Definition of ventilation
Pipeline and Hazardous Materials Safety Administration · Oct 16, 2007 · Reference No. 07-0155
Reggie’s quick take
A company asked PHMSA what "ventilated," "well ventilated," and "mechanically ventilated" mean for cargo holds on vessels under the Hazardous Materials Regulations. PHMSA said there are two kinds of ventilation: mechanical, which is power-generated, and natural, which is not. For SOLAS vessels, mechanical ventilation typically means at least six air changes per hour in an empty cargo space, as set out in SOLAS Chapter II-2 Regulation 19. Under the HMR, "ventilated" means removing dangerous vapors and gas from the cargo hold, and either natural or mechanical ventilation can meet that standard if it can prevent a dangerous buildup. For Part 176, "well ventilated" means the same thing as "ventilated."
Written by AI (Claude) on Oct 22, 2026. Robots make mistakes and leave things out, so the letter below is what counts. Not legal advice.
This responds to your August 9, 2007 fax requesting clarification on the definition of ventilation related terms under the Hazardous Materials Regulations (HMR; 49 CFR Parts 171-180). Specifically, you ask for the definitions of "ventilated", "well ventilated", and "mechanically ventilated" holds in vessels under the HMR.
Generally, there are two types of ventilation for a cargo hold: mechanical ventilation and natural ventilation. Mechanical ventilation can be simply defined as power-generated ventilation. For Safety of Life at Sea (SOLAS) vessels, this typically means at least six air changes per hour in the cargo space, based on an empty cargo space, and the removal of vapors from the upper or lower parts of the cargo space. This definition can be found in SOLAS Chapter II-2 Regulation 19. Natural ventilation is simply ventilation that is not power-generated. This type of ventilation usually takes the form of openings, vents, or goosenecks in the cargo hold.
Under the HMR, "ventilated" means the removal of dangerous vapors and gas from the cargo hold. Either natural or mechanical ventilation may be used provided the ventilation system has the capacity to prevent a dangerous accumulation of vapors or gases. For purposes of the Part 176 of the HMR, "well ventilated" has the same meaning as "ventilated."
A letter of interpretation applies the regulations to the facts the requester described. It isn’t a regulation and isn’t legal advice. The agency’s copy.