You can legally mail CO2 cartridges in the United States, and for the small cartridges most people ship — the 12-gram and 16-gram sizes used in airsoft guns and bike inflators — a federal exemption removes almost all of the hazmat paperwork on ground service. Larger cartridges, like the ones for soda makers, are still mailable, but you’ll need to package, label, and route them under the Limited Quantity rules. Here is how to mail CO2 cartridges through USPS, FedEx, and UPS without running into a compliance problem.
Check the Size First
Carbon dioxide is a Division 2.2 non-flammable compressed gas under federal hazardous materials law, which normally triggers packaging, labeling, and documentation requirements before any carrier will accept the package.1eCFR. 49 CFR 173.115 – Classes of Gases The person dropping off the package is legally the shipper, and the shipper carries the liability for what’s inside.
Under 49 CFR 173.306(j), a small gas cartridge holding no more than 50 mL (1.7 fluid ounces) with an internal pressure at or below 141 psig at 131°F is essentially exempt from hazmat shipping requirements, provided the only hazardous content is a Division 2.2 gas like CO2.2eCFR. 49 CFR 173.306 – Limited Quantities of Compressed Gases Standard 12-gram airsoft cartridges and most 16-gram bike inflators sit well under that 50 mL volume limit.
If your cartridges qualify, ground shipment needs no hazmat labels, no special container certification, and no shipping papers. One caveat: even exempt cartridges cannot travel in passenger cargo — by air, they move as cargo only, never in carry-on or checked baggage.2eCFR. 49 CFR 173.306 – Limited Quantities of Compressed Gases Cartridges over 50 mL, including the 60-gram and 130-gram soda-maker cylinders, don’t get the exemption and fall under the rules below.
Packaging Larger Cartridges as a Limited Quantity
Cartridges that miss the small-cartridge exemption ship as “Limited Quantity” hazmat, and the packaging rules are specific. Metal cartridges can hold up to 33.8 fluid ounces (1 liter), and the contents cannot completely fill the receptacle at 130°F. Pressure determines the container:
- Between 140 and 160 psig at 130°F: DOT 2P container required.
- Between 161 and 180 psig at 130°F: DOT 2Q container required.
- Over 180 psig at 130°F: prohibited from the mail.3Postal Explorer. Publication 52 – 342 Gases (Hazard Class 2)
You can put multiple non-flammable gas cartridges in one package as long as each cartridge is individually cushioned and the total package weight stays at or below 25 pounds. The outer box has to be rigid and strong enough to hold everything, and USPS sets minimum burst-test or edge-crush ratings depending on whether the package is above or below 20 pounds.4Postal Explorer. Publication 52 – Packaging Instruction 2B
For surface shipment, apply the DOT Limited Quantity ground mark on the address side. It’s a diamond (square on point) with black top and bottom portions and a white center. Each side must be at least 100 mm (3.9 inches); a reduced-size mark down to 50 mm (1.97 inches) per side is allowed on smaller packages.5Postal Explorer. Publication 52 – Hazardous, Restricted, and Perishable Mail Include a complete return and delivery address.4Postal Explorer. Publication 52 – Packaging Instruction 2B
Sending Them Through USPS
USPS accepts Limited Quantity CO2 shipments on surface transport only. That means no Priority Mail or Priority Mail Express, which move by air. Take the package to a post office counter rather than a blue collection box, and tell the clerk what’s inside so it gets routed correctly.
Air shipment through USPS is possible for certain consumer commodities, but the requirements jump sharply: the air-specific Limited Quantity mark with a “Y” in the center, a DOT Class 9 hazard label, identification number ID8000, the shipping name “Consumer Commodity,” and a Shipper’s Declaration for Dangerous Goods prepared in triplicate and attached in an accessible envelope on the outside of the package.3Postal Explorer. Publication 52 – 342 Gases (Hazard Class 2)6Postal Explorer. Publication 52 – 326 Shipping Documentation For most personal shipments, ground is simpler and legal.
Sending Them Through FedEx
FedEx Ground applies its own version of the federal framework. Shippers of fully regulated hazardous materials must complete a qualification process with a FedEx account executive before their first shipment. Limited Quantity ground shipments are lighter on paperwork: no shipping paper or hazmat certification form is required.7FedEx. FedEx Ground Hazardous Materials Shipping Guide If your cartridges qualify as Limited Quantity under 49 CFR, the process is substantially simpler than fully regulated hazmat.
Sending Them Through UPS
UPS does not accept hazardous materials at The UPS Store, UPS Customer Centers, Air Letter Centers, or Authorized Shipping Outlets. The exception: Limited Quantity ground shipments within the contiguous 48 states may be accepted at some locations.8UPS. Hazardous Materials Shipping Service Definition For fully regulated hazmat, use a staffed UPS facility or arrange a pickup. UPS classifies small non-refillable gas cartridges with a capacity of 1 liter or less under UN2037, Division 2.2, with the exception provision at 49 CFR 173.306.9UPS. 49 CFR Chemical Table – Ground and Air Packages
What Happens If You Skip the Rules
PHMSA investigates hazmat shipping violations and issues enforcement actions from warning letters up to six-figure civil penalties.10Pipeline and Hazardous Materials Safety Administration (PHMSA). HazMat Enforcement Actions Search Civil fines can exceed $100,000 per violation per day, with higher amounts when a violation causes death or serious injury.
Federal law also carries criminal penalties. Knowingly violating hazmat transportation law, or acting recklessly enough that a reasonable person would have known better, carries up to five years in prison, a fine, or both. If the violation causes a release resulting in death or bodily injury, the maximum doubles to ten years. You do not need to know the specific regulation exists — acting with knowledge of the underlying facts is enough.11Office of the Law Revision Counsel. 49 USC 5124 – Criminal Penalty
For someone mailing a few CO2 cartridges without proper labeling, the practical risk isn’t prison — PHMSA focuses enforcement on commercial shippers moving volume. But a carrier that finds undeclared hazmat in transit will refuse the package, and repeat violations from the same shipper draw scrutiny. Packaging and labeling correctly the first time costs almost nothing next to that.