A study of environmental and physiological effects associated with silo unloading was conducted. Air samples were collected at the base and within the headspace of five dairy silos as they were being unloaded; samples were analyzed for total suspended particulates, respirable dust, and viable microorganisms. Total dust concentrations ranged from 0.2 to 138 milligrams per cubic meter (mg/m3). Respirable dust concentrations ranged from 0.2 to 24mg/m3. Microorganism concentrations ranged from 100,000 to 1 billion organisms per cubic meter. Very low concentrations, 100 to 200 parts per billion, of a single mycotoxin, deoxynivalenol (51481108), were detected in one silo. Spirometry and serum complement concentrations in farm workers were not significantly changed during exposure. White cell counts were increased during uncapping. Two farmers in the dustiest silo experienced symptoms of illness. This silo did not contain detectable mycotoxins. The authors conclude that silo unloading may generate high concentrations of respirable dust containing large numbers of bacteria and fungi, despite the absence of mycotoxins.