Hazmat Class 3: DOT Flammable and Combustible Liquids Explained

Short answer: Hazard Class 3 in the US Department of Transportation (DOT) system covers flammable liquids, defined in 49 CFR 173.120 as liquids with a flash point of not more than 60 °C (140 °F). Liquids with a flash point above 60 °C and below 93 °C (200 °F) that meet no other hazard class are “combustible liquids”, handled in the same part of the regulations. This page is general regulatory information for lab and clinic staff, not a substitute for the current regulations or for shipper training.

Definition and scope

49 CFR 173.2 lists nine hazard classes: 1 explosives; 2 gases (divisions 2.1 flammable, 2.2 non-flammable compressed, 2.3 poisonous); 3 flammable and combustible liquids; 4 (4.1 flammable solid, 4.2 spontaneously combustible, 4.3 dangerous when wet); 5 (5.1 oxidizer, 5.2 organic peroxide); 6 (6.1 poisonous, 6.2 infectious substance); 7 radioactive; 8 corrosive; 9 miscellaneous. Class 3 has no divisions. Divisions exist only for classes 1, 2, 4, 5 and 6.

Under 49 CFR 173.120(a), a Class 3 flammable liquid is a liquid with a flash point of not more than 60 °C (140 °F). Five exceptions in that paragraph take a liquid out of Class 3:

• it meets the definition of another hazard class under 173.115 (for example a Class 6.1 material);
• it is a mixture with one or more components having a flash point of 60 °C or higher that meets the 99 percent by volume condition set out in the paragraph;
• it has a flash point above 35 °C (95 °F) and does not sustain combustion, as determined by ASTM D 4206;
• it has a flash point above 35 °C and a fire point greater than 100 °C (ISO 2592);
• it has a flash point above 35 °C and is in a water-miscible solution containing more than 90 percent water by mass.

The last exception is the one dilutions in a laboratory tend to meet: a dilute aqueous solution of a water-miscible flammable liquid can fall outside Class 3 if it satisfies that paragraph exactly.

How the Class 3 rules fit together in practice

The rules sit in separate sections of 49 CFR, which is why single-topic pages tend to answer only one piece. The table below follows one Class 3 material through them, each row tied to the section it comes from.

Question	What the regulation says	Source
Is it Class 3?	Flash point not more than 60 °C (140 °F), unless one of the five exceptions applies.	49 CFR 173.120(a)
Is it only “combustible”?	Flash point above 60 °C (140 °F) and below 93 °C (200 °F), and not meeting another hazard class.	49 CFR 173.120(b)(1)
What packing group?	PG I: initial boiling point of 35 °C (95 °F) or less. PG II: flash point below 23 °C (73 °F) and initial boiling point above 35 °C. PG III: flash point 23 °C to 60 °C (73 °F to 140 °F) and initial boiling point above 35 °C. Flash point is closed-cup.	49 CFR 173.121
What identification number?	Each proper shipping name in the Hazardous Materials Table has an ID number. A “UN” prefix means the entry is suitable for domestic and international transport; “NA” means it is not recognized outside the US.	49 CFR 172.101
Which placard?	FLAMMABLE placard for Class 3; COMBUSTIBLE placard for combustible liquids.	49 CFR 172.504; 172.542; 172.544
Can it go by mail?	Depends on channel: see the USPS section below.	USPS Publication 52, section 343

The packing group, the degree of danger in the Hazardous Materials Table, is the same I/II/III scale described in the dictionary entry for packing group; Packing Group I indicates the highest danger. The table also carries special provisions for each entry, defined in 49 CFR 172.102.

What are the UN numbers used for HAZMAT items?

They are the identification numbers in column 4 of the Hazardous Materials Table (49 CFR 172.101), assigned per proper shipping name. A “UN” prefix marks entries valid for domestic and international transport; “NA” entries are not recognized outside the United States. Paint, for example, appears in the USPS rules under UN1263.

Does Class 3 flammables require placards?

Class 3 is listed in Table 2 of 49 CFR 172.504, so a FLAMMABLE placard is required on each side and each end of bulk packagings, freight containers, unit load devices, transport vehicles and rail cars carrying it. Combustible liquids use the COMBUSTIBLE placard. There is a size-based exception for highway and rail: a transport vehicle or freight container holding less than 454 kg (1,001 lb) aggregate gross weight of Table 2 materials needs no placards, though placards may still be displayed. That exception does not apply to bulk packaging. FLAMMABLE may be substituted for COMBUSTIBLE on cargo tanks, portable tanks and compartmented tank cars carrying both.

Placards relate to the transport vehicle or container, not to an individual bottle. Labelling and marking of packages is a separate set of requirements; see hazmat shipping papers under 49 CFR for the documentation side.

Can I mail paint with USPS?

Often yes, within limits. USPS Publication 52, section 343 (“Flammable and Combustible Liquids (Hazard Class 3)”), states that paint (UN1263) is mailable if it meets the Limited Quantity surface criteria. Latex or water-based paint that is non-flammable and non-combustible is unrestricted and not hazmat. Other points from the same section:

• International mail: flammable liquids are prohibited, and combustible liquids are also barred.
• Domestic air: flammable liquids are prohibited; combustible liquids may go by air if they are ID8000 materials.
• Surface: flammable liquids with a flashpoint at or below 20 °F (-7 °C) are prohibited; others are mailable if they qualify as Limited Quantity surface material and meet Exhibit 343.21.
• Gasoline is nonmailable.
• Mailable Class 3 packages must be marked on the address side; surface packages need the approved DOT Limited Quantity marking and no shipping paper.

Laboratory solvents are a different matter from consumer paint; for lab and medical items see hazmat shipping for medical supplies.

Where it matters for labs and clinics

• Solvent and alcohol shipments are classified by the flash point and boiling point of the actual product or mixture, not by the name on the label. The exceptions in 173.120(a) mean concentration matters. Product-specific hazard data sits in the SDS; see acetone SDS and methanol SDS for how those documents present flammability information.
• Air and sea carriage use related but separate rule sets; see IATA dangerous goods regulations.
• Laboratory hazard classes in storage and handling follow a different framework from transport classes; the common lab chemical hazard classes primer covers that side.

Common misunderstandings

• “Class 3 has divisions like 3.1.” It does not; 49 CFR 173.2 lists no divisions for Class 3.
• “Flammable and combustible are the same.” The regulatory cut-off is 60 °C (140 °F): at or below is flammable (Class 3), above 60 °C and below 93 °C (200 °F) is combustible.
• “A placard is needed on every package.” Placards are required on vehicles and bulk containers, subject to the 1,001 lb exception for highway and rail.
• “All paint is hazmat.” Water-based, non-flammable, non-combustible latex paint is unrestricted under USPS section 343.
• “A UN number is the same as a class.” The ID number identifies the proper shipping name; the class is a separate column in the table.

What are the different classes and divisions of dangerous goods?

Nine classes, with divisions in classes 1, 2, 4, 5 and 6, as listed in 49 CFR 173.2 and summarized in the definition section above.

FAQ

Does Class 3 flammables require placards?

Yes on bulk packagings and vehicles carrying them (FLAMMABLE placard, 49 CFR 172.504), except that highway or rail vehicles with under 454 kg (1,001 lb) aggregate gross weight of Table 2 materials need none, and bulk packaging is not covered by that exception.

Can I mail paint USPS?

Paint (UN1263) is mailable by surface if it meets the Limited Quantity criteria; water-based non-flammable latex paint is unrestricted. Flammable liquids cannot go by domestic air or international mail (Publication 52, section 343).

What are the UN numbers used for HAZMAT items?

They identify the proper shipping name in the Hazardous Materials Table; “UN” entries are valid internationally, “NA” entries only in the US (49 CFR 172.101).

What are the different classes and divisions of dangerous goods?

Nine classes; classes 1, 2, 4, 5 and 6 have divisions, Class 3 does not (49 CFR 173.2).

Sources

• 49 CFR 173.120, Class 3 definitions (Cornell LII copy), read 2026-11-07: law.cornell.edu/cfr/text/49/173.120
• 49 CFR 173.121, Class 3 assignment of packing group, read 2026-11-07: law.cornell.edu/cfr/text/49/173.121
• 49 CFR 172.504, general placarding requirements, read 2026-11-07: law.cornell.edu/cfr/text/49/172.504
• 49 CFR 172.101, Hazardous Materials Table, read 2026-11-07: law.cornell.edu/cfr/text/49/172.101
• 49 CFR 173.2, hazardous materials classes, read 2026-11-07: law.cornell.edu/cfr/text/49/173.2
• USPS Publication 52, section 343, read 2026-11-07: pe.usps.com/text/pub52/pub52c3_022.htm