Interpretation Response #25-0133

Below is the interpretation response detail and a list of regulations sections applicable to this response.

Interpretation Response Details

Response Publish Date: 01/27/2026

Company Name: Currie Associates

Individual Name: Drew Watts

Location State: NY Country: US

View the Interpretation Document

Response text:

January 27, 2026

Drew Watts
Regulatory Compliance Specialist
Currie Associates
101 Ridge Street Suite I
Glens Falls, NY  12801

Reference No. 25-0133

Dear Mr. Watts:

This letter is in response to your October 23, 2025 letter requesting clarification of the Hazardous Materials Regulations (HMR; 49 CFR Parts 171-180) applicable to exceptions for compressed gases. Specifically, you are seeking confirmation of your understanding of the requirements outlined in § 173.307(a)(4)(iii) and (a)(4)(v) as they relate to specific hazardous materials. In addition, you reference a previously published letter of interpretation—Reference No. 21-0011—to further confirm your understanding of applicable requirements.

We have paraphrased and answered your questions as follows:

Q1. Can compressed gases that exist in both gaseous and liquefied states—such as "UN1075, Petroleum gases, liquefied or Liquefied petroleum gas, 2.1," "UN1978, Propane, see also Petroleum gases, liquefied, 2.1," "UN3161, Liquefied gas, flammable, n.o.s., 2.1," and "UN3358, Refrigerating machines, containing flammable, non-toxic, liquefied gas, 2.1"—qualify for the exception in § 173.307(a)(4)(iii), if they are not transported by aircraft?

A1. Yes. A compressed gas that can be in either a gaseous or a liquefied state may qualify for the exception in § 173.307(a)(4)(iii), if it meets all the conditions specified in the paragraph—including quantity limitations, vapor pressure or pressure requirements, cylinder specifications, and is not transported by aircraft.

Q2. If the answer is yes, does the same criterion apply to the exception in § 173.307(a)(4)(v)?

A2. Yes. A material that can exist in either a gaseous or a liquefied state may qualify for the exception in § 173.307(a)(4)(v), regardless of the mode of transportation, provided it complies with all conditions specified in that section.

Q3. Are the exceptions provided in § 173.307(a)(4)(i) through (a)(4)(v) designed to operate independently? If so, does satisfying the conditions of any one paragraph allow a shipment of "UN1075," "UN1978," "UN3161," or "UN3358" to be excepted from being fully regulated under the HMR?

A3. Yes. The exceptions in § 173.307(a)(4)(i) through (a)(4)(v) operate independently. In § 173.307(a)(4), the word "or" is used between paragraphs (a)(4)(iv) and (a)(4)(v), which signifies the provisions are independent of each other. Therefore, if a shipment of "UN1075," "UN1978," "UN3161," or "UN3358" meets all the criteria in any individual subparagraph in paragraph (a)(4), then the materials would be excepted from the HMR. However, each paragraph contains specific limitations—such as capacity restrictions and hazard class applicability—and if none of the conditions are met, the shipment remains fully regulated under the HMR.

Q4. Are the exceptions found in § 173.307(a)(4)(iii) and (a)(4)(v) consistent with similar exceptions found in international regulations—such as the International Civil Aviation Organization Technical Instructions (ICAO TI), the International Maritime Dangerous Goods (IMDG) Code, the Agreement concerning the International Carriage of Dangerous Goods by Road (ADR), and the Transportation of Dangerous Goods (TDG) Regulations?

A4. The international regulations you referred to generally contain the same exceptions for specified quantities of a non-flammable, non-toxic gas, ammonia solutions, and flammable gases, but in some cases, they also contain additional requirements (e.g., SP 291 of the IMDG Code and SP A103 of the ICAO TI contain certain design type criteria).

I hope this information is helpful. Please contact us if we can be of further assistance.

Sincerely,

Dirk DerKinderen
Chief, Standards Development Branch
Standards and Rulemaking Division

Regulation Sections