Disposable Filter Elements (DFEs) are used by the underground coal mining industry to control worker exposure to diesel particulate matter (DPM) emissions from mobile diesel engines. This study investigated the actual in-service filtration efficiency of DFEs when installed for immediate use as is the current practice. Four different DFEs were tested using statutory and in-service methods and the exhaust results analysed for elemental carbon, a surrogate for DPM. Using the statutory test method, two of the four DFEs had a filtration efficiency that was below the industry-accepted six-hour average efficiency of >85% in the first 15 to 30 minutes of testing while one DFE did not reach the required efficiency 45 minutes to one hour into testing. In-service testing of two of the DFEs showed even lower filtration efficiencies of 62% and 69%, respectively. The “lag period” in DFEs reaching >85% filtration efficiency exposes workers to higher than expected levels of DPM, a known carcinogen.