http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=FEDERAL_REGISTER&p_id=21365
“3301.1.5 Ammonium nitate. The storage and handling of ammonium nitrate shall comply with the requirements of NFPA 490, Chapter 2 and 40. Ammonium nitrate shall be classified in accordance with Table 3301.1.5(1) and stored in accordance with Section 3301.8 when classified as an explosive material.”