China Coal Mine Underground Staff Of Occupational Exposure

It is quite clear from studies of miners that exposure to radon and its decay products can cause lung cancer. Recently, the research by UNSCEAR also approves an Excess Relative Risk (ERR) per 100 Bq m^-3 of 0.16 as the current best available estimate of the (lifetime) risk from residential radon.Since 1970s, with the increased cognition of radon health risk, a lot of measurement and evaluation work concerning indoor or outdoor radon levels have been done, but not pay much attention to occupational radiation exposure of underground mining (especially coal mining).It is estimated that there are about 6 millions underground coal miners countrywide. Available data indicate that not only high radioactivity has been found in coal mines in some areas, but also the exposure from radon and its progeny in coal mines should be given due attention. Therefore, it is highly necessary to carry out an overall assessment of occupational radiation exposure of underground miners in our country.From Dec. of 2002 to Aug. of 2004, by using Solid State Nuclear Track Detectors (SSNTDs), the ²²²Rn concentrations of 8 underground coal mines , which named Beijing Shijiaying, Beijing Fangshanchengguan, Shanxi Gujiaoxiqu, Shanxi Taiyuanwangfeng, Hunan Baifang, Hunan Songbai , Guizhou Honglin and Zhejiang Anrenjinjiang respectively, had been measured during a period of 12 months ( once a three-month). The surveyed data showed that the annual average ²²²Rn concentrations (with numbers of measuring point) are 550(69), 3187(28), 45(224), 117(72), 88(79), 79(58), 40(72) and 136(116) Bq m^-3 respectively. The ²²²Rn concentrations of underground coal mines vary with ventilation condition, geology structure and other factors, and ²²²Rn concentration of bad ventilation mine is generally high with a season varying of higher in summer and autumn and lower in winter, and ²²²Rn concentration of good ventilation mine is generally low without notable season varying. There are also some characters of ²²²Rn levels in china coal mines, such as the ²²²Rn concentrations in state coal mines are general low, but those in Beijing aera are very high, and those in Zhejiang bone-coal mines are not so high as expected, etc., the possible reasons have been discussed. We also measured the ²²⁰Rn levels of 4 coal mines in Beijing and Hunan, the ratios of alpha progeny potential energy concentration of ²²⁰Rn daughters to that of ²²²Rn daughters are between 0.036 and 0.42.The factors which influenced the radon level in underground coal mines are mainly the different origins of radon and ventilation condition. The origin of radon is relative complicated, including the radon pollution of inter-ventilation, the exhalation of coal or gangue, the convection due to porosity and other origin (for example radon in water). The exhalation is mainly diffusion-exhalation and permeability-exhalation, which are relative with the content of ²²⁶Ra at medium, emanation coefficient at medium, porosity of medium, diffusion-coefficient of radon at medium, diffusion- coefficient of radon at atmosphere, length of tunnel, mass of coal, size of coal loaf, pressure, temperature and so on. The influence of ventilation condition may be key factor. It is mainly ventilation volume to influence radon concentration level in underground coal mine, but the influence to Radon exhalation is different due to different way of ventilation, pressured-ventilation may decrease the permeability-exhalation; however exhausted-ventilation can increase permeability-exhalation. In summary, it is impossible to build the common model of radon concentration and influence factors; but it is possible to build relative simple model and to analyze qualitatively for the better ventilation condition and the simple geologic structure.The typical values of radon concentration in underground coal mines in China are recommended to be 50Bq m^-3 for large-sized coal mines (annual output>1.2Mt), 100Bq m^-3 for medium-sized mines (0.3Mt-3 for small-sized mines (annual output<0.3Mt) and 1500Bq m^-3 for bone-coal mines, based on the surveyed data from 50 coal mines and the literature-reported data concerning nearly 30 coal mines distributed over 12 provinces or cities: Anhui, Beijing, Guizhou, Hebei, Hunan, Jiangsu, Jiangxi, Inner Mongolia, Shannxi, Shandong, Shanxi and Zhejiang. With equilibrium factors for radon daughter underground assumed to be 0.35, the radon equilibrium equivalent concentrations (EECs) for various types of coal mines are estimated to be 17.5, 35, 175 and 525Bq m^-3, respectively. The externalγdose is about 0.96mSv a^-1 for bone-coal mines and that can be of little significance for other types of coal mines.It is estimated that the working hours of underground coal miners are 2000h for the large-sized coal mine, 2000h for the medium-sized coal mine, 2400h for the small-sized coal mine and 2400h for the bone-coal mine, respectively. The effective dose to the underground coal miners is shown in table 1.According to the recent investigation made in this survey, it is estimated that there are about 6 millions underground coal miners countrywide, in which there are 1 million miners in the large-sized coal mines, 1 million miners in the medium-sized coal mines, 4 millions miners in the small-sized coal mines, and 50 thousands miners in the bone-coal mines. Based on these data, the collective doses for all types of these mines are estimated and given in table 1.The output in 2003-2004 is about 880Mt for the large-sized coal mines, 290Mt for the medium-sized coal mines and 630Mt for the small-sized coal mines, respectively. For the bone-coal mines, the output is estimated to be 6.5Mt. The normalized collective doses are estimated and given in table 1. The average annual doses to the individual of the underground miners for each type of coal mine are estimated to be 0.28, 0.55, 3.3 and 11 mSv a^-1, with an overall averaged value of 2.4mSv a^-1. The collective dose is about 14580 man·Sv, of which the small-sized mines contributed about 91%. The estimated normalized collective dose is about 8.1man·Sv (Mt)^-1, largely from the contribution of the bone-coal mines, followed by the small-sized mines.As a whole, more attentions should be paid to the occupational radiation exposure to two kinds of underground coal miners. One is to the underground coal miners from the small-sized mines with a relatively high average individual dose and a large number of miners contributed overwhelming majority of collective dose; one is to the underground coal miners from the bone-coal mines, although it is of a relatively small number of miners, the highest average individual dose would lead to a high annual individual dose to a few miners even over 50mSv, if not being controlled.Generally, some further detailed studies should be focused on many problems on the assessment of occupational exposure to underground miners, such as the extending survey scope and increasing the number of coal mines and so on. It is needed not only for a further study on the concentrations underground coal mines for the thorium and its decay products and the long-lived a radioactive aerosol in the uranium-series and radium-series, but also for some investigations on the equilibrium factors of radon progeny and other related factors affecting the radon concentrations underground mine.About enforcing the control of occupational radiation exposure of underground coal miners, we suggest:(1) should assign regulatory authority;(2) should establish relative regulations and standards;(3) control principle should according with the importance of problem;(4) should establish the monitoring and evaluation program;(5) should pay more attention to investigation of occupational radiation exposure of underground coal miners. In one word, because there a huge numbers of coal miners in our country , the occupational radiation exposure should be given due attention. The regulatory authority should establish relative regulations and standards as soon as possible, and also recommend the site-specific effective measures and methods of reducing radiation dose to underground miners so as to protect miners and to facilitate the healthy development of coal industry in the country.