Standards & Rulemaking

Interpretation Letters

About Interpretation Letters

PHMSA 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.  While interpretations are one form of guidance provided by PHMSA, they do not create legally-enforceable rights or obligations and are provided to help the public understand how to comply with the HMR. PHMSA regularly reviews interpretations on this site for accuracy and applicability. Letters of interpretation which are found to be inaccurate or no longer applicable are removed and may be revised and reissued if warranted.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 (HMIC).


Contact the Hazardous Materials Information Center (HMIC) with questions on compliance with the hazardous materials regulations and interpretations of those regulations at:

    1-800-HMR-4922 or 1-800-467-4922

The HMIC is 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 during the next business day. Furthermore, if you would like to submit an interpretation letter request, please email it to: or mail it to:

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