Indoor radon levels in irbid and health risk from internal doses

Abstract

Radon gas is known to contribute a high internal dose to some people in some parts of the worlds. But globally, the average dose contributed from radon is estimated more than 40% of the total dose that some persons received. The risk concern has been resulted because of mortality increase due to lung cancer in Irbid (Jordan). Due to this, Irbid was divided into seven districts, namely: eastern, southern, northern, Zabdah, al-Barha, City center and al-Hussein area. About 678 radon monitors were distributed among semi-randomly selected houses (taking in considerations the age, number of floors, types of building materials and the inhabitants’ behaviors). The retrieved detectors were chemically etched and the track density $(\mathrm{tracks}{\mathrm{cm}}^{-2})$ was optically counted. The results obtained show that radon concentrations vary widely between districts $(7–230\mathrm{Bq}{\mathrm{m}}^{-3})$ and among different floors in the same district $(6–168\mathrm{Bq}{\mathrm{m}}^{-3})$. In spite of this, the mean values of radon in different regions were not widely scattered. The lowest value $(19\mathrm{Bq}{\mathrm{m}}^{-3})$ was found in al-Hussein area and the highest one $(83\mathrm{Bq}{\mathrm{m}}^{-3})$ was found in the southern district. However, the overall radon concentration in Irbid was about $44\mathrm{Bq}{\mathrm{m}}^{-3}$, which is around the national figure. Moreover, the lung cancer risk factor to the inhabitants of this city, according to some available models, was calculated about 0.32%.

Introduction

Natural radon isotopes (²¹⁹Rn, ²²⁰Rn, and ²²²Rn) are produced within the grains of the bed rock of the earth crust which contains the ores of uranium and thorium and their daughters in secular equilibrium. The ores’ concentrations are widely vary in the bed rock of the earth crust. Because of its short half-life (3.92s) and because of very low abundance of its forerunner, ²¹⁹Rn may be ignored entirely in the survey studies. However, the abundance of ²³⁸U in the earth crust is somewhat less than that of ²³²Th. But, the production rates of ²²⁰Rn and ²²²Rn are almost the same because ²³⁸U has shorter half-life and less abundance than ²³²Th. The half-life of ²²²Rn (3.82d) is much longer than that of ²²⁰Rn (55s) (Walker et al., 1984). Therefore, there is a great probability for ²²⁰Rn to decay in its cradle before finding its way out to the atmosphere. In consequence, the entrance rate of ²²²Rn to the environments is much greater than that of ²²⁰Rn. So, ²²⁰Rn and its daughters are normally neglected in studying the “radon problem” although their contribution is not trivial as that from ²¹⁹Rn.

Radon (²²²Rn) is a radioactive noble gas, colorless, odorless, and tasteless. It is a natural radioactive decay product of ²²⁶Ra, which is in turn a decay product of ²³⁸U. As ²³⁸U is a very widely distributed element in the earth's crust, hence, radon is a part of man's environment (Mogro-Campero and Fleischer, 1977). The health effect from radon comes from its short-lived daughters, which happened to be metallic radioactive nuclei. Two of these, ²¹⁸Po and ²¹⁴Po emit alpha particles with energies 6–7.78 MeV, respectively. If the decay process occurs inside the lung, the energy of decay will be deposited and absorbed in the tissue lining of the lung. So, living in an elevated level of radon concentration for a long time means that the probability of inducing lung cancer increases (NCRP, 1984).

Indoor air pollution has recently attracted a great deal of attention. With the trend toward reduction of ventilation and infiltration rates in buildings and with measured use of synthetic chemicals in the indoor environment, this problem has become even more serious. The subject of the indoor radioactivity evaluation due to radon in Jordan was the subject of many researchers. Al-Kofahi et al. (1992) studied radon level at Yarmouk University and its housing at Irbid city. Abumurad et al. (1997) estimated radon concentration in a few cities in Jordan, and Ismail et al. (1996) measured radon exhalation rates from building materials in Jordan. However, Irbid city is one of the heavily populated areas in Jordan. It was not a subject to a comprehensive serious radon survey before. Hence, this work is an attempt to estimate radon activity concentration, using plastic detectors (CR-39), in an area second in population after the capital of Jordan (Amman). Furthermore, this work will estimate the risk factor for radon induced lung cancer in Irbid region.