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The Wuda Coalfield, Inner Mongolia suffers from serious coal fires for more than half a century. Fire-extinguishing projects have been carried out to suppress the coal fires since the last decade, but sporadic surface fires still occur and underground fires are more prevailing. Here, we used a real-time RA-915M Mercury Analyzer with modified inlet to monitor gaseous Hg concentrations in fumes emitted from boreholes that were designed to detect and control the underground coal fires. Meanwhile, offline methods were used to collect the fumes and analyze the contents of the gases including CO, CO 2 , CH 4 , C 2 H 6 , C 2 H 4 and C 2 H 2. The results showed that gaseous Hg concentrations in fumes from boreholes ranged from 6.42 ± 0.73 to 123.53 ± 34.66 ng m−3, with an average value of 49 ± 44 ng m−3. We suggest that the amounts of coal left for burning or smoldering mainly accounted for the large variation in fume Hg concentrations of underground coal fires. The gaseous Hg concentrations in near-surface air surrounding boreholes varied from 2.38 ± 0.28 to 13.10 ± 0.97 ng m−3, with a mean value of 6.68 ± 3.09 ng m−3. They were higher than the ambient air Hg concentrations measured at a background site near the Yellow River (<2 ng m−3), suggesting underground coal fires were one significant Hg pollution source. Importantly, we found that gaseous Hg concentrations in the boreholes had significantly positive correlations with temperatures and CO (a traditional coal-fire index gas) contents, implying that Hg has the potential to serve as an index gas to monitor the occurrences of underground coal fires in mining goafs. Image 1 • Gaseous Hg in underground fire fumes were measured by a RA-915M with modified inlet. • Gaseous Hg concentrations in fire fumes from the mining goaf were elevated. • Hg has the potential to monitor fire occurrence and intensity in coal mining goafs. [ABSTRACT FROM AUTHOR]