Background: a city marked by dioxin contamination
Lausanne has long wrestled with a legacy of dioxin contamination linked to its former Vallon incinerator. Soil samples once showed contamination levels up to 32 times the federal standard, fueling public concern and sparking calls for scientific clarity about any health risks. In response, the cantonal health department commissioned a study led by Unisanté to gauge real-world exposure through diet among residents.
The study design and its reported findings
Published in August 2024, the so-called exposure or impregnation study compared 100 Lausanne residents split into two groups. The exposure group consumed vegetables and eggs from their home gardens suspected of carrying dioxins, while the control group relied on groceries bought commercially. The researchers reported a 21% higher dioxin level in the blood of the exposed group but described this difference as non-significant, concluding a “minimal” cancer risk for residents overall.
The study’s authors argued that, despite the apparent difference, the result did not demonstrate a clear health signal. Officials framed the findings as reassuring, with the local medical officer at the time suggesting that Lausanne residents were not at greater risk than others. Critics, however, saw a more nuanced story behind the numbers.
Expert critiques raise methodological red flags
The critique from European experts centers on two core methodological concerns that they say undermine the study’s conclusiveness.
Group composition and exposure assessment
Professor Alfred Bernard, a Belgian toxicologist known for his work during Belgium’s 1999 dioxin crisis, contends that mixing eggs and vegetables in the same exposed group muddled exposure assessment. Dioxins concentrate in animal fats, so a diet combining high-fat animal products with plant-based foods could dilute the signal from the most at-risk items. Bernard suggests a design focusing on older individuals with high-fat local foods to better capture real exposure differences.
Choice of control group and sample size
Dr. Agostino di Ciaula of Bari argues that the control group should have been drawn from a non-exposed population outside Lausanne to establish a cleaner contrast. He also notes the sample size—100 participants, down from an initial plan of 200—reduces statistical power to detect meaningful differences. In his view, the study remains highly preliminary and not capable of robust conclusions about health outcomes such as cancer risk.
Contractual framework and what would justify further research
Questions about why additional cancer incidence investigations were not pursued hinge on the contract between the Canton and Unisanté. According to documents obtained by Temps Présent, the mandate allowed further research only if a statistically significant difference emerged. Since the 21% blood-dioxin difference was deemed non-significant, further inquiry was not automatically triggered, a decision criticized by the experts who see insufficient evidence to close the case definitively.
Response from authorities and the path forward
When confronted with the critiques, Deputy Cantonal Doctor Stéphanie Boichat Burdy defended the study as conducted “in the rules of the art” and scientifically valid, while acknowledging its limitations. She noted that individual health questions from residents are understandable but beyond the scope of this single assessment. Councilor Natacha Litzistorf, responsible for Environment, said she had no reason to doubt the researchers’ integrity and remained open to additional scientific expertise.
Publication and key takeaway
The impregnation findings appeared in the International Journal of Hygiene and Environmental Health, emphasizing the role of egg consumption in dioxin contamination. The broader takeaway is a call for more rigorous, adequately powered studies if communities seek definitive assurances about long-term health risks. As Lausanne continues to grapple with its dioxin heritage, the debate highlights how study design, statistical power, and population selection shape what is considered “conclusive.”
By: Jean-Philippe Ceppi
