Thatcher Company of California — Hazardous Materials Safety Interpretation

Page 11200 New Jersey Avenue, SE Washington, DC 20590 U.S. Department of Transportation Pipeline and Hazardous Materials Safety Administration March 9, 2026 Ames Ettinger Laboratory Manager Thatcher Company of California 8625 Unsworth Ave Sacramento, CA 95828 Reference No. 25-0141 Dear Mr. Ettinger: This letter is in response to your November 14, 2025 email requesting clarification of the Hazardous Materials Regulations (HMR; 49 CFR Parts 171-180) applicable to the use of Department of Transportation (DOT)-4L welded insulated cylinders for the transportation of a 5% solution of “UN1833, Sulfurous acid, 8, PG II” in accordance with packaging authorized in § 173.202. You explain that § 173.202(c) authorizes the following single packagings: “Cylinders, specification, as prescribed for any compressed gas, except for Specifications 8 and 3HT.” In addition, you state while the requirements in § 173.202(c) do not explicitly prohibit the use of DOT-4L specification cylinders you ask whether these cylinders may still be used because they are designed for the transportation of liquefied gases and “UN1833” is a liquid hazardous material. We have paraphrased and answered your questions as follows: Q1. Is a DOT-4L cylinder an authorized cylinder specification under § 173.202(c) for the transportation of “UN1833”? If not, would this require submitting a special permit request to perform this function? A1. Yes. Section 173.202 authorizes non-bulk packagings for transportation of liquid hazardous materials in Packing Group II. Thus, in accordance with § 173.202(c) which authorizes specification cylinders, and except for transportation by passenger aircraft, a DOT-4L cylinder adhering to applicable requirements of the HMR (e.g., § 173.24(b)) may be used for the transportation of a “UN1833” material. Q2. If a DOT-4L cylinder is considered an authorized packaging under § 173.202, are the cylinders excepted from requalification requirements in accordance with § 180.209, even though they are used for a liquid?#

Page 2A2. Yes. When a DOT-4L cylinder is used for liquid transport under § 173.202, it is considered an authorized packaging. Because DOT-4L cylinders are excluded from the requalification table listed in § 180.209(a), they are not subject to periodic requalification testing. However, this does not except the offeror or carrier from other HMR safety standards, such as the general inspection and maintenance requirements found in § 180.205(e). 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 3Baker, Y. 25-0141 From: INFOCNTR (PHMSA) To: Baker, Yul (PHMSA) Cc: Hazmat Interps Subject: 173.202 Date: Friday, November 14, 2025 13:58:02 Letter of Interpretation Request - FW: Request for Formal Interpretation - Use of DOT-4L Cylinders under 49 CFR Good afternoon, Please see the following interpretation request. Let us know if you need anything else. Best, Janaye From: Ames Ettinger <Ames.Ettinger@tchem.com> Sent: Friday, November 14, 2025 11:00 AM To: INFOCNTR (PHMSA) <INFOCNTR.INFOCNTR@dot.gov> Cc: John Riley <John.Riley@tchem.com> Subject: Request for Formal Interpretation - Use of DOT-4L Cylinders under 49 CFR 173.202 You don't often get email from ames.ettinger@tchem.com. Learn why this is important CAUTION: This email originated from outside of the Department of Transportation (DOT). Do not click on links or open attachments unless you recognize the sender and know the content is safe. Dear PHMSA Standards and Rulemaking Division, I am requesting a formal letter of interpretation regarding the use of DOT-4L cylinders for transportation of sulfurous acid, 5% solution (UN 1833) under 49 CFR 173.202 - Non-bulk packaging requirements for liquid hazardous materials in Packing Group II. Specifically, 49 CFR 173.202(c) authorizes “cylinders, specification, as prescribed for any compressed gas, except for Specifications 8 and 3HT,” without explicitly prohibiting the DOT- 4L specification. DOT-4L cylinders, however, are designed for cryogenic refrigerated liquefied gases under 49 CFR 178.57. We request PHMSA’s interpretation on the following point: 1. Is a DOT-4L cylinder considered an authorized cylinder specification under 49 CFR 173.202 for the transportation of sulfurous acid (UN 1833), or would this constitute a non-standard use requiring a PHMSA special permit? 2. If DOT-4L cylinders are considered authorized under 173.202, would they continue to be exempt from requalification under 49 CFR 180.209, as currently specified for DOT-4L cylinders?#

Page 4Thank you for your clarification. Please advise if any additional information is needed. Ames Ettinger Laboratory Manager Thatcher Company of California Direct: 435-668-5369 Mailing: 8625 Unsworth Ave, Sacramento CA 95828 The information contained in this e-mail message, and any attachments to it, contain confidential, proprietary, privileged, or otherwise secret information. If you are not the intended recipient, or believe that you may have received this email or attachments in error, any dissemination, use, review, distribution, printing or copying of the information contained in this e-mail message and any attachments to it are strictly prohibited. If you have received this communication in error, please notify us by reply e-mail or telephone and immediately and permanently delete the message and any attachments. Please include in your notice that the message and any attachments have been deleted. Thank you.#