Dry ice

Introduction

At normal temperature and pressure, carbon dioxide (CO₂) is a gas. When cooled, CO₂ solidifies at approximately – 78 °C and is known as solid CO₂, often referred to as dry ice. At temperatures warmer than – 78 °C it turns directly from a solid into a gas (sublimation). A small amount of solid CO₂ will produce a large volume of gas, approximately in a ratio of 1:850; for example, 10kg of dry ice sublimes into approximately 5.4 m³ of carbon dioxide gas. 

Dry ice appears as a translucent white solid, similar in appearance to water ice and is usually manufactured into blocks, slices or pellets. Common uses of dry ice in the University include the preservation and transportation of biological samples and the generation of cooling baths to maintain reaction mixtures at cryogenic temperatures.

Hazards

The main hazards when dealing with dry ice:

• Asphyxiation: Large volumes of CO₂ gas are produced from small volumes of dry ice and this can increase CO₂ concentrations in air to dangerous levels in poorly ventilated areas leading to the asphyxiation. The CO₂ formed is heavier than air and will tend to form higher concentrations at floor level. At high concentrations CO₂ also acts as a toxicant and can cause unconsciousness and respiratory arrest almost instantaneously. CO₂ has workplace exposure limits (WELs) of:
  • 15min short-term exposure limit (STEL) 15000ppm (1.5%)
  • 8h time-weighted average (TWA) 5000ppm (0.5%)
• Extreme cold: Contact or close proximity with solid CO₂ or surfaces cooled by the solid (e.g. when used in a dry ice/acetone bath) can cause cold burns. Prolonged exposure may lead to frostbite.

Other hazards include: 

• Pressure: if placed in an unvented container, pressure will build up, potentially resulting in rupture/explosion. Dry ice, must not, therefore, be stored in fridges, freezers or other sealed containers.
• Cold - inhalation: inhalation of either the cold gas, or the local cold atmosphere, which can damage the lungs and may aggravate existing health conditions. 
• Reduced visibility: a vapour cloud may form resulting in the potential for confusion, slips, trips and falls.
• Cold – ingestion: Ingestion of solid CO₂ will cause severe damage to internal organs and has the potential to be fatal.
• Embrittlement of materials: soft materials e.g. rubber and plastics may become brittle when cooled by dry ice and may shatter unexpectedly.

Risk control

The following safety measures should be applied by all those involved in the handling, storage and disposal of dry ice.

Handling

• Only trained, competent users who have read and understood the relevant risk assessment(s) should handle dry ice.
• Do not handle dry ice with bare hands. Use loose-fitting, thermally insulated gloves suitable for the extreme cold temperature or dry ice tongs. Eye protection should be worn.
• Only use dry ice in open or well-ventilated areas. Do not store in confined spaces.
• Do not put dry ice in an air tight container. Excess pressure build up can cause containers to rupture/explode.

Storage

Factors that can affect the rate at which dry ice sublimes into gaseous CO₂ include: ambient temperature and humidity, the quality of the storage container, the number of times the container is opened and closed. 

• Always store dry ice in a properly designed container. Do not store dry ice in a tightly sealed container such as a plastic or glass bottle, or any container with a screw-top lid that will not allow produced gas to escape.
• Containers should be stored in dry, well-ventilated areas away from sources of heat, ignition and direct sunlight.
• Do not store dry ice in freezers, refrigerators, cold rooms, or any other unventilated room.
• Keep the container lid closed when not in use.
• Avoid leaning into the container for longer than necessary.
• Do not expose dry ice to high ambient temperatures unnecessarily as this increases the sublimation rate and thereby the risk of forming carbon dioxide rich atmospheres.
• Store dry ice in a secure area and prevent unauthorised access.

Transportation 

• Only package dry ice in containers that are appropriate (i.e., non-sealable and not damaged by cold temperatures). 
• Do not transport dry ice in closed vehicles. Avoid transporting dry ice in the cab of a truck or the passenger compartment of a car. If this is not possible, the load should be well insulated (but not sealed) and adequate ventilation must be maintained i.e. forced air from outside using fan (not recycled air) and/or an open window.
• Dry ice load must be secured to prevent it falling over, which could lead to spillage and result in possible burns and increased production of asphyxiating gas.
• Always carry a carbon dioxide (solid) Safety Data Sheet in the cab or driver’s compartment of any vehicle that is carrying dry ice.
• Always unload product as soon as possible at the end of the journey and move to a suitable storage location
• Wear suitable gloves and exercise caution when transferring dry ice from delivery bags to storage containers.
• If you receive or see a container that appears swollen or bulging, or if you suspect dry ice is improperly placed in a sealed container, secure the area and ensure the area is cleared of people. Contact your local School Safety Adviser who will then inform the Corporate H&S Department if required. Do not try to release pressure on your own, which can cause serious injury. The risk assessment should include any emergency procedures to be followed.

Disposal

• Dispose of dry ice in a ducted fume cupboard or another well ventilated area away from the public.

Useful links

BCGA TIS 7 – Guidelines for the safe transportation, storage, use and disposal of solid carbon dioxide (dry ice) 

BCGA Guidelines for the safe transportation, storage, use and disposal of dry ice products