There is mounting evidence that bacteria originating from pigs degrade the environment of the pig shed and adversely affect the health of the animals and the pig-shed workers. α-haemolytic cocci (AHC) occur in pig-shed environments, but are regarded as commensals. Ammonia is also a component of the pig-shed environment, and is known to damage upper respiratory tract epithelia. The aim of this study was to determine whether polluted air in pig sheds adversely affected performance indicators in pigs. Modelling revealed a direct effect of AHC on voluntary feed intake and hence AHC are not commensal. No direct effect of ammonia on the pigs was detected, but the combination of AHC and ammonia stimulated the immune system in a progressive manner, and there were direct effects of immune stimulation on food intake and growth resulting in poorer feed-conversion efficiency, even though the effects remained subclinical. The authors conclude that exposure of the respiratory epithelia of pigs to viable AHC in the presence of ammonia redirects nutrients away from production and towards the immune system, explaining the impact of poor pig-shed hygiene on production parameters.