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Below are the available interpretations and guidance for the given section. To return to the list of parts, use the Parts link above. If you do not see any guidance or interpretation information below, that means there is none available for the given part/section. Return to the search page to browse additional regulations sections.

PHMSA has published several Frequently Asked Questions (FAQ) documents on regulatory topics. Please review our FAQ documents below:

Additionally PHMSA has Hazardous Materials FAQs based on letters of interpretation on the following topics:

This section of the website provides the ability to find regulations information including regulations parts, content of sections, and interpretations for regulations sections. Below is a list of parts, identified by the part number and subject. To browse sections by part, click on the subject link below. You will be taken to the eCFR website to view sections for a part.

A written regulatory interpretation, response to a question, or an opinion concerning a pipeline safety issue may be obtained by submitting a written request to the Office of Pipeline Safety (PHP–30), PHMSA, U.S. Department of Transportation, 1200 New Jersey Avenue SE., Washington, DC 20590–0001 or at the email address: In addition to email address, the requestor must include his or her return address and should also include a daytime telephone number. Requests should be submitted at least 120 days before the time the requestor needs a response.

Note: electronic submission is provided as alternative to the 49 CFR Part 190.11(b) mail-in requirement.

The regulations text of the section can be found on the eCFR website. To view the regulations text, use the link below. For assistance, please send an email to

View part 173 on the ECFR website

Interpretation Responses

Ref ID Date Company Name Individual Name Response Document
24-0009 Entegris, Inc. Stefanie Yarbrough View
23-0058 Carbochem Inc, Mr. Gavin Kahn View
20-0076 Packgen Keith Russ View
20-0046 R.B. Natali Consulting, Inc. Ronald B. Natali View
19-0113 Pristine World Academy Sean Kelly View
19-0058 Mark Hawk View
18-0088 Settoon Towing Nicholas Kohnke View
18-0014 Nuclear Energy Institute Janet Schlueter View
18-0004 Commercial Vehicle Safety Alliance View
08-0274R Alcor Life Extension Foudation Tanya Jones View
17-0074 Western International Gas & Cylinders, Inc. Ms. Kathy McKenzie View

Regulation Sections

Section Subject
173.1 Purpose and scope
173.2 Hazardous materials classes and index to hazard class definitions
173.2a Classification of a material having more than one hazard
173.3 Packaging and exceptions
173.4 Small quantities for highway and rail
173.4a Excepted quantities
173.4b De minimis exceptions
173.5 Agricultural operations
173.5a Oilfield service vehicles, mechanical displacement meter provers, and roadway striping vehicles exceptions
173.5b Portable and mobile refrigeration systems
173.6 Materials of trade exceptions
173.7 Government operations and materials
173.8 Exceptions for non-specification packagings used in intrastate transportation
173.9 Transport vehicles or freight containers containing lading which has been fumigated
173.10 Tank car shipments
173.11 Exceptions for shipment of light bulbs containing hazardous materials
173.12 Exceptions for shipment of waste materials
173.13 Exceptions for Class 3, Divisions 4.1, 4.2, 4.3, 5.1, 6.1, and Classes 8 and 9 materials
173.21 Forbidden materials and packages
173.22 Shipper's responsibility
173.22a Use of packagings authorized under special permits
173.23 Previously authorized packaging
173.24 General requirements for packagings and packages
173.24a Additional general requirements for non-bulk packagings and packages
173.24b Additional general requirements for bulk packagings
173.25 Authorized packagings and overpacks
173.26 Quantity limitations
173.27 General requirements for transportation by aircraft
173.28 Reuse, reconditioning and remanufacture of packagings
173.29 Empty packagings
173.30 Loading and unloading of transport vehicles
173.31 Use of tank cars
173.32 Requirements for the use of portable tanks
173.33 Hazardous materials in cargo tank motor vehicles
173.35 Hazardous materials in IBCs
173.36 Hazardous materials in Large Packagings
173.37 Hazardous Materials in Flexible Bulk Containers
173.40 General packaging requirements for toxic materials packaged in cylinders
173.41 Sampling and testing program for unrefined petroleum-based products
173.50 Class 1-Definitions
173.51 Authorization to offer and transport explosives
173.52 Classification codes and compatibility groups of explosives
173.53 Provisions for using old classifications of explosives
173.54 Forbidden explosives
173.56 New explosives-definition and procedures for classification and approval
173.57 Acceptance criteria for new explosives
173.58 Assignment of class and division for new explosives
173.59 Description of terms for explosives
173.60 General packaging requirements for explosives
173.61 Mixed packaging requirements
173.62 Specific packaging requirements for explosives
173.63 Packaging exceptions
173.64 Exceptions for Division 1.3 and 1.4 fireworks
173.65 Exceptions for Division 1.4G consumer fireworks
173.66 Requirements for bulk packagings of certain explosives and oxidizers
173.115 Class 2, Divisions 2.1, 2.2, and 2.3-Definitions
173.116 Class 2-Assignment of hazard zone
173.120 Class 3-Definitions
173.121 Class 3-Assignment of packing group
173.124 Class 4, Divisions 4.1, 4.2 and 4.3-Definitions
173.125 Class 4-Assignment of packing group
173.127 Class 5, Division 5.1-Definition and assignment of packing groups
173.128 Class 5, Division 5.2-Definitions and types
173.129 Class 5, Division 5.2-Assignment of packing group
173.132 Class 6, Division 6.1-Definitions
173.133 Assignment of packing group and hazard zones for Division 6.1 materials
173.134 Class 6, Division 6.2-Definitions and exceptions
173.136 Class 8-Definitions
173.137 Class 8-Assignment of packing group
173.140 Class 9-Definitions
173.141 Class 9-Assignment of packing group
173.144 Other Regulated Material (ORM)-Definitions
173.145 Other Regulated Materials-Assignment of packing group
173.150 Exceptions for Class 3 (flammable and combustible liquids)
173.151 Exceptions for Class 4
173.152 Exceptions for Division 5.1 (oxidizers) and Division 5.2 (organic peroxides)
173.153 Exceptions for Division 6.1 (poisonous materials)
173.154 Exceptions for Class 8 (corrosive materials)
173.155 Exceptions for Class 9 (miscellaneous hazardous materials)
173.156 Exceptions for limited quantity and ORM
173.157 Reverse logistics-General requirements and exceptions for reverse logistics
173.158 Nitric acid
173.159 Batteries, wet
173.159a Exceptions for non-spillable batteries
173.160 Bombs, smoke, non-explosive (corrosive)
173.161 Chemical kits and first aid kits
173.162 Gallium
173.163 Hydrogen fluoride
173.164 Mercury (metallic and articles containing mercury)
173.165 Polyester resin kits
173.166 Safety devices
173.167 Consumer commodities
173.168 Chemical oxygen generators
173.170 Black powder for small arms
173.171 Smokeless powder for small arms
173.172 Aircraft hydraulic power unit fuel tank
173.173 Paint, paint-related material, adhesives, ink and resins.
173.174 Refrigerating machines
173.175 Permeation devices
173.176 Capacitors
173.181 Pyrophoric materials (liquids)
173.182 Barium azide-50 percent or more water wet
173.183 Nitrocellulose base film
173.184 Highway or rail fusee
173.185 Lithium cells and batteries
173.186 Matches
173.187 Pyrophoric solids, metals or alloys, n.o.s
173.188 White or yellow phosphorus
173.189 Batteries containing sodium or cells containing sodium
173.192 Packaging for certain toxic gases in Hazard Zone A
173.193 Bromoacetone, methyl bromide, chloropicrin and methyl bromide or methyl chloride mixtures, etc
173.194 Gas identification sets
173.195 Hydrogen cyanide, anhydrous, stabilized (hydrocyanic acid, aqueous solution)
173.196 Category A infectious substances
173.197 Regulated medical waste
173.198 Nickel carbonyl
173.199 Category B infectious substances
173.201 Non-bulk packagings for liquid hazardous materials in Packing Group I
173.202 Non-bulk packagings for liquid hazardous materials in Packing Group II
173.203 Non-bulk packagings for liquid hazardous materials in Packing Group III
173.204 Non-bulk, non-specification packagings for certain hazardous materials
173.205 Specification cylinders for liquid hazardous materials
173.206 Packaging requirements for chlorosilanes
173.211 Non-bulk packagings for solid hazardous materials in Packing Group I
173.212 Non-bulk packagings for solid hazardous materials in Packing Group II
173.213 Non-bulk packagings for solid hazardous materials in Packing Group III
173.214 Packagings which require approval by the Associate Administrator
173.216 Asbestos, blue, brown or white
173.217 Carbon dioxide, solid (dry ice)
173.218 Fish meal or fish scrap
173.219 Life-saving appliances
173.220 Internal combustion engines, vehicles, machinery containing internal combustion engines, battery-powered equipment or machinery, fuel cell-powered equipment or machinery
173.221 Polymeric beads, expandable and Plastic molding compound
173.222 Dangerous goods in equipment, machinery or apparatus
173.223 Packagings for certain flammable solids
173.224 Packaging and control and emergency temperatures for self-reactive materials
173.225 Packaging requirements and other provisions for organic peroxides
173.226 Materials poisonous by inhalation, Division 6.1, Packing Group I, Hazard Zone A
173.227 Materials poisonous by inhalation, Division 6.1, Packing Group I, Hazard Zone B
173.228 Bromine pentafluoride or bromine trifluoride
173.229 Chloric acid solution or chlorine dioxide hydrate, frozen
173.230 Fuel cell cartridges containing hazardous material
173.231 Ammonium nitrate emulsion, suspension or gel
173.240 Bulk packaging for certain low hazard solid materials
173.241 Bulk packagings for certain low hazard liquid and solid materials
173.242 Bulk packagings for certain medium hazard liquids and solids, including solids with dual hazards
173.243 Bulk packaging for certain high hazard liquids and dual hazard materials which pose a moderate hazard
173.244 Bulk packaging for certain pyrophoric liquids (Division 4.2), dangerous when wet (Division 4.3) materials, and poisonous liquids with inhalation hazards (Division 6.1).
173.245 Bulk packaging for extremely hazardous materials such as poisonous gases (Division 2.3)
173.247 Bulk packaging for certain elevated temperature materials
173.249 Bromine
173.251 Bulk packaging for ammonium nitrate emulsion, suspension, or gel
173.301 General requirements for shipment of compressed gases and other hazardous materials in cylinders, UN pressure receptacles and spherical pressure vessels
173.301a Additional general requirements for shipment of specification cylinders
173.301b Additional general requirements for shipment of UN pressure receptacles
173.302 Filling of cylinders with nonliquefied (permanent) compressed gases or adsorbed gases
173.302a Additional requirements for shipment of nonliquefied (permanent) compressed gases in specification cylinders
173.302b Additional requirements for shipment of non-liquefied (permanent) compressed gases in UN pressure receptacles
173.302c Additional requirements for the shipment of adsorbed gases in UN pressure receptacles
173.303 Charging of cylinders with compressed gas in solution (acetylene).
173.304 Filling of cylinders with liquefied compressed gases
173.304a Additional requirements for shipment of liquefied compressed gases in specification cylinders
173.304b Additional requirements for shipment of liquefied compressed gases in UN pressure receptacles
173.305 Charging of cylinders with a mixture of compressed gas and other material
173.306 Limited quantities of compressed gases
173.307 Exceptions for compressed gases
173.308 Lighters
173.309 Fire extinguishers
173.310 Exceptions for radiation detectors
173.311 Metal hydride storage systems
173.312 Requirements for shipment of MEGCs
173.313 UN Portable Tank Table for Liquefied Compressed Gases and Chemical Under Pressure
173.314 Compressed gases in tank cars and multi-unit tank cars
173.315 Compressed gases in cargo tanks and portable tanks
173.316 Cryogenic liquids in cylinders
173.318 Cryogenic liquids in cargo tanks
173.319 Cryogenic liquids in tank cars
173.320 Cryogenic liquids; exceptions
173.321 Ethylamine
173.322 Ethyl chloride
173.323 Ethylene oxide
173.334 Organic phosphates mixed with compressed gas
173.335 Chemical under pressure n.o.s
173.336 Nitrogen dioxide, liquefied, or dinitrogen tetroxide, liquefied
173.337 Nitric oxide
173.338 Tungsten hexafluoride
173.340 Tear gas devices
173.401 Scope
173.403 Definitions
173.410 General design requirements
173.411 Industrial packages
173.412 Additional design requirements for Type A packages
173.413 Requirements for Type B packages
173.415 Authorized Type A packages
173.416 Authorized Type B packages
173.417 Authorized fissile materials packages
173.418 Authorized packages-pyrophoric Class 7 (radioactive) materials
173.419 Authorized packages-oxidizing Class 7 (radioactive) materials
173.420 Uranium hexafluoride (fissile, fissile excepted and non-fissile)
173.421 Excepted packages for limited quantities of Class 7 (radioactive) materials
173.422 Additional requirements for excepted packages containing Class 7 (radioactive) materials
173.423 Requirements for multiple hazard limited quantity Class 7 (radioactive) materials
173.424 Excepted packages for radioactive instruments and articles
173.425 Table of activity limits-excepted quantities and articles
173.426 Excepted packages for articles containing natural uranium or thorium
173.427 Transport requirements for low specific activity (LSA) Class 7 (radioactive) material and surface contaminated objects (SCO)
173.428 Empty Class 7 (radioactive) materials packaging
173.431 Activity limits for Type A and Type B packages
173.433 Requirements for determining basic radionuclide values, and for the listing of radionuclides on shipping papers and labels
173.434 Activity-mass relationships for uranium and natural thorium
173.435 Table of A1 and A2 values for radionuclides
173.436 Exempt material activity concentrations and exempt consignment activity limits for radionuclides
173.441 Radiation level limitations and exclusive use provisions
173.442 Thermal limitations
173.443 Contamination control
173.447 Storage incident to transportation-general requirements
173.448 General transportation requirements
173.453 Fissile materials-exceptions
173.457 Transportation of fissile material packages-specific requirements
173.459 Mixing of fissile material packages with non-fissile or fissile-excepted material packages
173.461 Demonstration of compliance with tests
173.462 Preparation of specimens for testing
173.465 Type A packaging tests
173.466 Additional tests for Type A packagings designed for liquids and gases
173.467 Tests for demonstrating the ability of Type B and fissile materials packagings to withstand accident conditions in transportation
173.468 Test for LSA-III material
173.469 Tests for special form Class 7 (radioactive) materials
173.471 Requirements for U.S. Nuclear Regulatory Commission approved packages
173.472 Requirements for exporting DOT Specification Type B and fissile packages
173.473 Requirements for foreign-made packages
173.474 Quality control for construction of packaging
173.475 Quality control requirements prior to each shipment of Class 7 (radioactive) materials
173.476 Approval of special form Class 7 (radioactive) materials
173.477 Approval of packagings containing greater than 0.1 kg of non-fissile or fissile-excepted uranium hexafluoride
173.232 Articles containing hazardous materials, n.o.s.