Mixtures of nitrogen and oxygen containing not less than 19.5% and not more than 23.5% oxygen by volume may be transported under this entry when no other oxidizing gases are present. A Division 5.1 subsidiary hazard label is not required for any concentrations within this limit. Compressed air containing greater than 23.5% oxygen by volume must be shipped using the description “Compressed gas, oxidizing, n.o.s., UN3156.”