Agricultural work is generally performed outdoors. Major outdoor work activities leading to dust and chemical exposure by farm workers include preparation of soil for field crops, growing, harvesting, transport, storage of agricultural products, and fieldwork activities such as plowing, tilling, and haying. Fieldwork also has the potential to expose agricultural workers to inorganic dusts as well as various pesticides and chemicals. Exposures may be particularly significant in dry, semiarid, and desert climates and under windy conditions. The bulk of inorganic dusts are composed of silicates. These include crystalline silica (quartz) and noncrystalline amorphous silica (diatomite). Dust samples from outdoor agricultural environments may be composed of approximately 10% to 20% or greater concentrations of crystalline silica. Workers performing fieldwork may develop clinically significant exposures to various silicates including respirable fibrous minerals and to nonfibrous silicate materials, including mica and clay silicates, known to cause pulmonary fibrosis (27-30).
Airborne mineral dust concentrations and exposure potential may vary with many environmental variables, such as regional geological and climate conditions, amount of rainfall, type of crops grown, and the specific agricultural practices employed. Aerosolized dusts with a median diameter of 4 to 5|im or less can penetrate deep into the respiratory system and have pathophysiological effects after deposition in the gas-exchange areas of the terminal bronchioles and alveoli (12,17).
In general, inorganic dusts do not contribute to agricultural respiratory disease to the same degree as organic dusts. However, occupational exposures to mineral particles from inorganic dusts and crystalline silica may stimulate release of reactive oxygen species (ROS) in the lung. Reactive oxygen species may play a key role in the mechanisms of disease initiation and progression subsequent to inhalational exposures to these particles. In fact, multiple pathways may be facilitated to produce ROS, which may lead to inflammation, resulting in production of diseases such as pneumoconiosis and carcinogen-esis (31).
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