DOT Hazard Classes Overview

The shipper (offeror) decides it. Under 49 CFR 173.22(a)(1), whoever offers a hazardous material for transport must "class and describe the hazardous material in accordance with parts 172 and 173". In practice that means testing or assessing the material against each class definition in Part 173. If it meets more than one, the 173.2a precedence rules pick the primary class. The 172.101 Hazardous Materials Table then gives the shipping name, class and packing group to use.

Approach: The datasets I can search hold only the 49 CFR 172.101 Hazardous Materials Table. The classification rules themselves (173.22, 173.2a and the 172.101 instructions) come from the current eCFR text.

1. Compare the material with the class definitions

• Each class and division has its own definition and test criteria in Part 173 (for example 173.120 for Class 3 and 173.136 for Class 8), as listed in the 173.2 index from my last answer.
• If the hazard class is uncertain, for example with samples or wastes, 172.101(c)(11) lets the shipper assign a tentative description. It must be based on the defining criteria, the hazard precedence rules and the shipper's own knowledge of the material. The description must include the word "Sample", and the package may hold no more than 2.5 kg net.

2. If it meets more than one definition, apply the precedence rules in 49 CFR 173.2a

• 173.2a(a) order, highest first: Class 7, then Divisions 2.3, 2.1 and 2.2. Next comes Division 6.1 PG I when it is poisonous by inhalation only, then pyrophoric materials (4.2) and self-reactive materials (4.1). After that, Class 3, Class 8 and Divisions 4.1, 4.2, 4.3, 5.1 and 6.1 are ranked against each other using the Precedence of Hazard Table. Combustible liquids come next, and Class 9 is last.
• 173.2a(b) notes:
  • The most stringent packing group assigned to any of the hazards takes precedence.
  • A Class 8 material that meets PG I for inhalation toxicity is classed as 6.1 if its oral or dermal toxicity meets PG I or II.
• 173.2a(c) exceptions, where the table is not used:
  • Explosives get only a Class 1 division.
  • Organic peroxides stay 5.2.
  • Infectious substances stay 6.2, except where they also meet Class 7.
  • Specifically listed or approved wetted explosives stay 4.1.
  • Limited-quantity Class 7 materials follow 173.423.

3. Find the entry in the Hazardous Materials Table (172.101)

• If the material is listed by name, column 3 gives its class or division. Column 5 gives its packing group: PG I, II or III means great, medium or minor danger. Where an entry shows more than one packing group, you assign it using the criteria in Part 173, subpart D.
• If it is not listed by name, use the generic or n.o.s. entry that best describes it (172.101(c)(12)(ii)). For a material with several hazards, the precedence result decides which n.o.s. name applies (172.101(c)(12)(iii)). For example:
  • "Flammable liquids, corrosive, n.o.s." (UN2924) is Class 3 with labels 3 and 8 ^[1].
  • "Corrosive liquids, flammable, n.o.s." (UN2920) is Class 8 with labels 8 and 3 ^[2].
  • Which of the two applies depends on where the 3/8 pair falls in the precedence table.

4. The test data can override the Table, except for "+" entries

• A material listed under one class but found to meet another class's definition must be described under the correct entry instead. A material that meets no class definition is not a hazardous material (172.101(c)(12)).
• A "+" in column 1 fixes the shipping name, class and packing group "without regard to whether the material meets the definition" (172.101(b)).
• Entries marked "Forbidden" in column 3 cannot be transported unless the material is diluted, stabilized or built into a device, and then classed under the Part 173 definitions (172.101(d)).
• A Class 3 material reclassified as a combustible liquid under 173.150(e)-(f), or with a flash point above 60°C and below 93°C, has its column 3 entry read "Combustible liquid".

Sources: eCFR 49 CFR 173.22; eCFR 49 CFR 173.2a; eCFR 49 CFR 172.101

Division 6.1 is non-gaseous poisonous material, and Class 8 is material that corrodes skin or metal. In the 49 CFR 172.101 Hazardous Materials Table, Division 6.1 has 534 entries (530 UN/NA numbers) and Class 8 has 302 entries (283 numbers). I've put every entry, with its packing groups, labels and a citation, in the panel document.

Approach: I took the entries from the 49 CFR 172.101 Hazardous Materials Table. The definitions and packing-group criteria (173.132, 173.133, 173.136, 173.137) and the special-provision meanings (172.102) aren't in the datasets, so they come from the current eCFR. An "entry" here is one shipping name with its ID number. Some names appear on more than one table line, one per packing group.

Division 6.1: Poisonous materials

What qualifies (173.132):

• It is a material other than a gas, known to be so toxic to humans that it is a health hazard in transport.
• Without human data, it qualifies if it meets any of these limits:
  • oral LD50 of 300 mg/kg or less
  • dermal LD50 of 1000 mg/kg or less
  • dust or mist inhalation LC50 of 4 mg/L or less
  • vapour inhalation LC50 of 5000 mL/m³ or less, if its vapour concentration is high enough
• Irritating materials similar to tear gas also qualify.
• PHMSA's Associate Administrator can exempt a material that will not cause serious sickness or death.

Packing groups (173.133):

Vapours are ranked differently: the most toxic go to PG I, Hazard Zone A (LC50 ≤200 mL/m³) or Zone B (≤1000 mL/m³).

What the table holds:

• Lines by packing group: 742 lines in total, 220 at PG I, 310 at PG II, 208 at PG III and 4 with no packing group.

• Generic entries: 89 entries are n.o.s.

• Subsidiary labels:

  Label	Entries
  3 (flammable)	85
  8 (corrosive)	52
  5.1	6
  4.3	6
  4.1	4
  4.2	3
  7	1

• Inhalation hazard: 95 entries carry special provision 1, 2, 3, 4, 5 or 6. Provisions 1 to 4 put the material in Hazard Zones A to D, and 6 marks it as poisonous by inhalation. Provision 5 is conditional: the shipping name must show the inhalation hazard only if the material meets the definition.

Examples:

• Hydrogen cyanide, stabilized (UN1051), PG I, labels 6.1 and 3, special provision 1 (Zone A) ^[3]
• Acrolein, stabilized (UN1092), PG I, special provision 1 ^[4]
• Chloropicrin (UN1580), PG I, special provision 2 (Zone B) ^[5]
• Arsenic trioxide (UN1561), PG II ^[6]
• Zinc cyanide (UN1713), PG I ^[7]
• Phenol solutions (UN2821), PG II or III ^[8]
• Pesticides, liquid, toxic, n.o.s. (UN2902), PG I, II or III ^[9]

Class 8: Corrosive material

What qualifies (173.136):

• A liquid or solid that causes irreversible damage to human skin within a set contact time.
• Or a liquid, or a solid that may become liquid in transport, that severely corrodes steel or aluminium.
• PHMSA can change the class where human experience shows more or less hazard than the tests do.

Packing groups (173.137):

• PG I: irreversible skin damage after 3 minutes' exposure or less, observed for up to 60 minutes.
• PG II: the same damage after more than 3 and up to 60 minutes' exposure, observed for up to 14 days.
• PG III: either of these:
  • the same damage after more than 60 minutes and up to 4 hours' exposure
  • no skin damage, but corrosion of more than 6.25 mm (0.25 inch) a year on both steel and aluminium at 55 °C

What the table holds:

• Lines by packing group: 385 lines in total, 62 at PG I, 202 at PG II, 114 at PG III and 7 with no packing group (articles such as wet batteries).

• Generic entries: 37 entries are n.o.s.

• Subsidiary labels:

  Label	Entries
  3	31
  6.1	30
  5.1	8
  4.2	2
  4.3	2
  4.1	1

• Inhalation hazard: 11 entries also carry an inhalation-hazard special provision.

Examples:

• Sulfuric acid, more than 51% acid (UN1830), PG II ^[10]
• Hydrochloric acid (UN1789), PG II or III ^[11]
• Sodium hydroxide solution (UN1824), PG II or III ^[12]
• Nitric acid other than red fuming, more than 70% (UN2031), PG I, labels 8 and 5.1 ^[13]
• Hydrofluoric acid, more than 60% (UN1790), PG I, labels 8 and 6.1 ^[14]
• Bromine (UN1744), PG I, labels 8 and 6.1, special provision 1 ^[15]
• Batteries, wet, filled with acid (UN2794), no packing group ^[16]

Where the two overlap: Many materials are both toxic and corrosive, and which one is primary follows the 173.2a precedence rules from my last answer. Red fuming nitric acid (UN2032) shows how far this goes: it is Class 8 with labels 8, 5.1 and 6.1 and special provision 2 ^[17].

Sources: eCFR 49 CFR 173.132; eCFR 49 CFR 173.133; eCFR 49 CFR 173.136; eCFR 49 CFR 173.137; eCFR 49 CFR 172.102