Explanation
While you should not add the risk of smoking in extremely close proximity of any class of hazardous material, the additional rules are: no smoking within 25 feet of classes 3 and 2.1 (flammable materials and gases) and no smoking or holding a lighted smoking object such as a cigar or a pipe within 25 feet of classes 1, 3, 4, or 4.2 (explosives, flammable liquids and solids, and spontaneous combustibles).