Coal mining waste material (coal gangue) is a serious problem in all coal-producing countries, consuming land resource and posing occupational and environmental hazards. In China alone, coal gangue inventory reaches 4.5-5 Gt, with an annual increase of 0.37-0.55 Gt. Coal gangue is commonly stored as gangue hills and known for the inclination to spontaneous combustion. This study targeted the largest gangue hill with 100 m in height, 2000 m in foot perimeter and a combustion history over 50 years in Wuda coalfield, Inner Mongolia of China a global hot spot for underground coal fire. Lumex RA-915+ portable mercury analyzer was employed in the in-situ real-time measurement of exhaust gas at 46 accessible vents on the gangue hill slope, and a total of 2760 mercury concentration data showed an average mercury concentration of 5908 ng/m(3) (1022-31,750ng/m(3), n = 2760) in the exhaust gas, which is 12.7 times that measured in underground coal fire in the same coalfield. Moreover, an elevated mercury content (43.2 ng/m(3)) was observed in the near-surface air in the vicinity of gangue hill, which is 15-30 times of the atmospheric background level. This findings suggest that the coal gangue may also be an important contributing factor to the global anthropogenic mercury inventory. (C) 2016 Published by Elsevier Ltd.