Risk of carbon monoxide release during the storage of wood pellets

Health and Safety Executive - Safety alert

Department Name:
Operational Strategy Division - Manufacturing Sector (General Manufacturing Team)

Bulletin No:
OPSTD 3-2012

Issue Date:
5 November 2012

Target Audience:

  1. Users/installers/maintainers/distributors of wood pellet boilers
  2. Manufacturers/storers/distributors of wood pellets

Key Issues:
Storage of wood pellets: Risk of death from carbon monoxide


The HSE is issuing this notice to those who use, install, maintain or distribute wood pellet boilers or manufacture/store/distribute wood pellets. Since 2002 there have been at least nine fatalities in Europe caused by carbon monoxide poisoning following entry into wood pellet storage areas. Although there have not been any incidents so far in the UK the use of wood pellets is increasing and awareness of this danger is required. Wood pellet boilers are used in homes and businesses as an alternative to oil or gas fired boilers. They are also being installed to replace coal-fired boilers, particularly in schools.

Carbon monoxide can kill quickly without warning. It is a colourless, odourless and tasteless gas that is highly toxic. When carbon monoxide enters the body, it prevents the blood from bringing oxygen to cells, tissues, and organs.


Wood pellets are made from dried and milled sawdust and wood shavings that have been compressed into pellets, typically 10-20mm long and 3-12mm in diameter. They do not typically contain any additives or binders.

They are classed as a biofuel, a non-fossil heating fuel. The main countries of manufacture are Canada, North America and the Scandinavian countries within Europe. In 2000, the annual production of wood pellets in Europe and North America was about 1.5 million tons. This was expected to increase to around 16 million tons by 20111. Some wood pellet manufacture is now taking place in the UK.

Fatality details:

Fatalities caused by the release of carbon monoxide from wood pellets have previously been reported2 in Europe following personnel entering ships cargo holds (four) or storage silos (two).

Since 2010 there have also been three deaths caused by entry into wood pellet storage facilities in domestic sites3. Two were associated with a work activity and the other was a householder. In each case, the entry had been to resolve a technical problem. Details:

  • In January 2010, a 43-year-old engineer died in Germany after he opened a pellet bunker door. A second worker who was standing right behind him was also affected but still able to call the emergency services. The pellet bunker had a storage capacity of approximately 155 tonnes of pellets, supplying about 700 households.
  • In November 2010 a 38-year-old male householder in Ireland died after entering the 7 tonne wood pellet storage room for his boiler. His wife and another man were treated in hospital after trying to pull him to safety.
  • In February 2011, the 28-year-old pregnant wife of a caretaker, acting on his behalf, died in Switzerland after entering an 82m3 pellet storeroom that supplied 60 households.

Factors affecting the amount of carbon monoxide released from wood pellets

Wood pellets for boilers are normally stored in a large sealed hopper/tank or a storage room that has a screw feeder (auger) connected to the boiler. Alternatively, the hopper/tank can be mounted over the boiler for gravity feeding. Due to the enclosed nature of these hoppers/tanks/rooms, the atmosphere inside can become oxygen depleted and a toxic atmosphere containing carbon monoxide can accumulate. The chemical reactions responsible for carbon monoxide production from wood pellets are assumed to be an auto-oxidation process, especially oxidation of the fatty acids to be found in wood4.

Experimentation has shown3 that small quantities of wood pellets can produce life-threatening quantities of carbon monoxide in a confined space and that there are various factors that will affect the amount of carbon monoxide produced:

  • Age - pellets will produce more carbon monoxide within the first six weeks of being manufactured.
  • Temperature - more carbon monoxide is produced at higher temperatures.
  • Wood type - pellets made from pine contain more unsaturated fatty acids than spruce so produce more carbon monoxide.
  • Other factors - carbon monoxide levels will also increase with the amount of available oxygen present, exposed pellet surface area and amount of mechanical abrasion of the pellets that has taken place.

Note: In addition to the risk of carbon monoxide from wood pellets there is also a possibility of carbon monoxide being present because of a back-flow of flue gases via the fuel supply mechanism from the boiler. Causes for this include inadequate equipment being installed or a poorly designed flue.

Action required:

The HSE is advising all those who use, install, maintain or distribute wood pellet boilers and/or manufacture/store/distribute wood pellets to consider the following:

  • Wood pellet hoppers/tanks/storage rooms and boilers should always be installed and commissioned by a competent person, normally approved by the manufacturer/supplier. This is particularly important if the installation involves the replacement of a coal-fired boiler, where existing boiler room and storerooms are often utilised.
  • Do not enter the pellet storage area or place your head into a wood pellet hopper as they can contain toxic gases. No personnel should enter the hopper/tank unless fully trained and competent in confined space entry procedures. Controls should be put in place to ensure safe entry as per the HSE's Code of Practice for Working in Confined Spaces5. This should include adequately ventilating the storage area and checking carbon monoxide and oxygen levels with an appropriate device prior to entry. It is recommended that the store room is ventilated at all times, either mechanically or by being designed to have a through draught.
  • Ensure that the boiler and pellet feed mechanism etc. is cleaned and serviced by a competent person as specified by the manufacturers' instructions.
  • If any problems are encountered with the unit, such as the system not heating correctly or flue gas is flowing into the boiler room, turn the unit off and contact the supplier and/or manufacturer and request assistance.
  • Duty holders who store wood pellets, particularly in bulk should have a suitable risk assessment and safe system of work in place.
  • Manufacturers, suppliers and distributors of wood pellets should provide adequate health and safety information to the user in their materials safety data sheet.
  • Warning signs should be placed on the pellet storage area access door, ideally on both sides so it can be seen when the door is open. The warning sign should include the following information:
    • DANGER - RISK OF CARBON MONOXIDE POISONING - There is a danger to life from odourless carbon monoxide and lack of oxygen. Check atmosphere before entry with an appropriate device. No entry for unauthorised persons. Keep children away from the storeroom.
    • No smoking, fires or naked flames.
    • The room should be adequately ventilated before entering. Keep the door open whilst inside.
    • There is a danger of injury from movable parts.
    • Filling procedures should be carried out accordance to the instructions of the heating installation company and the pellet suppliers.


  1. Information taken from Wood Markets Monthly International Report Volume 16, Number 10, December 2011 - January 2012. Back to reference of footnote 1
  2. Wood Pellet Association of Canada - Review of Off-gassing from Wood Pellets - A Canadian Perspective - Staffan Melin, Research Director, February 2010. Back to reference of footnote 2
  3. Saskia Gauthier, Hildegard Gras et al (2012) Lethal Carbon Monoxide Poisoning in Wood Pellet Storerooms - Two Cases and a Review of the Literature. Annals of Occupational Hygiene, Vol. 56, No. 7, pp. 755-763, 567755-763. Back to reference of footnote 3
  4. Kuang X, Shankar TJ, Bi XT et al. (2008) Characterization and kinetics study of off-gas emissions from stored wood pellets. Annals of Occupational Hygiene; 52: 675-83.Back to reference of footnote 4
  5. HSE's Code of Practice for Working in Confined Spaces. Back to reference of footnote 5
  6. Silo Fires Require Specific Response Tactics - Henry Persson, Project Leader, Fire Dynamics section SP Swedish Technical Research Institute, October 2011. Back to reference of footnote 6

Further information:

General note:

Please pass on this information to anyone to whom it may be relevant.

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