Hydrogen transportation is subject to stringent international regulations to ensure safety, given its flammability and potential risks. The United Nations frameworks, including the European Agreement concerning the International Carriage of Dangerous Goods by Road (ADR) and the Regulations concerning the International Carriage of Dangerous Goods by Rail (RID), establish comprehensive rules for hydrogen logistics. These regulations cover classification, packaging, labeling, and handling, with specific provisions for compressed and liquid hydrogen.

**Classification of Hydrogen Under UN Regulations**
Hydrogen is classified under UN numbers based on its physical state. Compressed hydrogen gas falls under UN 1049, while liquid hydrogen is designated as UN 1966. Both are categorized as Class 2 dangerous goods, specifically Division 2.1 (flammable gases). The classification reflects hydrogen’s low ignition energy, wide flammability range (4% to 75% in air), and high diffusivity, necessitating strict containment measures.

**Packaging Requirements for Compressed Hydrogen**
For road and rail transport, compressed hydrogen must be stored in pressure receptacles compliant with ISO 11119 or EN 12245 standards. These cylinders are typically made of high-strength materials like carbon fiber-reinforced composites or steel alloys to withstand pressures up to 700 bar. The ADR and RID mandate periodic inspections, including hydrostatic testing every 5 years, to ensure integrity. Valves and pressure relief devices must meet ISO 10297 standards, with burst discs rated to prevent over-pressurization beyond 110% of the vessel’s working pressure.

**Packaging Requirements for Liquid Hydrogen**
Liquid hydrogen requires cryogenic containers capable of maintaining temperatures below -253°C. Double-walled vacuum-insulated tanks, conforming to ISO 21028 or EN 13458, are mandatory to minimize boil-off losses. The ADR specifies that tanks must withstand a minimum pressure of 1.5 times the working pressure, typically 4–10 bar. Pressure relief systems must activate at 110% of the design pressure to prevent rupture. Rail transport under RID imposes additional requirements for tank car design, including thermal protection and roll-over safeguards.

**Labeling and Marking**
All hydrogen shipments must display hazard labels: a red diamond with the number 2 (flammable gas) and the UN number (1049 or 1966). Packages must include the proper shipping name (“Hydrogen, compressed” or “Hydrogen, refrigerated liquid”) and the Class 2 label. For road transport, vehicles must carry orange placards with the UN number and hazard class. Rail shipments require identical markings on tank cars. Additional labels indicating “Cryogenic Liquid” are mandatory for liquid hydrogen.

**Handling and Operational Controls**
Loading and unloading hydrogen require grounding and bonding to prevent static discharge. ADR Article 5.3.2 prohibits open flames, smoking, or spark-producing equipment within 15 meters of hydrogen handling areas. Transport personnel must undergo specialized training under ADR 1.3 and RID 1.4, covering emergency response, leak detection, and first aid. Vehicles must carry fire extinguishers rated for Class B and C fires, and drivers must adhere to speed limits (typically 80 km/h for road transport).

**Exemptions and Special Provisions**
Small quantities of hydrogen benefit from limited quantity exemptions under ADR 3.4 and RID 3.4. For compressed hydrogen, cylinders with water capacity ≤1 liter and pressure ≤200 bar are exempt from full regulations if the total load per transport unit is ≤1000 liters. Liquid hydrogen shipments ≤20 liters per package are exempt if the total load is ≤500 liters. These exemptions do not apply to tunnels with restrictions (categories B–E under ADR 1.9.5).

**International Logistics and Multimodal Transport**
For cross-border shipments, ADR and RID align with the UN Model Regulations, ensuring harmonization with other frameworks like the International Maritime Dangerous Goods (IMDG) Code and IATA Dangerous Goods Regulations. Hydrogen transported via multiple modes must comply with the strictest applicable standards. For example, a road-to-rail shipment must meet both ADR and RID requirements, including documentation (transport documents, safety data sheets) in languages understood by all transit countries.

**Emergency Response Protocols**
In case of leaks or fires, ADR 8.5 and RID 8.5 mandate immediate isolation of the area (minimum 100 meters for liquid hydrogen, 50 meters for compressed gas). First responders must use thermal imaging to detect invisible hydrogen flames and avoid water jets, which can disperse gas. Approved mitigation measures include controlled venting for compressed hydrogen and controlled burn-off for liquid hydrogen spills.

**Material Compatibility and Long-Term Safety**
Hydrogen embrittlement of metals is addressed in ADR 6.2 and RID 6.2, requiring materials like 316L stainless steel or aluminum alloys for valves and fittings. Composite materials must pass permeation tests per ISO 11114-4. Storage systems must include hydrogen sensors with alarms triggered at 1% concentration (10% of the lower flammability limit).

**Regulatory Updates and Future Directions**
Recent amendments to ADR (2023) and RID (2023) incorporate advances in hydrogen technology, such as Type IV composite tanks and passive autocatalytic recombiners for leak control. Ongoing revisions aim to address hydrogen blends (e.g., 20% hydrogen in natural gas pipelines) and large-scale export projects, with proposed updates to pressure thresholds and inspection intervals.

The ADR and RID frameworks provide a robust foundation for hydrogen transportation, balancing safety with logistical feasibility. Compliance ensures the safe integration of hydrogen into global energy systems while mitigating risks across supply chains. Future regulatory developments will likely focus on standardization for emerging technologies like liquid organic hydrogen carriers and large-scale cryogenic transport.