How dangerous, within a hazard class? The PG assignment drives packaging strength, quantity limits, and labeling for nearly every regulated material under 49 CFR.
A hazardous material's hazard class tells you what kind of danger it presents — flammable, toxic, corrosive, oxidizer. The packing group tells you how dangerous it is within that class. Packing Group I is the most hazardous tier, Packing Group III the least, with PG II in between. The assignment is made by quantitative criteria specific to each class, codified in 49 CFR Part 173 and harmonized with the UN Recommendations on the Transport of Dangerous Goods.
PG drives more than a label. It determines the minimum performance standard of the packaging (UN-spec X, Y, or Z), eligibility for limited quantity and excepted quantity exceptions, per-mode quantity caps, and in some cases the proper shipping name itself. A material at PG I and the same material at PG II are treated as effectively different shipments — different packaging, different paperwork, different limits.
Not every hazard class uses packing groups. Classes 1, 2, 6.2, and 7 use other categorization systems entirely — divisions, compatibility groups, categories, label types. The full breakdown of which classes use what is in the hazard class chart.
PG I covers the most hazardous materials within their respective classes — substances whose physical or toxicological properties present great danger during transport. Examples include extremely flammable liquids with low flashpoints and low boiling points (Diethyl Ether, Carbon Disulfide), highly toxic materials (Hydrogen Cyanide), and strongly corrosive substances that destroy skin tissue within minutes.
PG I assignment triggers the strictest packaging, labeling, and mode-of-transport requirements. UN-specification packaging must be X-marked, indicating it passed the most stringent drop, leakproofness, and hydrostatic pressure tests. Limited quantity exceptions are generally not available for PG I materials, and excepted quantities are sharply restricted. Air shipments of PG I are subject to the tightest IATA per-package limits — many PG I substances are forbidden on passenger aircraft entirely.
PG I criteria by hazard class
Class
Criterion for PG I
CFR
3 — Flammable Liquid
Initial boiling point ≤ 35 °C (95 °F)
§173.121
4.1 — Flammable Solid
Self-reactive Types A-B; certain readily combustible solids by burn rate
§173.124
4.2 — Pyrophoric / Spontaneously Combustible
Pyrophoric liquids or solids; substances that ignite within 5 minutes of air contact
§173.124
4.3 — Dangerous When Wet
Reacts with water spontaneously at ambient temp and gives off ≥ 10 L flammable gas per kg per minute
§173.124
5.1 — Oxidizer
Greater oxidizing effect than 50% perchloric acid in standardized test
Heads up: Some materials are forbidden on passenger aircraft and have severe quantity limits on cargo aircraft. Always check the §172.101 Hazardous Materials Table for mode-specific restrictions before tendering a PG I shipment.
IIMedium Danger
Packing Group II
Substances presenting medium danger
Moderate hazard tier · Y-marked UN packaging · Standard hazmat ops
PG II is the workhorse tier — covering most of the chemicals that move every day in commerce. Common solvents (Acetone, Methanol, Isopropanol), industrial acids and bases (Hydrochloric Acid, Sodium Hydroxide solution), gasoline, and a long list of regulated chemistries fall here. The criteria across classes share a common shape: moderate flashpoints, moderate toxicity, materials that destroy skin in minutes-to-an-hour rather than minutes.
Packaging must be Y-marked (or X, since X-marked packaging is authorized for any lower group). Limited quantity exceptions are widely available — most PG II liquids can ship up to 1 L per inner packaging and 30 kg gross per outer using LQ provisions, dramatically simplifying labeling and paperwork for small quantities. Excepted quantity codes apply for very small amounts.
PG II criteria by hazard class
Class
Criterion for PG II
CFR
3 — Flammable Liquid
Flashpoint < 23 °C (73 °F) AND initial boiling point > 35 °C
§173.121
4.1 — Flammable Solid
Readily combustible solids with moderate burn rate; self-reactive Types C-F
§173.124
4.2 — Spontaneously Combustible
Solids that exhibit self-heating but don't ignite spontaneously in 5 min
§173.124
4.3 — Dangerous When Wet
Reacts with water and gives off ≥ 20 L flammable gas per kg over one hour
§173.124
5.1 — Oxidizer
Oxidizing effect between 50% perchloric acid and 40% sodium chlorate solution
§173.127
6.1 — Toxic
Oral LD50 > 5 to ≤ 50 mg/kg, dermal LD50 > 50 to ≤ 200 mg/kg, or inhalation LC50 > 200 to ≤ 1000 ppm
§173.133
8 — Corrosive
Destroys intact skin tissue in observation period of > 3 min to ≤ 60 min after exposure of ≤ 14 days
Lowest regulated tier · Z-marked UN packaging · Broadest exceptions
PG III is the lowest tier within the regulated range. Materials here are still hazardous — they wouldn't be regulated otherwise — but they present the least danger of the three groups. Diesel fuel, dilute ethanol solutions, mild corrosives, and many environmentally hazardous substances live here.
PG III gets the broadest set of regulatory exceptions. Z-marked packaging is the minimum requirement, performance-tested at the lowest standard. LQ exceptions are generous — typical PG III liquids can ship up to 5 L per inner packaging and 30 kg gross per outer. Many PG III materials qualify for the "Combustible Liquid" reclassification under domestic ground transport (49 CFR §173.150) when the flashpoint is between 38 °C and 93 °C, which exempts them from most hazmat marking and paperwork requirements.
PG III criteria by hazard class
Class
Criterion for PG III
CFR
3 — Flammable Liquid
Flashpoint 23 °C to 60 °C (73 °F to 140 °F)
§173.121
4.1 — Flammable Solid
Readily combustible solids with slow burn rate; self-reactive Type G
§173.124
4.2 — Spontaneously Combustible
Solids exhibiting limited self-heating effect
§173.124
4.3 — Dangerous When Wet
Reacts with water and gives off ≥ 1 L flammable gas per kg per hour
§173.124
5.1 — Oxidizer
Oxidizing effect between 65% nitric acid and 40% sodium chlorate (lower than PG II)
§173.127
6.1 — Toxic
Oral LD50 > 50 to ≤ 300 mg/kg, dermal LD50 > 200 to ≤ 1000 mg/kg, or inhalation LC50 > 1000 to ≤ 5000 ppm
§173.133
8 — Corrosive
Destroys intact skin tissue in observation period of > 60 min to ≤ 14 days; OR causes severe corrosion to steel/aluminum (> 6.25 mm/year)
Every UN-specification package carries a marking code molded, printed, or stenciled on the container — usually on the bottom or side. The code identifies the package type, material, performance standard, year of manufacture, and country of approval. Buried in that string is a single letter X, Y, or Z indicating which packing groups the package is authorized for.
Example marking on a UN-spec steel drum:
UN 1A1/Y1.4/150/26/USA/M4567
That "Y" in the middle means this drum is certified for PG II and PG III liquids — but not PG I. Trying to ship a PG I material in a Y-marked package is a violation, regardless of how sturdy the drum looks.
X
Authorized for PG I · II · III
Highest performance standard. Required for PG I materials; usable for any lower group. Passes the most stringent drop, leakproofness, hydrostatic, and stack tests.
Y
Authorized for PG II · III
Moderate performance standard. Most common UN-spec packaging in commerce, since PG II covers the bulk of regulated chemicals.
Z
Authorized for PG III only
Lowest performance standard in the UN-spec system. Suitable only for the least-hazardous regulated materials.
The full marking decoded: The example above reads — UN (United Nations standard), 1A1 (closed-head steel drum), Y1.4 (PG II/III rated, specific gravity 1.4), 150 (hydrostatic test pressure in kPa), 26 (year of manufacture 2026), USA (country of approval), M4567 (manufacturer code). The full specification rules are in 49 CFR §178.503.
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Why PG Matters
What changes when the packing group changes
The same chemical at a different packing group is a different shipment. Five places PG drives the rules:
1. Packaging performance
The X/Y/Z system above. PG I requires X-marked. PG II accepts X or Y. PG III accepts X, Y, or Z. Buying lower-rated packaging "to save money" is one of the most common violations — and one of the easiest for an inspector to spot from the markings.
2. Limited Quantity (LQ) eligibility
The limited quantity exception in 49 CFR §173.150–155 dramatically simplifies hazmat shipping for small inner packagings. PG I is rarely eligible. PG II usually allows 0.5–1 L per inner. PG III usually allows 1–5 L per inner. The exact limit is in column 9a of the §172.101 Hazardous Materials Table.
3. Excepted Quantity (EQ) codes
EQ codes E0 through E5 cap inner/outer quantities for the smallest hazmat shipments. PG I is mostly E0 (excepted quantities not permitted) or E1. PG II is usually E2–E4. PG III is usually E1 or E2 — counterintuitively, EQ codes don't simply scale with PG. They reflect class-specific hazard profiles independent of PG.
4. Per-mode quantity limits
IATA DGR (air) and the IMDG Code (sea) publish per-package and per-mode-of-transport limits driven by class and PG. PG I in cargo aircraft is severely restricted; many PG I materials are forbidden on passenger aircraft outright. PG III typically gets the highest per-package allowances.
5. Combustible liquid reclassification (PG III, Class 3 only)
Under 49 CFR §173.150(f), a Class 3 PG III flammable liquid with a flashpoint of 38 °C to 93 °C may be reclassified as a "Combustible Liquid" for domestic ground transport. Combustible Liquids are exempt from most hazmat marking, labeling, and shipping paper requirements — a major simplification used heavily for diesel fuel and similar materials. The exception does not apply to air or marine shipments.
The Exceptions
Hazard classes that don't use packing groups
Four hazard classes use different categorization systems entirely. If you're trying to find the "packing group" for an explosive, gas, infectious substance, or radioactive material — you're looking for the wrong field.
Class 1
Explosives
Uses divisions (1.1 through 1.6) for explosion behavior and compatibility groups (letters A through S) for storage/transport segregation. A typical Class 1 entry reads "1.4G" — Division 1.4, Compatibility Group G.
Class 2
Gases
Uses divisions only: 2.1 Flammable Gas, 2.2 Non-Flammable Non-Toxic Gas, 2.3 Toxic Gas. The hazard tier is encoded directly in the division number rather than a separate PG field.
Class 6.2
Infectious Substances
Uses Category A (causes permanent disability or life-threatening disease — e.g. UN2814 Infectious Substance, Affecting Humans) and Category B (less hazardous — UN3373 Biological Substance, Category B).
Class 7
Radioactive Material
Uses label categories (Radioactive White-I, Yellow-II, Yellow-III) reflecting external radiation level and a Transport Index for criticality and dose-rate control. Package type (Excepted, Industrial, Type A, Type B, Type C) is a separate axis.
Where PG Rules Come From
Regulatory anchors
The PG concept originates with the UN Recommendations on the Transport of Dangerous Goods (the "Orange Book"). Every modal and national regulation adopts the framework, with the specific assignment criteria appearing in their hazard-class chapters:
49 CFR (United States, ground)
Pipeline and Hazardous Materials Safety Administration (PHMSA) enforces. PG criteria for each class appear in Part 173: §173.121 (Class 3), §173.124 (Classes 4.1–4.3), §173.127 (Class 5.1), §173.133 (Class 6.1), §173.137 (Class 8). The §172.101 Hazardous Materials Table assigns PG per UN number in column 5.
IATA DGR (air)
The IATA Dangerous Goods Regulations align with PHMSA PG assignments but tighten the per-package quantity limits in columns I, J, K, L, M of the DGR list. PG I in air is often forbidden or severely limited — particularly in passenger aircraft.
IMDG Code (sea)
The International Maritime Dangerous Goods Code uses identical PG assignments and adds marine pollutant and segregation rules. PG affects stowage category, with PG I typically requiring "Away from" or "Separated from" rules against incompatible cargo.
ADR (European road)
ADR — the European Agreement concerning the International Carriage of Dangerous Goods by Road — uses the identical Packing Group I, II, III system. The "ADR packing groups" are the same three tiers defined on this page, drawn from the same UN Recommendations, so a material's packing group under 49 CFR generally matches its packing group under ADR. The differences between ADR and 49 CFR lie in documentation, placarding, and tunnel-restriction codes — not in the packing group assignment itself.
FAQ
Common questions
What are the three packing groups in hazardous materials shipping?
PG I (great danger), PG II (medium danger), and PG III (minor danger). The assignment is made based on the physical and chemical properties of the material as defined in 49 CFR Part 173. PG I requires the strongest UN-specification packaging (X-marked), PG II requires Y-marked packaging, and PG III requires Z-marked packaging.
How is a packing group assigned to a hazardous material?
Packing group assignment is determined by quantitative criteria specific to each hazard class. Flammable liquids (Class 3) are assigned by flashpoint and initial boiling point under §173.121. Toxic materials (Class 6.1) are assigned by oral, dermal, or inhalation LD50/LC50 values under §173.133. Corrosives (Class 8) are assigned by skin-destruction observation times under §173.137. Oxidizers, flammable solids, and other classes have their own rules.
What's the difference between PG I, PG II, and PG III?
PG I materials present a great danger and require X-marked UN-spec packaging that passes the most stringent drop, leakproofness, and stack tests. PG II materials present medium danger; Y-marked packaging meets the lower performance standard. PG III materials present minor danger; Z-marked packaging is acceptable. PG also drives LQ and EQ eligibility, and per-mode shipping limits — air shipments are particularly affected.
Why don't all hazard classes have packing groups?
Class 1 (Explosives) uses divisions (1.1 through 1.6) and compatibility groups (letters A through S). Class 2 (Gases) uses divisions only. Class 6.2 (Infectious Substances) uses Category A and Category B. Class 7 (Radioactive) uses label categories (White-I, Yellow-II, Yellow-III) and a Transport Index. These alternative systems describe the hazards more usefully than the PG I/II/III scale.
What do the X, Y, and Z markings on UN packaging mean?
X-marked packaging is authorized for PG I, II, and III. Y-marked is authorized for PG II and III. Z-marked is authorized for PG III only. The markings appear in the UN packaging code molded or printed on the container per 49 CFR §178.503, along with the packaging type, material, year of manufacture, and country of approval.
UNLookup is a reference utility. The PG criteria and packaging information above summarize 49 CFR Parts 173 and 178 but are not a substitute for the current edition of those regulations, the IATA DGR, the IMDG Code, or the manufacturer's UN-specification documentation. Always verify against the official source applicable to your mode of transport before tendering a shipment. Regulations change; this page is updated periodically and may not reflect the most recent amendments.