PHMSA letter of interpretation

Transportation of Zinc Dust

Pipeline and Hazardous Materials Safety Administration · Jun 2, 1993 · Reference No. 93-0102

PHMSA says to use caution with this letter.

Interprets: 49 CFR § 172.101

Reggie’s quick take

Someone asked whether zinc dust that fails the Division 4.3 water-reactivity test must still be regulated under the Hazardous Materials Regulations. The agency said it depends on the material. Zinc dust may be shipped as "Zinc dust, 4.3, UN1436," as "Environmentally hazardous substance, solid, n.o.s., 9, UN3077, PG III," or as not regulated. For the tested material, the agency agreed it does not meet the 4.3 criteria in § 173 Appendix E.

Written by AI (Claude) on Oct 22, 2026. Robots make mistakes and leave things out, so the letter below is what counts. Not legal advice.

This is in response to your letter dated January 19, 1993, concerning the applicability of the Hazardous Materials Regulations (HMR; 49 CFR Parts 171-180) to the transportation of zinc dust that does not meet the test criteria for Division 4.3.

Depending on the properties of the material, zinc dust may be described, as appropriate, as "Zinc dust, 4.3, UN1436", "Environmentally hazardous substance, solid, n.o.s., 9, UN3077, PG III", or as not regulated. The test method and criteria for classifying a material as Division 4.3 are contained in 49 CFR Part 173, Appendix E, paragraph 4. If your material liberates hydrogen gas when in contact with water, to the extent that it meets the prescribed criteria, it would be classed as Division 4.3. Based on the test results you submitted on zinc dust, we agree that this particular tested material does not meet the Division 4.3 criteria.

If the diameter of the zinc particles is less that 100 micrometers (0.004 inches) and the quantity in one package, or transport vehicle if not packaged, equals or exceeds 1000 pounds, the material is a hazardous substance. If your material meets neither criteria, then it is not subject to the HMR.

A letter of interpretation applies the regulations to the facts the requester described. It isn’t a regulation and isn’t legal advice. The agency’s copy.