An underrated risk in the wood pellet industry
Why pellets emit carbon monoxide
Over the past twenty years or so, documented fatal accidents in Sweden, Finland, and Germany have highlighted a phenomenon that had until then been underestimated: wood pellets spontaneously emit carbon monoxide (CO) while in storage, particularly in the first months after production. The mechanism is well documented by academic research (work published on diva-portal.org, academic.oup.com, mdpi.com): auto-oxidation of the polyunsaturated fatty acids present in softwood (pine, spruce) generates CO, CO2, and aldehydes (formaldehyde, acetaldehyde, hexanal).
The phenomenon is especially pronounced with pellets made from recently produced softwood. Concentrations can reach levels above 10,000 ppm in a closed, unventilated silo, more than 100 times the lethal dose for a human. The aggravating factors are high temperature, humidity, and above all a confined, unventilated space.
Accidents documented since 2002
Academic research began taking up the subject after several deaths in 2002 and the following years: deliveries of maritime pellets in Scandinavia, agricultural silos, industrial boiler rooms. In France, the industry is relatively spared (storage in bags or in smaller silos, regular deliveries), but a few incidents have been reported, and the issue is gaining visibility as the installed base grows.
The concrete risks for a residential installation
Silo built into the boiler room: the riskiest scenario
In a house fitted with a pellet boiler, the silo is often installed in a plant room adjacent to the boiler room, sometimes even sharing the same walls. If this room isn't ventilated, the CO emitted by the pellets can migrate towards living spaces through pipe penetrations, electrical conduits, or simply by diffusion through non-airtight walls. The risk is even higher when large deliveries (tonnes of freshly produced pellets) temporarily saturate the air in the room.
Working on a silo: the golden rule — never enter without a measurement
For an installer or a chimney sweep working on a silo, the rule is clear and non-negotiable: never enter a pellet silo without having measured the CO concentration first. A portable CO detector (€20 to 150) must be used systematically. Before any intervention, ventilate the silo (doors open, temporary extractor) for at least 30 minutes, then measure again. If the concentration exceeds 35 ppm (the alert threshold), increase ventilation until it drops back below that level.
Storage best practices recommended by research
Ventilating the room, keeping storage away from living spaces
Academic work converges on several practical recommendations. First, ventilate the storage room: natural high and low ventilation (grilles sized to the volume) is enough in most cases. Second, keep the storage away from living spaces: an unventilated basement silo under a bedroom is a scenario to avoid. Third, favour an outdoor silo (textile silo, ventilated underground silo, separate concrete silo) rather than one built into the dwelling.
Testing before any intervention, airing thoroughly
Before opening a silo, air the room for at least 30 minutes, with windows open and an extractor running if possible. Measure with a portable CO detector. The measurement should be taken at floor level and at height, since CO has a density very close to that of air and can accumulate unevenly. Record the concentration before and after ventilation, and keep a log to track how the risk evolves with the seasons.
For the tradesperson: building prevention in from the design stage
CO detector: €20 to 50 against a tragedy
For any retrofit or installation of a pellet boiler room, always plan for a CO detector at the bottom of the storage room and a second one in the boiler room. EN 50291 detectors (the European standard) cost €20 to 50 at major retailers, and certify reliable triggering at 50 ppm in under 60 minutes. It's a minimal investment given the risk.
For larger installations (collective, commercial, industrial), plan for wired detectors linked to an alarm system with automatic shut-off of the silo's supply, in addition to controlled mechanical ventilation.
Ventilating the room: smart silo sizing
Silo sizing must take the CO risk into account, not just the heating volume. An oversized silo extends the period during which the pellets emit CO: pellets stored for 6 months emit much less than freshly delivered ones, but a silo that turns over slowly exposes the space to the risk zone for longer. Favouring two deliveries a year rather than one large one can improve safety while also smoothing out the cost for the client.
Informing the client: usage recommendations
A good installer hands the client a best-practices document: never enter the silo, air the room before any technical intervention, check the CO detector once a year, call a professional in case of doubt. This step carries responsibility for the tradesperson but strengthens the client relationship and documents the service in case of a dispute. It fits into the advisory approach developed in our guide to pellet stoves.
A worrying regulatory gap in France
No labelling, no production date, no dedicated NF standard
The finding is striking: in France, no obligation currently applies to pellet producers regarding the CO risk. No labelling on bags, no production date, no NF standard specifically addressing this issue (the ENplus certification and the international ISO 17225-2 standard cover fuel quality, not CO emission during storage). The end consumer therefore has no objective way of knowing whether their pellets are recent (higher risk) or well stabilised (lower risk).
This is a topic that could evolve in the coming years, under pressure from accident cases and the growing scale of the industry. In the meantime, the duty to advise falls on the installer.
A real risk, but not a reason to reject pellets
The right balance needs to be kept. Pellets remain an excellent renewable energy source in carbon terms, with a competitive running cost and a level of comfort close to gas. The CO risk is real but fully manageable: an outdoor or ventilated silo, a €50 CO detector, rules for working on a silo. For the tradesperson, it's a technical point to master that strengthens professional credibility and protects civil liability.
Key takeaways
Wood pellets spontaneously emit CO through auto-oxidation of the fatty acids in softwood, with concentrations that can exceed 10,000 ppm in a closed, unventilated silo in the first months. Best practices come down to four points: ventilate the storage room, keep the silo away from living spaces, install a €20-50 CO detector, never enter a silo without a prior measurement. The French regulatory gap (no labelling, no production date) reinforces the installer's duty to advise. This issue, documented since 2002 by Scandinavian and German research, remains little known in France: a professional who masters it and builds it into their services gains a commercial edge and protects their liability.
Key figures
> 10,000 ppm
Maximum CO concentration observed in an unventilated closed silo
€20-50
Cost of a consumer EN 50291 CO detector
50 ppm
Trigger threshold of an EN 50291 CO detector
Frequently asked questions
Yes, it's a risk documented since 2002 by academic research (studies published on diva-portal.org, annweh, mdpi). Deaths have been reported in Sweden, Finland, and Germany during interventions on silos or in unventilated storage rooms. In France, the risk mainly concerns silos built into the boiler room without adequate ventilation, and technical interventions carried out without a prior measurement. A €50 CO detector and ventilating the room are enough to cancel out the risk in almost all cases.
Pierre-Louis Guhur
CEO of Argile
