Currie Associates — Hazardous Materials Safety Interpretation

Page 11200 New Jersey Avenue, SE Washington, DC 20590 U.S. Department of Transportation Pipeline and Hazardous Materials Safety Administration 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.#

Page 2Q3. 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, and Transport Canada’s Transportation of Dangerous Goods 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., Special Provision 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#

Page 3Baker, Y. 25-0133 October 23, 2025 Mr. Matthew Nickels Acting Director, Standards and Rulemaking Division Pipeline and Hazardous Materials Safety Administration Attn: PHH-10 U.S. Department of Transportation East Building, 1200 New Jersey Ave., SE Washington, D.C. 20590-0001 Submitted: Via Email cc: infocntr@dot.gov Dear Mr. Nickels, Currie Associates respectfully requests a formal Letter of Interpretation (LOI) regarding the application of § 173.307 - Exceptions for compressed gases. Specifically, we seek confirmation that our interpretation of §173.307(a)(4)(iii) and (v) is consistent with PHMSA’s intent, as outlined in our discussion below and supported by PHMSA LOI Reference No. 21- 0011 (issued April 12, 2021), which provides guidance related to this topic. HMR Verbiage for Exceptions for Compressed Gases And Refrigerating Machines § 173.307(a)(4)(iii) & (v) and the applicable preliminary text reads as follows: “§ 173.307 Exceptions for compressed gases. (a) The following materials are not subject to the requirements of this subchapter: … (4) Refrigerating machines, including dehumidifiers and air conditioners, and components thereof, such as precharged tubing containing: … (iii) Except when offered or transported by air, 12 kg (25 pounds) or less of a flammable, non-toxic gas; … (v) 100 g (4 ounces) or less of a flammable, non-toxic liquefied gas. …” § 173.307(a)(4)(iii) & (v) in its entirety is displayed in Attachment 1 (located in the attachments at the end of this document). LOI Supports Applicability to Gases & Liquefied Gases Regarding § 173.307, LOI Ref. No. 21-0011 Question 1 & Answer 1 (Attachment 2) clarified that the exception within § 173.307(a)(4)(iii) is applicable to both flammable, non-toxic gas as well as liquefied gas when offered for all modes of transport except by aircraft. 101 Ridge Street Suite I, Glens Falls, NY 12801 | Phone: 1-518-761-0668 | mail@currieassociates.com#

Page 4Use of the Exception for Gases Capable of Both States Currie Associates requests confirmation that a gas which is capable of existing in both a gas & a liquefied state (for example, R-1234yf or D290 propane) may utilize the exception under §173.307(a)(4)(iii), if it is not transported by aircraft. Our understanding is based on the language of §173.307(a)(4)(iii), which references flammable, non-toxic gases but does not explicitly exclude liquefied forms. Meanwhile, LOI Ref. No. 21-0011 (Questions 1 & Answer 1) supports this interpretation by clarifying that liquefied gases are included within the scope of this exception. Therefore, we conclude that a gas capable of existing in both a liquefied and non- liquefied state qualifies for the §173.307(a)(4)(iii) exception when transported by surface modes. Intended Applicability of §173.307(a)(4)(v) If the conclusion that § 173.307(a)(4)(iii) applies to both gases and liquefied gases is correct, Currie Associates requests confirmation on the intended applicability of § 173.307(a)(4)(v). It is our understanding that Subparagraph (v) applies to all modes of transport, including air, while Subparagraph (iii) applies to all modes except air. Thus, we interpret the HMR as follows: a) Subparagraph (iii) - Authorizes an exception for up to 12 kilograms of flammable, non-toxic gas (either liquefied or non-liquefied) when transported by highway, rail, or vessel (excluding air). b) Subparagraph (v) – Authorizes an exception for up to 100 grams of liquefied flammable, non-toxic gas contained in a refrigerating machine when transported by any mode, including air. Independence of Meeting Exception Conditions Currie Associates requests confirmation on our understanding that the exceptions listed in § 173.307(a)(4)(i) through (v) are independent, and meeting the conditions of any single subparagraph removes the shipment from full regulation requirements under the HMR. For example, compliance with (iii) would not require additional compliance with (v) and vice versa. Based on this interpretation, Currie Associates concludes that refrigerating machines containing no more than 12 kilograms of a flammable, non-toxic liquefied gas (such as R-1234yf or D290 propane), when they are not transported by air, are not subject to the HMR in accordance with §173.307(a)(4)(iii), even if the amount exceeds 100 grams. Harmonization with International Regulations Currie Associates also requests confirmation that §173.307(a)(4)(iii) and (v) are consistent with the applicable provisions in the various international regulations. Based on our review, ICAO Technical Instructions Special Provision A103 reads as follows: “A103 (≈ 291) Flammable liquefied gases must be contained within refrigerating machine components. These components must be designed and tested to at least three times the working pressure of the machinery. The refrigerating machines must be designed and constructed to contain the liquefied gas and preclude the risk of bursting or cracking of the pressure-retaining components during normal conditions of transport. Refrigerating#

Page 5machines and refrigerating machine components are considered not subject to these instructions if containing less than 100 g flammable, non-toxic, liquefied gas.” Based on our review, this aligns with § 173.307(a)(4)(v) since both authorize refrigerating machines or components containing less than 100 grams of flammable, non-toxic liquefied gas when they are transported by air. Except for this situation, air transport of these machines appears to be restricted otherwise. For surface modes, the IMDG Code and ADR Special Provision 291 (Attachment 3 & 4) and TDGR Special Provision 104 (Attachment 5) appear to be consistent with §173.307(a)(4)(iii), providing an exception for machines or components containing less than 12 kilograms of flammable, non-toxic gas. Special Provision 291 for both IMDG and ADR states that “refrigerating machines and refrigerating-machine components are not subject to the requirements of this Code (or ADR) if they contain less than 12 kg of gas.” Additionally, TDGR SP104 provides a similar exemption for ground, rail and vessel transport within Canada. Summary of Confirmation Requests Currie Associates respectfully requests PHMSA’s confirmation that the interpretations outlined above accurately reflect the correct intent and applicability of § 173.307(a)(4)(iii) and (v). We also seek confirmation that these provisions are consistent with the corresponding international regulations that are referenced in this request. Please see the following attachments referenced within this document: - Attachment 1: § 173.307(a)(4)(iii) & (v) - Attachment 2: LOI Ref. No. 21-0011 - Attachment 3: IMDG Code Special Provision 291 - Attachment 4: ADR Special Provision 291 - Attachment 5: TDGR Special Provision 104 Currie Associates appreciates your review of this interpretation request and we look forward to your response. Sincerely, Drew Watts Regulatory Compliance Specialist Currie Associates www.currieassociates.com drew@currieassociates.com O: 518-761-0668#

Page 6Attachment 1: § 173.307(a)(4)(iii) & (v)#

Page 7Attachment 2: LOI Ref. No. 21-0011 of Transportation U.S. Department 1200 New Jersey Avenue, SE Pipeline and Hazardous Washington, DC 20590 Administration Materials Safety April 13, 2021 Helen Walter-Terrinoni VP Regulatory Affairs Air-Conditioning, Heating, and Refrigeration Institute (AHRI) 2311 Wilson Boulevard Suite 400 Arlington, VA 22201 Reference No. 21-0011 Dear Ms. Walter-Terrinoni: This letter is in response to your February 4, 2021, email requesting clarification of the compressed gases in refrigerating machines and their components. Specifically, you ask Hazardous Materials Regulations (HMR; 49 CFR Parts 171-180) applicable to exceptions for questions regarding the exceptions provided in §§ 173.307(a) (4)(iii) and (v). We have paraphrased and answered your questions as follows: Q1. You ask whether the exception provided in § 173.307(a)(4)(iii) would apply to refrigerating machines containing 12 kg or less of a flammable, non-toxic liquefied gas, such as "UN3252, Difluoromethane, 2.1" or "UN3161, Liquefied gas, flammable, n.o.s., 2.1," so long as such units are not offered or transported by air. Al. The answer is yes. Refrigerating machines and components thereof, containing 12 kg or less of a flammable, non-toxic gas, including a liquefied gas, are not subject to the requirements of the HMR provided they are not offered or transported by aircraft. Q2. You ask whether the exception provided in § 173.307(a)(4)(v) would apply to such as "UN3252, Difluoromethane, 2.1" or "UN3161, Liquefied gas, flammable, n.o.s., refrigerating machines containing 100 g or less of a flammable, non-toxic liquefied gas, 2.1," regardless of the mode of transportation.#

Page 8The answer is yes. Refrigerating machines and components thereof, containing 100 g or HMR, regardless of the mode of transportation. less of a flammable, non-toxic liquefied gas, are not subject to the requirements of the I hope this information is helpful. Please contact us if we can be of further assistance. Sincerely, Dirk Der Kinderen Chief, Standards Development Branch Standards and Rulemaking Division#

Page 9Attachment 3: IMDG Code Special Provision 291 Attachment 4: ADR Special Provision 291 Attachment 5: TDGR Special Provision 104#