A new test method has been developed by the National Institute for Occupational Safety and Health (NIOSH) and Clean Air Filter (CAF) for quantifying the outside air leakage into environmental cab filtration systems. This method uses specially configured filter cartridges to remove carbon dioxide (CO2) from the environmental cab's air filtration system. Real-time gas monitors are used to measure the outside and inside cab CO2 concentrations after the cab reaches steady-state equilibrium conditions inside an unoccupied cab. Cab filtration system leakage can be mathematically determined using the measured cab penetration (inside to outside cab concentration ratio) and the special CO2 filter cartridge efficiency. Examination of several CO2 sampling instruments for this type of testing showed them to be relatively precise, but exhibited noticeable variations in accuracy. These results indicate that frequent gas calibration checks of comparative sampling instruments would be needed to ensure their accuracy, which is not well-suited for cab field testing. Thus, a single-instrument, multiple-sample-location cab testing methodology was devised to eliminate multiple-instrument sampling biases and frequent calibrations during testing. This methodology was examined and can provide accurate measurements of filtration system leakage into enclosed cabs with a precise instrument at or near steady-state test conditions. The new leak test method provides cab manufacturers, cab service personnel, and industrial hygienists with a measurement tool to ensure environmental cab integrity and minimize worker exposure to outside airborne substances.