Total and respirable dust particles, ammonia and endotoxin were measured in the nursery and grower areas of 4 swine confinement houses during January-March and April-June. Increased ventilation in the spring-summer relative to that in winter reduced concentrations of large dust particles more rapidly than it did smaller particles and ammonia. The greater decrease in large particles correlating to increased room air velocity may reflect the momentum of larger particles causing impaction on surfaces. There was a significant spatial variation in the concentration of airborne endotoxin within individual swine rooms and pens reflecting different mixing of large feed and smaller manure particles. Smaller particles had a 4-fold higher concentration of endotoxin than did larger particles, suggesting they had higher faecal material concentrations. Total airborne endotoxin and total suspended particulates correlated to the fraction of functional endotoxin contained in large particles, suggesting that small particles (0.5-2.0 um) collide with large particles (50 um). These results indicate that large non-respirable particles remove smaller respirable particles from indoor atmospheres due to kinematic coagulation.