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Hazmat Interpretations

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An unsigned text version with a signed PDF of each letter is provided. To view interpretation letters, click the links below the Search box.

In the Search box, you can also enter a word(s) and click Search to locate that term within the letters.

All letters of interpretation posted to the website are fully searchable. This means you can enter a word(s) in the search field and click "Search" to locate that term within a letter.
Contact the Hazardous Materials Information Center with questions on compliance with the hazardous materials regulations (49 CFR parts 171 through 180) and interpretations of those regulations.

Hazardous Materials Information Center
1-800-HMR-4922 OR 1-800-467-4922
202-366-4488 for Washington DC residents

Available Monday through Friday, 9:00 am to 5:00 pm (ET), except Federal holidays. After hours, you can leave a recorded message and your call will be returned by the next business day.

Send a letter, with your return address and a daytime telephone number, to:

PHMSA Office of Hazardous Materials Standards
Attn: PHH-10
East Building
1200 New Jersey Avenue, SE.
Washington, DC 20590-0001


The Pipeline and Hazardous Materials Safety Administration, Office of Hazardous Materials Safety (OHMS) provides written clarifications of the Hazardous Materials Regulations (HMR; 49 CFR Parts 100-185) in the form of interpretation letters.

These letters reflect the agency's current application of the HMR to the specific facts presented by the person requesting the clarification.  Interpretations are one form of Guidance provided by OHMS.

Interpretations do not create legally-enforceable rights or obligations and are provided to help the public understand how to comply with the HMR. 

OHMS regularly reviews interpretations on this site for accuracy and applicability.  Interpretation letters from the last ten years are published here to provide the public with a greater awareness and understanding of the HMR.  Letters of interpretation which are found to be inaccurate or no longer applicable are removed and may be revised and reissued if warranted.  Letters which do not appear on this website may no longer be valid.

We welcome feedback from the hazmat safety community to ensure the most recent and accurate letters of interpretation are provided here.  Questions or concerns about a specific letter or topic should be directed to the Hazardous Materials Information Center.

Related Links
Federal Hazardous Materials Transportation Law: An Overview
This guide summarizes federal hazmat transportation law, including regulatory authorities, scope, and enforcement procedures.