{"operation":"document","citation":"25-0133","title":"Currie Associates — Hazardous Materials Safety Interpretation","source_type":"guidance","agency":"Pipeline and Hazardous Materials Safety Administration","status":"guidance","official":true,"published_on":"2026-01-27","effective_on":null,"summary":"25-0133 response to Currie Associates concerning 173.307.","machine_formats":{"json":"https://regulus.evalyn.ai/document/phmsa-interpretation-25-0133.json","markdown":"https://regulus.evalyn.ai/document/phmsa-interpretation-25-0133.md"},"app_url":"https://regulus.evalyn.ai/document/phmsa-interpretation-25-0133","source_url":"https://www.phmsa.dot.gov/sites/phmsa.dot.gov/files/2026-02/25-0133.pdf","body":"<<<PAGE 1>>>\n\n1200 New Jersey Avenue, SE\nWashington, DC 20590\nU.S. Department\nof Transportation\nPipeline and Hazardous\nMaterials Safety\nAdministration\nJanuary 27, 2026\nDrew Watts\nRegulatory Compliance Specialist\nCurrie Associates\n101 Ridge Street Suite I\nGlens Falls, NY 12801\nReference No. 25-0133\nDear Mr. Watts:\nThis letter is in response to your October 23, 2025 letter requesting clarification of the Hazardous\nMaterials Regulations (HMR; 49 CFR Parts 171-180) applicable to exceptions for compressed\ngases. Specifically, you are seeking confirmation of your understanding of the requirements\noutlined in § 173.307(a)(4)(iii) and (a)(4)(v) as they relate to specific hazardous materials. In\naddition, you reference a previously published letter of interpretation—Reference No. 21-0011—\nto further confirm your understanding of applicable requirements.\nWe have paraphrased and answered your questions as follows:\nQ1. Can compressed gases that exist in both gaseous and liquefied states—such as “UN1075,\nPetroleum gases, liquefied or Liquefied petroleum gas, 2.1,” “UN1978, Propane, see also\nPetroleum gases, liquefied, 2.1,” “UN3161, Liquefied gas, flammable, n.o.s., 2.1,” and\n“UN3358, Refrigerating machines, containing flammable, non-toxic, liquefied gas,\n2.1”—qualify for the exception in § 173.307(a)(4)(iii), if they are not transported by\naircraft?\nA1. Yes. A compressed gas that can be in either a gaseous or a liquefied state may qualify for\nthe exception in § 173.307(a)(4)(iii), if it meets all the conditions specified in the\nparagraph—including quantity limitations, vapor pressure or pressure requirements,\ncylinder specifications, and is not transported by aircraft.\nQ2. If the answer is yes, does the same criterion apply to the exception in § 173.307(a)(4)(v)?\nA2. Yes. A material that can exist in either a gaseous or a liquefied state may qualify for the\nexception in § 173.307(a)(4)(v), regardless of the mode of transportation, provided it\ncomplies with all conditions specified in that section.\n\n<<<PAGE 2>>>\n\nQ3. Are the exceptions provided in § 173.307(a)(4)(i) through (a)(4)(v) designed to operate\nindependently? If so, does satisfying the conditions of any one paragraph allow a\nshipment of “UN1075,” “UN1978,” “UN3161,” or “UN3358” to be excepted from being\nfully regulated under the HMR?\nA3. Yes. The exceptions in § 173.307(a)(4)(i) through (a)(4)(v) operate independently. In\n§ 173.307(a)(4), the word “or” is used between paragraphs (a)(4)(iv) and (a)(4)(v), which\nsignifies the provisions are independent of each other. Therefore, if a shipment of\n“UN1075,” “UN1978,” “UN3161,” or “UN3358” meets all the criteria in any individual\nsubparagraph in paragraph (a)(4), then the materials would be excepted from the HMR.\nHowever, each paragraph contains specific limitations—such as capacity restrictions and\nhazard class applicability—and if none of the conditions are met, the shipment remains\nfully regulated under the HMR.\nQ4. Are the exceptions found in § 173.307(a)(4)(iii) and (a)(4)(v) consistent with similar\nexceptions found in international regulations—such as the International Civil Aviation\nOrganization Technical Instructions (ICAO TI), the International Maritime Dangerous\nGoods (IMDG) Code, the Agreement concerning the International Carriage of Dangerous\nGoods by Road, and Transport Canada’s Transportation of Dangerous Goods\nRegulations?\nA4. The international regulations you referred to generally contain the same exceptions for\nspecified quantities of a non-flammable, non-toxic gas, ammonia solutions, and\nflammable gases, but in some cases, they also contain additional requirements (e.g.,\nSpecial Provision 291 of the IMDG Code and SP A103 of the ICAO TI contain certain\ndesign type criteria).\nI hope this information is helpful. Please contact us if we can be of further assistance.\nSincerely,\nDirk DerKinderen\nChief, Standards Development Branch\nStandards and Rulemaking Division\n\n<<<PAGE 3>>>\n\nBaker, Y.\n25-0133\nOctober 23, 2025\nMr. Matthew Nickels\nActing Director, Standards and Rulemaking Division\nPipeline and Hazardous Materials Safety Administration\nAttn: PHH-10\nU.S. Department of Transportation\nEast Building, 1200 New Jersey Ave., SE\nWashington, D.C. 20590-0001\nSubmitted: Via Email\ncc: infocntr@dot.gov\nDear Mr. Nickels,\nCurrie Associates respectfully requests a formal Letter of Interpretation (LOI) regarding the\napplication of § 173.307 - Exceptions for compressed gases. Specifically, we seek\nconfirmation that our interpretation of §173.307(a)(4)(iii) and (v) is consistent with PHMSA’s\nintent, as outlined in our discussion below and supported by PHMSA LOI Reference No. 21-\n0011 (issued April 12, 2021), which provides guidance related to this topic.\nHMR Verbiage for Exceptions for Compressed Gases And Refrigerating Machines\n§ 173.307(a)(4)(iii) & (v) and the applicable preliminary text reads as follows:\n“§ 173.307 Exceptions for compressed gases.\n(a) The following materials are not subject to the requirements of this subchapter:\n…\n(4) Refrigerating machines, including dehumidifiers and air conditioners,\nand components thereof, such as precharged tubing containing:\n…\n(iii) Except when offered or transported by air, 12 kg (25\npounds) or less of a flammable, non-toxic gas;\n…\n(v) 100 g (4 ounces) or less of a flammable, non-toxic\nliquefied gas.\n…”\n§ 173.307(a)(4)(iii) & (v) in its entirety is displayed in Attachment 1 (located in the attachments\nat the end of this document).\nLOI Supports Applicability to Gases & Liquefied Gases\nRegarding § 173.307, LOI Ref. No. 21-0011 Question 1 & Answer 1 (Attachment 2) clarified that\nthe exception within § 173.307(a)(4)(iii) is applicable to both flammable, non-toxic gas as well as\nliquefied gas when offered for all modes of transport except by aircraft.\n101 Ridge Street Suite I, Glens Falls, NY 12801 | Phone: 1-518-761-0668 | mail@currieassociates.com\n\n<<<PAGE 4>>>\n\nUse of the Exception for Gases Capable of Both States\nCurrie Associates requests confirmation that a gas which is capable of existing in both a gas & a\nliquefied state (for example, R-1234yf or D290 propane) may utilize the exception under\n§173.307(a)(4)(iii), if it is not transported by aircraft. Our understanding is based on the\nlanguage of §173.307(a)(4)(iii), which references flammable, non-toxic gases but does not\nexplicitly exclude liquefied forms. Meanwhile, LOI Ref. No. 21-0011 (Questions 1 & Answer 1)\nsupports this interpretation by clarifying that liquefied gases are included within the scope of this\nexception. Therefore, we conclude that a gas capable of existing in both a liquefied and non-\nliquefied state qualifies for the §173.307(a)(4)(iii) exception when transported by surface modes.\nIntended Applicability of §173.307(a)(4)(v)\nIf the conclusion that § 173.307(a)(4)(iii) applies to both gases and liquefied gases is correct,\nCurrie Associates requests confirmation on the intended applicability of § 173.307(a)(4)(v). It is\nour understanding that Subparagraph (v) applies to all modes of transport, including air, while\nSubparagraph (iii) applies to all modes except air. Thus, we interpret the HMR as follows:\na) Subparagraph (iii) - Authorizes an exception for up to 12 kilograms of flammable,\nnon-toxic gas (either liquefied or non-liquefied) when transported by highway,\nrail, or vessel (excluding air).\nb) Subparagraph (v) – Authorizes an exception for up to 100 grams of liquefied\nflammable, non-toxic gas contained in a refrigerating machine when transported\nby any mode, including air.\nIndependence of Meeting Exception Conditions\nCurrie Associates requests confirmation on our understanding that the exceptions listed in §\n173.307(a)(4)(i) through (v) are independent, and meeting the conditions of any single\nsubparagraph removes the shipment from full regulation requirements under the HMR. For\nexample, compliance with (iii) would not require additional compliance with (v) and vice versa.\nBased on this interpretation, Currie Associates concludes that refrigerating machines containing\nno more than 12 kilograms of a flammable, non-toxic liquefied gas (such as R-1234yf or D290\npropane), when they are not transported by air, are not subject to the HMR in accordance with\n§173.307(a)(4)(iii), even if the amount exceeds 100 grams.\nHarmonization with International Regulations\nCurrie Associates also requests confirmation that §173.307(a)(4)(iii) and (v) are consistent with\nthe applicable provisions in the various international regulations. Based on our review, ICAO\nTechnical Instructions Special Provision A103 reads as follows:\n“A103 (≈ 291) Flammable liquefied gases must be contained within refrigerating machine\ncomponents. These components must be designed and tested to at least\nthree times the working pressure of the machinery. The refrigerating\nmachines must be designed and constructed to contain the liquefied gas\nand preclude the risk of bursting or cracking of the pressure-retaining\ncomponents during normal conditions of transport. Refrigerating\n\n<<<PAGE 5>>>\n\nmachines and refrigerating machine components are considered not\nsubject to these instructions if containing less than 100 g flammable,\nnon-toxic, liquefied gas.”\nBased on our review, this aligns with § 173.307(a)(4)(v) since both authorize refrigerating\nmachines or components containing less than 100 grams of flammable, non-toxic liquefied gas\nwhen they are transported by air. Except for this situation, air transport of these machines\nappears to be restricted otherwise.\nFor surface modes, the IMDG Code and ADR Special Provision 291 (Attachment 3 & 4) and\nTDGR Special Provision 104 (Attachment 5) appear to be consistent with §173.307(a)(4)(iii),\nproviding an exception for machines or components containing less than 12 kilograms of\nflammable, non-toxic gas. Special Provision 291 for both IMDG and ADR states that\n“refrigerating machines and refrigerating-machine components are not subject to the\nrequirements of this Code (or ADR) if they contain less than 12 kg of gas.” Additionally, TDGR\nSP104 provides a similar exemption for ground, rail and vessel transport within Canada.\nSummary of Confirmation Requests\nCurrie Associates respectfully requests PHMSA’s confirmation that the interpretations outlined\nabove accurately reflect the correct intent and applicability of § 173.307(a)(4)(iii) and (v). We\nalso seek confirmation that these provisions are consistent with the corresponding international\nregulations that are referenced in this request.\nPlease see the following attachments referenced within this document:\n- Attachment 1: § 173.307(a)(4)(iii) & (v)\n- Attachment 2: LOI Ref. No. 21-0011\n- Attachment 3: IMDG Code Special Provision 291\n- Attachment 4: ADR Special Provision 291\n- Attachment 5: TDGR Special Provision 104\nCurrie Associates appreciates your review of this interpretation request and we look forward to\nyour response.\nSincerely,\nDrew Watts\nRegulatory Compliance Specialist\nCurrie Associates\nwww.currieassociates.com\ndrew@currieassociates.com\nO: 518-761-0668\n\n<<<PAGE 6>>>\n\nAttachment 1: § 173.307(a)(4)(iii) & (v)\n\n<<<PAGE 7>>>\n\nAttachment 2: LOI Ref. No. 21-0011\nof Transportation\nU.S. Department\n1200 New Jersey Avenue, SE\nPipeline and Hazardous\nWashington, DC 20590\nAdministration\nMaterials Safety\nApril 13, 2021\nHelen Walter-Terrinoni\nVP Regulatory Affairs\nAir-Conditioning, Heating, and Refrigeration Institute (AHRI)\n2311 Wilson Boulevard Suite 400\nArlington, VA 22201\nReference No. 21-0011\nDear Ms. Walter-Terrinoni:\nThis letter is in response to your February 4, 2021, email requesting clarification of the\ncompressed gases in refrigerating machines and their components. Specifically, you ask\nHazardous Materials Regulations (HMR; 49 CFR Parts 171-180) applicable to exceptions for\nquestions regarding the exceptions provided in §§ 173.307(a) (4)(iii) and (v).\nWe have paraphrased and answered your questions as follows:\nQ1. You ask whether the exception provided in § 173.307(a)(4)(iii) would apply to\nrefrigerating machines containing 12 kg or less of a flammable, non-toxic liquefied gas,\nsuch as \"UN3252, Difluoromethane, 2.1\" or \"UN3161, Liquefied gas, flammable, n.o.s.,\n2.1,\" so long as such units are not offered or transported by air.\nAl.\nThe answer is yes. Refrigerating machines and components thereof, containing 12 kg or\nless of a flammable, non-toxic gas, including a liquefied gas, are not subject to the\nrequirements of the HMR provided they are not offered or transported by aircraft.\nQ2.\nYou ask whether the exception provided in § 173.307(a)(4)(v) would apply to\nsuch as \"UN3252, Difluoromethane, 2.1\" or \"UN3161, Liquefied gas, flammable, n.o.s.,\nrefrigerating machines containing 100 g or less of a flammable, non-toxic liquefied gas,\n2.1,\" regardless of the mode of transportation.\n\n<<<PAGE 8>>>\n\nThe answer is yes. Refrigerating machines and components thereof, containing 100 g or\nHMR, regardless of the mode of transportation.\nless of a flammable, non-toxic liquefied gas, are not subject to the requirements of the\nI hope this information is helpful. Please contact us if we can be of further assistance.\nSincerely,\nDirk Der Kinderen\nChief, Standards Development Branch\nStandards and Rulemaking Division\n\n<<<PAGE 9>>>\n\nAttachment 3: IMDG Code Special Provision 291\nAttachment 4: ADR Special Provision 291\nAttachment 5: TDGR Special Provision 104","truncated":false,"body_characters":13338}