Real-time metallic mercury vapor levels of the indoor air were monitored at several mercury spill sites around the US in order to evaluate the effectiveness of site cleanup operations. Mercury readings taken in the field with a Jerome 431 (TM) Mercury Vapor Analyzer were compared with laboratory analysis using a modified National Institute for Occupational Safety and Health (NIOSH) 6009 method. Statistical evaluation showed that the data were highly comparable except at low concentrations, due to the large degree of uncertainty associated with measuring low levels of mercury with the Jerome analyzer. Regression analysis indicated that Jerome measurements of 10 mug/m(3) or greater are comparable for field analysis of mercury vapor in air. Published by Elsevier Science B.V.