Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, R01-OH-007850, 2008 Oct; :1-284
Agriculture has been established as a hazardous work environment for those who work in it. Due to cultural and economic reasons, it is widely accepted to assign agricultural tasks to youths. Generally, work assignments tend to be decided based on the experience of supervisory adults rather than documented guidelines or legislative regulations. This common practice may cause overestimation of the capabilities of youth workforces and exposure to greater dangers. Tractors, one of the most common and frequently used pieces of equipment in agriculture, are no exception to this phenomenon. The significance of young victims of tractor-related accidents has attracted the attention of occupational health epidemiologists and the public. Tractor operation depends on the operator's mental and physical capabilities; however, it is important to realize that apparent maturity or exceptional mental capabilities do not compensate for the lack of physical capabilities and vice versa. This study focuses on the evaluation of physical mismatches between operational demands and the operator's capabilities in tractor operations in order to evaluate NAGCAT Tractor Guidelines from ergonomics standpoint. More than 80 tractors of varying age and size from California and Wisconsin were assessed in this study. The tractor selection process was guided by objective data on the most ROPS and non-ROPS tractors most used in the US. The most-used 25 ROPS and 25 non-ROPS tractors were selected for the focused evaluation in this study. The overall evaluation was simulated by an ergonomic CAD system in conjunction with a photogrammetry software. Photogrammetry derives geometrical information of an object from digital images taken at various angles. The overall quality of the virtual 3-D models created by the photogrammetry software was satisfactory with relatively high accuracy - less than one pixel of residual error, on average. Using photogrammetry is advantageous, especially in connection with an ergonomic evaluation tool, graphic software, or CAD systems. An ergonomic evaluation tool called SAMMIE CAD was implemented for the dimensional evaluations and field of vision analysis. Reach analysis is one of the important parameters in workplace assessment and displays the dimensional characteristics of the operator and the workplace. Digital human mockups for 12, 14, and 16 year old males and females were created using an anthropometry data set of US children, and the reach capabilities of youths on popular tractors were simulated. The results indicate, in general, that hand-operated controls tend to be out of reach for both genders with the exception of steering wheels. The worst controls were hand throttle levers; literally no tractor had a hand throttle lever within reach for all age groups tested. Only 60% and 36% of all tractors had hand throttles within reach for average-sized mature males and females, respectively. On the other hand, foot-operated controls yielded higher reachable percentages for both genders, while steering wheels showed a medium range of reachable percentages. The results of logistic regression evaluating the North American Guidelines for Children's Agricultural Tasks (NAGCAT) indicate that its demarcation by the size of the tractor does not, in general, significantly distinguish among the age groups. "Field of vision" implies the amount of visual information available to the tractor operator. Visibilities for youths were simulated in accordance with the field of vision module in SAMMIE CAD and compared with the visibilities for same-gender adults in order to display visual disadvantage of the youth operator. In general, youth operators showed relatively restricted fields of vision at 30, 90, and 150 degrees compared to the adults, and their fields of vision decreased as obstacles were closer to the tractor. Differences in field of vision between an adult and a child may look small, i.e., a child has more than 90% of the adult field in most cases; however, this small difference may prevent detection of an object around the tractor, such as a simple obstacle, a ditch, or even a bystander. The results of Kruskal-Wallis test on the visibility ratio between a youth operator and a mature operator indicate that the demarcation of the NAGCAT by the size of the tractor is significant for most youth operators. A Kruskal-Wallis test was also performed for other parameters, and the results suggest that the A-pillar of a roll-over protective structure can be used as an additional indicator of reduced visibility for youth operators. The strength of a youth operator dictates his/her capability for maneuvering various controls on a tractor. The physical strengths of children were compared to the activation forces for various controls to verify their maneuvering capabilities. The activation forces for controls were collected based on the popularity and accessibility of tractors, while the physical strengths of children were estimated from alternative sources, due to the lack of exact data for US children. Their maximum strengths easily surpassed the activation forces for controls in general; however, it was not recommended to exert more than 30% of maximum strength for frequently used controls. The results of statistical analysis indicate that foot-operated controls require more strength than the recommendation. In the meantime, the recommended physical strengths of children surpassed the activation forces for hand-operated controls on the average. This result suggests that youth operators could be exposed to muscle fatigue in the leg if the particular operation required the frequent use of foot clutches and brakes. Results of statistical analysis on the difference between physical capabilities and operational demands indicate that the demarcation of the NAGCAT by the size of the tractor is not significant for the majority of controls. Overall, the results of comparisons suggest that there are differences between operational demands and the physical/visual capabilities of youth operators, which do not necessarily indicate the need for a ban on tractor operation by youth operators, but urge the necessity of awareness by supervisory adults and the public. The primary purpose of this project is to confirm the difference between operational demands of agricultural tractors and the physical/visual capabilities of youth operators. Furthermore, it is expected that the confirmation of those differences will be used in related research and the development of guidelines; ultimately to contribute to decrease in agricultural injuries and deaths of children.