Dry ice is the solid form of carbon dioxide. At normal atmospheric pressure it turns directly into gas without melting (it sublimes) at about −78.5 °C; because it leaves no liquid behind, it is one of the most widely used coolants for shipping frozen biological samples. The same property makes it a regulated substance in transport: dry ice is a class 9 dangerous good with the UN number UN 1845.
This article explains why dry ice is treated as a dangerous good, the general principles of packaging and marking in a sample shipment and the safety points to keep in mind when working with dry ice. For binding details, the regulations in force and the carrier's instructions always apply.
Why is dry ice a dangerous good?
Dry ice is not toxic; its risks come from its physical properties:
- Gas pressure: Subliming dry ice turns into a very large volume of carbon dioxide gas. In a fully airtight container the pressure rises and the container can burst.
- Risk of asphyxiation: Carbon dioxide is heavier than air; it can build up in a closed vehicle, a lift or a small room and reduce the oxygen in the air.
- Cold burns: Touching dry ice at −78.5 °C with bare hands causes cold burns to the skin.
These three risks determine both how dry ice is packed and where it is placed in a vehicle.
Basic rules for air transport
In air transport, dry ice appears in the Dangerous Goods Regulations of the International Air Transport Association (IATA) as “Carbon dioxide, solid” or “Dry ice”, with the number UN 1845 and packing instruction 954. The general principles are:
| Subject | General rule |
|---|---|
| Packaging | Must allow gas to escape; must not be sealed airtight |
| Marking | UN 1845, the proper shipping name and the net quantity of dry ice in kilograms |
| Label | Class 9 hazard label |
| Document | The quantity of dry ice is also stated in the shipping documents |
| Legal basis | IATA Dangerous Goods Regulations, packing instruction 954 |
On the road (ADR) and at sea (IMDG), there are also warning and ventilation rules for dry ice used as a coolant, particularly against the risk of asphyxiation in closed vehicles and containers. In ADR these rules are set out in section 5.5.3; for dry ice (UN 1845) carried as a coolant, the other provisions of ADR do not apply and only this section is relevant. The general framework of the three regulations is explained in the article on ADR, IMDG and IATA.
Shipping a venom sample on dry ice
When a dangerous substance such as crude venom is shipped with dry ice, the package contains two separate dangerous goods: the contents are marked under their own class and the dry ice coolant under UN 1845. The transport classification of scorpion venom is covered in the article on UN 3462. In practice the package usually consists of the following layers:
- Sample tube or vial: a primary container with a leak-proof cap
- Absorbent material and a leak-proof secondary container
- Insulated box: the dry ice is placed around the secondary container, which is supported so that it does not shift as the dry ice dissipates
- Outer packaging: a sturdy box that lets gas escape and on which the marks and labels can be read
The amount of dry ice is set with the length of the journey and possible delays in mind; too little dry ice lets the sample thaw. The acceptance conditions of the carrier and the destination country for dry ice shipments should be checked in advance; some carriers accept packages containing dangerous goods only with certain services. The quantity of dry ice marked on the package must match the quantity stated in the documents. The choice of inner packaging is covered in the article on venom packaging, and keeping the temperature in storage and on the road in the article on the cold chain.
Which coolant when?
| Coolant | Approximate temperature | Typical use |
|---|---|---|
| Conditioned gel pack | 2-8 °C | Samples that only need to be kept cold |
| Dry ice | About −78.5 °C | Frozen samples |
| Liquid nitrogen vapour-phase shipper (dry shipper) | Below −150 °C | Cell and tissue samples that need very low temperatures |
Dry forms such as lyophilized powder do not always need to be shipped frozen; the shipping temperature is set according to the product's storage conditions and the length of the journey.
Safety when working with dry ice
- Handle dry ice with thick, insulated gloves; do not touch it with bare hands.
- Do not keep it in small, closed spaces; work in a ventilated area.
- Avoid carrying a box containing dry ice in the passenger compartment of a vehicle.
- Do not put dry ice in airtight glass or plastic containers.
- Open the received box in a ventilated place; do not pour leftover dry ice into a sink, but leave it to turn into gas on its own in a well-ventilated area.
