Aerosol instrumentation was used to evaluate air infiltration into tractor cabs that are used to protect the agricultural worker during pesticide applications. Preliminary surveys were conducted on three different manufactured agriculture enclosures. The results of these preliminary surveys indicated that aerosols are entering the cab through leak sources or are being generated inside the cab. These results identified the need for in-depth field evaluations of tractor cabs to identify any leak sources. To evaluate the ability of tractor cabs to reduce operator air contaminant exposure, field evaluations were conducted on two tractor cabs. Specifically, we evaluated: 1) the particle size distribution and the effectiveness of the filter system; and 2) air infiltration into the cab. These evaluations were also conducted to demonstrate the ease and practicality of using optical particle counters to evaluate the ability of cabin filtration systems. Pesticide particle size distribution during an air blast spray operation was also evaluated during the study. The field tests were conducted on a John Deere 7000 series tractor cab (tractor manufacturer's cab) and a Nelson spraycab (retrofit cab). Both cabs were equipped with high efficiency particulate air (HEPA) filter media which were assumed to be 99.97 percent efficient at removing the test aerosol, atmospheric condensation nuclei. Thus, the major source of aerosols inside the cab was assumed to be leakage around filters at the seals. Using a portable dust monitor (PDM), the ratio of the outside to inside aerosol measurements was used to calculate a cab protection factor. During the evaluations, one PDM was placed inside the tractor cab (near the tractor operator) and one PDM was placed outside (near the air intake) to count particles. During the evaluations, the instruments were switched to prevent instrument bias from affecting the findings. The ratio of the two measurements (i.e., protection factor = outside concentration / inside concentration) was used to calculate how efficient the tractor cab was at removing aerosols. The John Deere cab was more than 99 percent efficient at removing aerosols larger than 3.0 microm in diameter and had protection factors greater than 260 for particles larger than 3.0 microm (indicated by the PDM results). The Nelson cab was more than 99 percent efficient at removing aerosols larger than 3.0 microm in diameter and had protection factors greater than 200 for particles larger than 3.0 microm (indicated by the PDM results). For aerosols smaller than 1.0 microm in diameter (indicated by a PortaCount Plus instrument), the John Deere cab provided a mean protection factor of 43 and the Nelson cab provided a mean protection factor of 16. The results from this study indicate that tractor cabs can be effective at removing different size aerosols depending on the seals and filters used with the enclosure. This study has also demonstrated the practical use of real-time aerosol counting instrumentation to evaluate the effectiveness of enclosures and to help identify leak sources. The method used in this study can be applied to various cabs used in different industries including agriculture, construction, and manufacturing.