(a) When transported in cargo transport units, the cargo transport units must provide an adequate exchange of air in the unit. This adequate exchange of air may be accomplished by utilizing a ventilated container, an open-top container, or a container in one door off operation. When cargo transport units with venting devices are used these devices should be kept clear and operable. If mechanical devices are used for ventilation, they must be explosion-proof.

(b) As an alternative to the options presented in paragraph (a) of this section to ensure an adequate exchange of air; a refrigerated cargo transport unit may be used.

(c) The requirements in paragraph (a) and (b) of this section do not apply if the hazardous material is:

(1) Packed in hermetically sealed packagings or IBC's which conform to packing group II performance level for liquid dangerous goods with a total pressure in the packaging (i.e., the vapor pressure of the material plus the partial pressure of air or other inert gases, less 100kPa (15 psia)) at 55 °C (131 °F), determined on the basis of the hazardous material not completely filling the receptacle at a temperature of 55 °C (131 °C) or less at a filling temperature of 15 °C (59 °F), will not exceed two-thirds of the marked test pressure.

(2) [Reserved]

(d) Cargo transport units must be marked with a warning mark including the words “CAUTION—MAY CONTAIN FLAMMABLE VAPOR” or “CAUTION—MAY CONTAIN FLAMMABLE VAPOUR” with lettering having a height of at least 25 mm (1 inch). The mark must be affixed to each access point in a location where it will be easily seen by persons prior to opening or entering the cargo transport unit and must remain on the cargo transport unit until the following provisions are met:

(1) The cargo transport unit has been completely ventilated to remove any hazardous concentrations of vapor or gas;

(2) The immediate vicinity of the cargo transport unit is clear of any source of ignition; and

(3) The hazardous materials have been unloaded.