Health and safety oxygen depletion.

While wood fuels are in storage they consume some of the oxygen, which can become a problem in confined spaces. Ventilate confined spaces before entering.

Read more here:

Handling and using wood fuels in a safe manner with regard to Carbon Monoxide (CO)
http://www.hsa.ie/eng/Publications_and_Forms/Publications/Codes_of_Practice/COP_Confined_Space_Document.pdf