Dry ice (solid Carbon Dioxide)

Overview

Dry ice (solid Carbon Dioxide)

Dry ice is solid carbon dioxide (CO₂), made by compressing and cooling gaseous CO₂ until it liquefies. Expansion converts the liquid into the solid state which is compressed by a hydraulic press into dry ice blocks, slices or pellets.

Before handling and using dry ice it is important that you understand its properties, potential hazards and measures to be taken to reduce risk.

Two key precautions that must be followed by those using or handling dry ice are:

1. Do not put dry ice in a gas tight container. Excess pressure build up can cause containers to explode!
2. Do not store dry ice in cold rooms or any other unventilated room

Properties

• Dry ice appears as a translucent white solid which at normal temperatures sublimes from the solid state directly into a gas without passing through a liquid phase
• It is non-flammable and an asphyxiant
• It is a colourless gas with a slightly pungent odour which is only detectable in high concentrations

Hazards

• Asphyxiant: in high concentrations sublimed vapour may cause asphyxiation. Expansion ratio (relative increase in volume when evaporating to gas) for dry ice is 845; 10kg of dry ice sublimes into about 5.4 m³ of carbon dioxide gas
• Extremely cold: having a product temperature of -78°C dry ice is a good source of extreme cold but contact with the product can cause cold burns or frostbite.

Risk control

Risk control measures

The following represent generic safety measures to be applied for all workers involved in the handling, use, storage and transport of dry ice to minimise risk of harm. Further, more detailed consideration on suitable ventilation systems and possible need for dioxide monitors is required for activities involving the use / storage of large quantities of dry ice.

General

• Only experienced and properly instructed people should handle dry ice
• Do not handle dry ice with bare hands. It can cause sever cold burns and frostbite. Use insulated gloves suitable for the extreme cold temperature of dry ice is handling is necessary
• Never play games with dry ice
• Avoid adding water to solid CO₂ - this will increase sublimation with a corresponding higher risk of asphyxiation
• Dispose of dry ice in a well ventilated area away from the public. Do not discharge into any place where its accumulation could be dangerous
• Take care when carrying packages of dry ice
• Special care should be taken when storing and transporting dry ice

Storage

Many factors can affect the rate at which dry ice sublimes from the sold state into gaseous carbon dioxide. These include ambient temperature and humidity, the quality of the storage container, the number of times the container is opened and closed. The better the insulation, the slower the sublimation rate and the longer the quality of the product will be maintained.

Storage practices to be followed by all users of dry ice include:

• Do not put dry ice in a gas tight container. Excess pressure build up can cause containers to explode!
• Ensure adequate low level ventilation wherever dry ice is stored
• Do not store dry ice cold rooms or any other unventilated room
• Always store dry ice in a properly designed container
• 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 carbon dioxide atmospheres

Transport

Transport practices to be followed by all users of dry ice include:

• 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 and adequate ventilation must be maintained. It is preferable to transport dry ice in vehicles where the driver's cab is isolated from the load compartment
• Always ensure that there is adequate ventilation during transportation and before entering the load compartment to unload the product
• Always carry Carbon Dioxide (solid) Safety Data Sheet in the cab or driver's compartment of any vehicle carrying significant quantities of dry ice
• Always unload the product as soon as possible at the end of the journey and move it to a suitable storage location

Oxygen depletion

Effects and symptoms of Oxygen depletion / Carbon Dioxide enrichment

• The air we breathe normally contains 20.9% oxygen by volume. When liquid nitrogen boils or carbon dioxide sublimes, the increase in the concentration of these gases will reduce the concentration of oxygen in the air. It is this fact which is often the major hazard when dealing with these materials, particularly when used in a confined space.
• There is a simple formula for calculating the oxygen concentration in a confined space (such as a cold-room or lift) during a worst-case spillage scenario.
• Breathing even moderately elevated carbon dioxide levels (2-4%) is extremely unpleasant and initiates the "gasp for air" reflex even in the presence of elevated oxygen.
• Conversely, depleted oxygen in the absence of an increase in carbon dioxide causes hypoxia which is extremely dangerous because the victim floats off into a euphoric sleepy state.
• We have evolved to react to increasing levels of carbon dioxide proportionately to oxygen depletion - we have rapid detection of increased CO2 directly from the brain tissue itself.
• Oxygen receptors in the carotid bodies are extremely slow and only evoke physiological changes, i.e. no panic attack or asphyxiation response.
• Anaesthetic effects only occur at about 14% CO₂ - you would need a massive sudden release into a small volume room, e.g. from a cylinder plug blowing - in effect anyone in there would run out gasping for air in a state of total panic before they were in any danger.
• We all have the best CO₂ detectors invented to date as a by product of being air breathing mammals.
• The use of inert gases in a confined space is quite another matter if oxygen is depleted in the absence of an increased CO₂ it is potentially fatal.

Effects and symptoms of oxygen depletion

Effects and symptoms of carbon dioxide enrichment

Calculation

Oxygen depletion calculation

Method:

• Calculate the volume (Vr m³) of the confined space
• Calculate the volume of the released gas (Vg m³) by multiplying the volume of the liquid nitrogen (in m³) or weight of solid carbon dioxide (in Kg) by the expansion ratio (682 for LN and 845 for CO₂). (1,000 litres = 1 m³)
• Calculate the volume of available oxygen (Vo m³) as 0.2095 x (Vr-Vg)
• Calculate the % oxygen available to breathe as 100 x Vo / Vr

Example 1:

• A cold room with a nominal volume of 26.3 m³ (Vr)
• If 10 Kg of solid CO₂ (Vg = 8,450 litres or 8.45 m³) evaporates slowly into this sealed room, then the oxygen concentration will be depleted from 20.95% to 20.95 x (26.3 - 8.450) / 26.3 =14.2%

Using this calculation, if the oxygen concentration can fall to, or below, 18% then action needs taking to minimise risks. For this reason the use or storage of liquid nitrogen or solid carbon dioxide is prohibited in cold rooms.

Example 2:

• A number of specimens packed in 5 Kg of dry ice are being transported by car from one location to another. The container is well insulated and is positioned on the back seat of the car - the car windows are closed.
• Assume:
  • the journey takes 1 hour
  • the interior volume of the car is 2.8 m³
  • 1 kg of dry ice will produce 0.45 m³ of gas (figure from Gas Safety UK Ltd)
  • dry ice sublimation rate is approximately 1% of total mass per hour in an insulated container (figure from Federal Aviation Administration in the USA)
• A volume of 0.023 m³ of CO₂ gas is produced during the journey
• The concentration of CO₂ in the car atmosphere will reach 0.8% (between the short term (1.5%) and long term (0.5%) exposure limits) and within the range where it will not cause any adverse effects
• The concentration would be increased by any of the following:
  • larger quantity of dry ice
  • using a poorly insulated or open container
  • using a smaller car
  • increase journey time
• A concentration of 0.8% CO₂ will reduce the oxygen concentration from 20.95% to 20.78% - presenting a low risk to occupants of the car (i.e. above 20%)
• Providing all these factors are taken into account, a reasonable quantitative assessment of risk can be carried out