Soil-related pneumonia cases rise as authorities warn against inhaling dry potting mix dust

Finnish health and food safety agencies have issued a warning about the growing risk of legionella infections linked to potting soil, after cases of soil-related pneumonia reached record levels last year.

The number of legionella pneumonia cases in Finland surged from around 50 to over 80 in 2025, with roughly half of patients reporting exposure to soil or potting mix before falling ill, according to the Finnish Institute for Health and Welfare (THL) and the Finnish Food Authority. The agencies now caution that dry soil dust poses a significant inhalation risk, particularly for the elderly and those with weakened immune systems.

Legionella bacteria occur naturally in soil and fresh water but can cause severe pneumonia—known as Legionnaires’ disease—when inhaled. While most healthy individuals remain unaffected, the infection can become life-threatening for vulnerable groups, often requiring hospitalization or intensive care.

THL researcher Silja Mentula noted that soil-related cases were previously rare, with only sporadic incidents reported. However, recent years have seen a marked increase, mirroring trends in other Nordic countries. The rise has been linked primarily to commercial potting mixes and composts, though no single factor explains the surge.

To reduce risk, Mentula advises avoiding inhaling soil dust by working in well-ventilated areas or dampening dry soil before use. For waterborne exposure—a more common infection route—she recommends ensuring taps and showers are used regularly and monitoring water temperatures, as the bacteria thrive in lukewarm conditions (20–45°C). After prolonged absence, flushing pipes before use is also advised.

Diagnosis remains a challenge, as legionella requires specific testing not included in standard pneumonia screens. Mentula suspects underreporting in Finland, where case numbers lag behind those in Sweden, Norway, and much of Europe. Symptoms typically include dry cough, fever, and headache, indistinguishable from other pneumonias in early stages.