Wood pellet bio-fuel

HSE is aware of some incidents that have occurred as a result of the transporting and storing of wood pellets used as fuel.

Domestic and commercial heating

Wood Pellet Boilers are commonly used in homes and businesses as an alternative to oil or gas fired boilers. Wood pellets for these units are normally housed in a large sealed hopper/tank that is either fitted with screw feeder (auger) connected to the boiler, or the hopper/tank is mounted over the boiler for gravity feeding. Due to the enclosed nature of these hoppers/tanks the atmosphere inside can become oxygen depleted and a toxic atmosphere can accumulate. Such enclosed spaces would, by virtue of these hazards, be a confined space and subject to the Confined Spaces Regulations, where they are in premises subject to the Health and Safety at Work Act 1974.


The Irish regulator, the HSA, issued an urgent alert to operators, maintenance personnel and members of the public on the dangers associated with bulk feed hoppers/tanks normally used with Wood Pellet Boilers. The alert followed a fatal accident where a home owner entered a bulk wood pellet storage hopper/tank and was overcome by carbon monoxide (CO) gas. In addition two people were killed in Finland when doing work related to silo storage of wood pellets. The HSA offered the following advice to operators, maintenance personnel and users of this equipment:

  • DO NOT ENTER or place your head into the wood pellet hopper under any circumstances. The unit could contain toxic gases.
  • Ensure that your wood pellet hopper/tank and boiler has been installed and commissioned by a competent person. If in doubt, contact the supplier and/or manufacturer and request assistance.
  • Ensure that the boiler is cleaned and serviced by a competent person at the frequency required by the manufacturers' instructions.
  • If any problems are encountered with the unit, such as, system not heating correctly, flue gas is flowing into boiler room, turn unit off and seek assistance immediately.
  • No personnel should enter the hopper/tank unless fully trained and competent in confined space entry procedures. The hopper/tank should be fully ventilated and controls put in place to ensure safe entry ( see guidance on the Confined Spaces Regulations 1997)
  • Ensure the boiler room is well ventilated at all times to ensure no inadvertent build up of toxic gases.


The manufacture and carriage of wood pellets has increased in recent years because of their use as a non-fossil heating fuel.

The pellets are produced from sawdust and wood shavings and do not contain any additives or binders. The sawdust and shavings are dried, and milled into particles that are then compressed.

These compressed wood pellets are, of course, combustible and can be ignited by a range of ignition sources. In addition the dust associated with the pellets, when dispersed and ignited can give rise to a dust explosion under appropriate conditions of containment. Stored bulk piles of wood pellets can self-heat in parts with high moisture contents and it is reported that this process can lead to the spontaneous combustion of the material after a long period of time.

In addition to the combustion hazards, wood pellets also undergo oxidation to produce carbon monoxide and carbon dioxide. In a confined space such as an unventilated ship's hold, this can lead to a dangerous reduction in the oxygen concentration in the hold as well as the development of a dangerous concentration of carbon monoxide which is toxic (and flammable). In a recent case a carbon monoxide concentration of approximately 1% was measured in a sealed cargo hold of a ship containing wood pellets some 18 days after the cargo was loaded. The oxygen concentration at this time was less than 1%. Since the cargo hold was an enclosed space and there was a clear specific risk of oxygen depletion and potential noxious fume, this hold would constitute a confined space.

There is an entry in the International Maritime Solid Bulk Cargoes (IMSBC) Code, for wood pellets in which reference has been made to the hazard associated with the generation of carbon monoxide.

The hazards associated with oxygen depletion and the generation of carbon monoxide is now recognised by stevedores who routinely employ "gas doctors" to check spaces which contain or have contained wood pellets. HSE would encourage raising awareness of the issue both at ports and with ship's crews and others who may have need to enter a cargo hold which contains, or has recently contained wood pellets.

Since May 2002 there have been reports of fatalities throughout Europe. In Rotterdam one person was killed and two other people were severely injured as a result of exposure to carbon-monoxide when entering one of the cargo holds. In November 2006 in Helsingborg, despite clear instructions on-board and an elaborate system of temperature sensors in the cargo as well as gas sampling tubes at various levels of the cargo hold, one person was killed and several other people were injured, one seriously. In other incidents in Swedish ports, five people were reported killed during a period of 24 months. The main caused appears to be oxygen depletion and to a lesser extent carbon-monoxide.

Accidents have also been reported in Germany, including two fatal incidents and two near fatalities.

For more advice on working safely in, and the risk of working at, ports go to the HSE Ports website

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Updated 2023-09-08