Reports recently surfaced in the Danish newspaper Jyllands-Posten describing a cluster of localized health issues, including nosebleeds, mouth blisters, and respiratory irritation, reported by residents living near a newly commissioned pyrolysis plant in North Jutland. What was launched as a flagship project in Denmark’s green transition quickly became the center of a growing local controversy. Are these effects linked to the technology itself, or something else entirely?
The local story: what’s happening in North Jutland?
Shortly after the pyrolysis plant in Vendsyssel began operations in 2024, residents in nearby villages started reporting a nuisance odor from the plant, specifically from the drying process of residual fibers that the pyrolysis plant receives from the co-located biogas plant.
Later in 2025 some residents reported symptoms, including persistent headaches, respiratory irritation, skin rashes and burning sensations, nosebleeds and mouth ulcers.
Some residents claimed that symptoms appeared or worsened when the plant was active and subsided when it shut down or when they left the area.
Complicating the situation, the co-located biogas plant has its own odor profile, also at times presenting a nuisance to some neighbors, making it difficult for people to distinguish between odor sources. Descriptions ranged from agricultural waste to chemical and burnt smells, suggesting a complex mix of airborne compounds.
A regional hospital later examined 29 individuals from the area, noting an “unusually high number” of cases with similar symptoms. The findings did not establish a direct causal link, and the Danish health authorities assess that it is unlikely that any actual illness or lasting symptoms have developed, as some individuals have reported (Aalborg Universitetshospital, 2025). The health authorities noted, however, that the odor nuisance in itself could have provoked some of the symptoms reported.
The science: what we know about emissions & health
Scientific literature on pyrolysis and biochar indicates that the process can produce a variety of emissions, depending on feedstock, temperature, and operational controls.
Studies have identified potential release of:
- Particulate matter (PM)
- Volatile organic compounds (VOCs)
- Nitrogen oxides (NOx)
- Sulfur dioxide (SO₂)
Exposure to such substances is known to cause short-term health effects, including irritation of the eyes and respiratory system, headaches, and exacerbation of asthma (Kim et al., 2019; Liu et al., 2021; Wang et al., 2018).
Importantly, the pyrolysis process itself is not inherently hazardous when properly managed. Modern systems are designed to operate in closed environments with emission controls such as gas cleaning and filtration. However, real-world performance can vary, especially in large-scale, newly deployed facilities (ScienceDirect, 2020; Springer, 2024).
Another area of uncertainty lies in the composition of biochar. Depending on input materials, it may contain trace contaminants such as heavy metals or persistent organic pollutants. While most research suggests these are typically within safe limits, long-term environmental impacts remain under study (Liu et al., 2021; Wang et al., 2019).
Expert voices
To better understand what is happening in North Jutland, we spoke with a range of experts covering different perspectives: Jesper Ahrenfeldt, Chief Engineer at Stiesdal; Jakob Balling Bro Sørensen, founder of Balling Bro, providing on-the-ground view; and Jonathan Hessner Lindhardt, a researcher at the University of Copenhagen (UCPH) offering insights from environmental science and field trials. Together, their perspectives help distinguish between issues inherent to the technology and operational challenges such as odor management.
Jesper Ahrenfeldt, at Stiesal, emphasized that the odor issues reported by residents are not related to biochar production itself but rather to the drying of biogas residual fibers, which can produce odors. He highlighted that these challenges were not anticipated, but were addressed with a variety of measures after the problem became apparent. After implementation of an activated carbon filtration system in August 2025, complaints about the specific odor nuisance from the drying process subsided. Ahrenfeldt also pointed out that all research has been conducted scientifically and handed over to environmental authorities.
Besides carrying out regular analyses of the biochar, performed by independent laboratories and showing that the biochar complies with all limit values, the plant is also subject to an extended monitoring program with tests now showing compliance with limit values for both emissions of substances and odor.
On the safety of biochar for agriculture, he said, “We have all the knowledge we need… we comply with regulations, and we test very often… we can produce safe biochar that meets all the standards.”
Jakob Balling Bro Sørensen, founder and owner of Balling-Bro ApS, provided the on-the-ground perspective. Educated as a farmer and mechanical engineer, with over 20 years of experience in agricultural machinery production, Sørensen leads a team that develops practical solutions for agriculture. He explained that biochar is a new technology that should be reviewed by government authorities, but he stressed that the health concerns reported in North Jutland are not caused by gases from pyrolysis. Agreeing with Ahrenfeldt, the main challenge has been managing odors from drying residual fibers and ensuring safe ventilation, rather than any inherent danger from biochar itself.
Jonathan Hessner Lindhardt, added that biochars that they have tested in field trials, does not show negative effects on soil organisms or crops. In high-dose field trials, soil and crop health remained stable, suggesting that biochar is safe at the end-user stage. Hessner Lindhardt emphasized the importance of clear legislation and guidance for biochar application, noting that agricultural benefits vary depending on biochar type, soil conditions, and cropping systems. “Biochar is a safe and great alternative product that sequesters carbon,” he said, “but the agricultural benefits will differ a lot between biochar types, soil types, and cropping systems. I do not see health impacts as a potential showstopper in the biochar industry.”
So, what happened in this case?
The ongoing measurement of emissions from the North Jutland plant, mandated and controlled by the environmental authorities, revealed that emission limits for sulfur dioxide and nitrogen dioxide had been exceeded once. This was handled through the installation of a filter solution, and emissions from the pyrolysis process have been in compliance subsequently (Strøbech 2026).
The health authorities concluded that the exceeded sulfur dioxide and nitrogen dioxide emissions did not pose a health threat to neighbors (Styrelsen for Patientsikkerhed, 2025). Odor levels near neighboring homes from the biogas residual fiber drying process were also recorded significantly above acceptable thresholds until the establishment of the carbon filter solution in August 2025.
These findings confirm that residents were exposed to elevated odor emissions through the first part of 2025. However, authorities have stated that measured concentrations do not necessarily imply long-term health risks.
Medical experts involved in the case suggest a mixed explanation:
- Some symptoms are likely linked to real exposure to significant odor nuisances
- Others may be influenced by stress, concern, or heightened awareness
Crucially, both authorities and researchers agree on one point: More data is needed to fully understand the plant’s impact. Despite extensive monitoring carried out by independent experts, the complexity of the combined pyrolysis and biogas facility makes it difficult to isolate specific sources of emissions.
Conclusion
The North Jutland case highlights the complexity of introducing new biochar technologies. According to Ahrenfeldt, Sørensen, and Hessner Lindhardt, biochar production itself is scientifically studied, while odor issues from drying fibers require ongoing attention. Experts agree that continued monitoring, regulatory review, and research are essential to ensure safe operation and public confidence in biochar as a sustainable agricultural product.
Ultimately, the lesson learned is that while the core technology may be sound, implementation matters: early-stage operational challenges, transparent communication, and strong regulatory oversight are critical when scaling new climate solutions in real-world settings.
The Rockstart Biochar Network was created with the support of the Danish government’s GUDP – Green Development and Demonstration Program. The initiative aims to advance regenerative agriculture and promote the use of biochar within all stakeholders in the ecosystem. Biochar, produced through pyrolysis, can persist in soils for centuries, enhancing fertility and capturing carbon dioxide. The Network supports Denmark’s climate change mitigation strategies, including improving the fertility of coarse sandy soils and preparing for the government’s 2027 biochar objectives, which encompass CO₂ taxation policies and the integration of climate technologies. Participation in the Biochar Network is free and open to all stakeholders.
Join the Biochar Network if you would like to learn more about Denmark’s biochar ecosystem.
