The aromatic oils DO off-gas to some degree, so the older it is, the less likely it is to cause problems. Here's the research take on it. If your birds get a cold, don't blame the virus, blame the cedar for weakening their immune systems. Shavings are the worst, as they are off-gassing all at...
Can Cedar bedding harm chickens?
Dr. Richard Evans, a veterinary pathologist who is also associated with the Orange County (CA) Department of Public Health, responded to this question by discussing laboratory findings and practical experience in the use of cedar shavings as has been found by...