reality, since an aerosol is a collection of discrete charges and not a continuum, we propose that the collective image–particle interactions have to be analysed using statistical techniques based on configuration averaging. Two cases are considered: (i) charged particles confined to a conducting spherical cavity and (ii) those distributed outside a conducting sphere which is insulated from the ground. It is shown that in both the cases, a given charged particle experiences an average attractive electric field due to its image charge and the average field due to the images of other charged particles present in the aerosol vanishes. This arises because the position of the particle is delta correlated with the position of its image charge, whereas it is non-correlated with the positions of the image charges of other particles. The proposition of Vauge conforms to the case of an image field at an arbitrary point and not to that on a charged particle. The study also investigates, for the first time, the root mean square fluctuations in the electric fields arising due to particle–particle and image–particle interactions. However, for the charge densities normally encountered in aerosol systems, these contributions are found to be quite small.