Investigator G. Jules writes:
This 2002 study about cotton bales had a big effect. The authors demonstrated that cotton bales do not need to be shipped as a hazardous material under DOT [Department of Transportation] regulations. 
Note: Prior to 1999, cotton bales were in fact shipped as haz mat under DOT regs — this research is part of what supported a reclassification.
The research includes a discussion of cotton fires, calculations of air available within a cotton bale, assessments of the potential for spontaneous combustion, etc. Spoiler alert: cotton bales do not spontaneously combust.
The study is: “Evaluation of the Flammability of Cotton Bales,” P.J. Wakelyn1 and S.E. Hughs, Fire and Materials, 2002; 26: 183–189 (DOI: 10.1002/fam.795).