Before a puppy is big enough to rip a blanket into shreds and make his own bedding, he needs something that is soft, warm, absorbent and, most of all, safe. Pine bedding meets most of these requirements, but there is some evidence that the same compound that gives it a fresh smell may also be unhealthy for animals.
Soft woods like pine contain an acidic compound called phenols. Phenols give the wood its familiar scent and are often extracted for use in cleaning products. Besides smelling good, phenols are caustic and good for removing grease and grime. This makes pine bedding good for covering up urine and other bad odors, but it also means that young puppies are constantly breathing in the scent. This constant exposure can lead to respiratory irritation and lung damage. Phenols have also been implicated in liver damage in rabbits.
Dust and small sawdust particles in pine bedding can also cause respiratory irritation. The dust can cause sneezing, runny noses and coughing. It can even get into the eyes and cause further irritation. Since a puppy's immune system isn't fully developed, this small irritation can turn into an upper respiratory infection and even lead to pneumonia. If the phenols are already bothering the puppy, sawdust in the bedding may aggravate the problem.
Research on pine bedding is limited and mostly focused on small animals that spend most of their lives in contact with the bedding, such as rats, guinea pigs and rabbits. It's unclear whether the negative effects observed in these studies translate directly to puppies and dogs. Studies on humans, however, have shown that workers in wood industries, like at saw mills, have a higher than average rate of respiratory tract cancer.
Hardwood bedding doesn't contain as many phenols as soft wood like pine, but may still be dusty. Other alternatives include newspaper, fleece and old blankets, T-shirts or towels. Commercial bedding made from recycled materials, such as paper, wood and fabric, comes in pellet and chip form and contains less dust and odor than pine shavings. A good bedding should be soft, warm, absorbent and not contain any strong odors or dust that might irritate the puppy's respiratory system.