If you have flammables on site, you must follow the relevant requirements of the first few Articles in Chapter 5 of the NEC. If you apply Art. 505 as an alternative to the division classification system (covered in Art. 500 for Class I, Zone 0, 1, and 2), you could easily get confused about Material Groups [505.6].

"Material Groups" may not be the best wording for what this actually means, but it's not nearly as complex as it might seem. First, it doesn't address oxygen-enriched environments. Second, there are only three groups: A, B, and C (oddly enough, group A and C swap places in the NEC numbering system). Each Group addresses specific gases. Just look in the group listing for any gas you have, and apply Table 505.7(D) and Table 505.9(C)(1)(2) accordingly.