(a) The marking must be durable, legible and of a size relative to the package as to be readily visible and similar to the illustration shown in this paragraph with the minimum dimension of each side of the marking at least 100 mm (3.9 inches) as measured from the outside of the lines forming the border. The width of the border forming the square-on-point marking must be at least 5 mm. The marking must be displayed at each location (e.g., manhole, loading head) where exposure to hydrogen sulfide vapors may occur.

(b) The border of the square-on-point must be black or red on a white or other suitable contrasting background. The symbol must be black and located in the center of the square-on-point and be clearly visible as follows:

(c) As an alternative to the marking required in (a) and (b) of this section, a label, tag, or sign may be displayed at each location (e.g., manhole, loading head) where exposure to hydrogen sulfide vapors may occur. The label, tag, or sign must be durable, in English, and printed legibly and of a size relative to the package with a warning statement such as “Danger, Possible Hydrogen Sulfide Inhalation Hazard” to communicate the possible risk of exposure to harmful concentrations of hydrogen sulfide gas.