Abstract Until now, there is limited information about mercury exposures inside solid waste incineration plants although incineration has been considered as one of major solid waste treatments. This study investigated indoor air concentrations of gaseous elemental mercury (GEM), reactive gaseous mercury (RGM) and particulate mercury (Hgp) and indoor dust mercury concentrations in a municipal solid waste incineration (MSWI) plant and a hospital waste incineration (HWI) plant during December 2003 and July 2004. The final results showed that the employees in incineration plants are not only exposed to GEM, but also to RGM and Hgp. For the HWI plant, only concentration of total mercury (HgT) in operation center in summer was below 1000 ng m −3 due to frequent ventilation, while those of GEM and HgT in hospital waste depot exceeded 3000 ng m −3. For the MSWI plant, only concentration of HgT in workplace in winter exceeded 1000 ng m −3. Therefore, more attention should be paid to mercury exposures in HWI plants than in MSWI plants. Indoor dust containing ∼3968 μg HgT kg −1 (dry matter) possibly served as the potential source for indoor air mercury pollution, especially in the HWI plant.