Several work environments have the potential for organic dust exposure including agricultural fields, grain handling and storage areas, animal confinement areas, and dairy barns. Grain handling and storage has been associated with respiratory symptoms and illnesses (such as chronic cough, sputum production, chronic bronchitis, grain fever (organic dust toxic syndrome), and nasal and skin irritation in exposed workers. Similar findings were found in several global areas. Grain elevator workers frequently report symptoms such as cough, sputum production, wheezing, and dyspnea, and have obstructive or restrictive impairments on pulmonary function testing. Respiratory symptoms and worsening of ventilatory function have also been reported with workers exposed to dusts from soybeans and burned rice husk in rice farmers. Growing crops such as rice, soybeans, and flowers has been associated with lung disease (32-43).
Work in dairy barns is also related to respiratory illness. Hay and preserved grasses or corn (silage) are generally used to feed cows in dairy barns and may be a significant source of organic dust exposure to varying degrees based on the mechanism of preparation and storage. Rates of allergic alveolitis were found to be high in workers exposed to hay in small, tightly closed barns. Increased respiratory symptoms have been observed in workers in Finland who shake out hay to feed cows in dairy barns. Another source of exposure in the barn is the bedding chopper, which uses a series of rotating blades to cut bales of hay into smaller lengths, which are then blown into animals' stalls to serve as bedding. This practice can aerosolize hay and create significant amounts of respirable dust (44,45).
In addition, possible additive and/or synergistic toxic exposures and respiratory health effects may occur with dust exposure with coexistent toxic gases, especially in confined work spaces (12,19).
Major components of organic dusts include substances derived from bacterial and fungal organisms such as endotoxin from gram-negative bacteria, peptidoglycans from gram-positive bacteria, glucans, and mold and myco-toxins from fungi. Biologically active proteins of organic dusts may be aller-
genic and proinflammatory. Endotoxin from gram-negative bacteria has been particularly found to be an important causative agent in producing respiratory illness. Recent insights into the innate immune system from genetic research may help elucidate the biological mechanism(s) related to respiratory health effects from endotoxin and other inhaled toxins. The Toll-like group of receptors (TLRs) are receptors for specific components of pathogens such as lipopolysaccharide (LPS), peptidoglycan, and others. It appears that genetic variation in TLRs influences the response to inhaled endotoxin. Thus, variable pulmonary responses in individuals exposed to organic dusts may be due to polymorphisms in the TLR genes. For example, it has been demonstrated that common missense mutations in the Toll-like receptor 4 (TLR4) are associated with a blunted response to inhaled LPS (endotoxin). Consequently, some individuals may be more susceptible than others when exposed to organic dusts. Some authors have also suggested that grain dusts and extracts of grain dusts may play a role in activating inflammation in the lung independent of endotoxin, and that this mechanism may also involve the TLR receptors (46-50).
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