Humidity greater than 65% during the evening rest period is a major cause of stress. Essentially, the loft must provide a dry atmosphere without being dusty, while at the same time provide sufficient ventilation without exposing the birds to extremes of temperature and humidity. High humidity, apart from being stressful in itself, means that the droppings do not dry out overnight and, unless strict hygiene is applied, this provides an ideal environment for problems such as E. Coli and Coccidia.
Taken from the Flying Vet's Pigeon Health & Management
by Dr Colin Walker