Factors associated with personal dust exposure levels during agricultural field crop operations in California were examined. From April through November, dust concentrations were measured at a field crop farm during land planing, discing, other ground preparations, planting, harvesting, fertilizing, mowing, and cultivating. Farm workers wore either a four stage personal cascade impactor containing Mylar membranes coated with silicone spray or a personal respirable dust cyclone containing a 37 millimeter diameter polyvinyl-chloride filter. Dust mass on the membranes and filters was determined gravimetrically. Median sampling time was 55 minutes for the cascade impactor and 72 minutes for the cyclone. Dust levels were high during all the field crop operations, especially ground preparation activities. The presence of an enclosed cabin on the tractor caused significant reductions in dust exposure levels during discing, other ground preparations, and harvesting. In single predictor models, relative humidity explained 24% and 34% of the variance in dust exposure levels determined with the cascade impactor and cyclone, respectively. Tractor speed and type of operation were also associated with dust exposure levels. The small dust particle concentration measured with the cascade impactor was significantly associated with soil temperature. In covariance models, type of operation, tractor speed, and relative humidity explained 73% of the variance for dust collected on all impactor stages and 55% of the variance for dust particles smaller than 9.8 micrometers (microm). Soil temperature and humidity explained 36% of the variance for dust particles smaller than 3.5microm. The authors conclude that the above findings can be applied to dust exposure reduction interventions in the California agricultural industry.