Legionella bacteria cause record number of pneumonia in Finland

A record number of cases of pneumonia caused by legionella bacteria were detected in Finland last year, said the Finnish Institute for Health and Welfare (THL) and Finnish Food Authority in a joint press release on Wednesday.

The increase was mainly due to a rise in soil related infections.

More than 80 cases of Legionnaire’s Disease were identified in Finland in 2025. About half of the patients reported contact with soil or earth prior to falling ill.

In the two preceding years, the total annual number of cases was around 50.

“An increase in soil-related infections has recently been observed not only in Finland but also in other Nordic countries, particularly Sweden. The reason for the rise is still unknown. Some of the increase may be explained by more active testing,” said Riikka Ruotsalainen, specialist of THL.

Legionella bacteria are naturally present in small quantities in fresh water and soil. They can also be found in potting soil and compost products.

Exposure pathways in soil and earth related infections have varied. For example, patients may have changed potting soil or emptied a compost bin during the incubation period.

To prevent legionella bacteria from causing illness, dry soil and compost products should not be allowed to disperse dust into the air.

If the bacteria reach the lungs, they can cause severe pneumonia. Infection may also occur through wounds in the skin.

The risk of falling ill is highest among older adults and people with weakened immune systems.

Lung diseases and smoking also increase the risk. Most healthy individuals do not become ill even if they are exposed.

The authorities advised to avoid creating dust clouds from soil or compost to reduce the risk of infection when handling soil.

Using protective gloves and washing hands, opening soil bags, moistening soil or composting, clean soiling and storing soil bags out of direct sunlight were also advised.

The Finnish Food Authority reminded that consumers must be provided with sufficient information on the safe handling and storage of soil products in marketing materials.

Manufacturers have been instructed to add safety guidelines to product packaging.

Traditionally, water systems have been the most common source of legionella infections in Finland.

Legionella bacteria thrive in temperatures between 20–45 degrees Celsius and can grow in, for example, showers, hot tubs, humidifiers and cooling systems.

Legionella growth can be prevented by ensuring that cold water remains below 20 °C and hot household water stays sufficiently hot (at least 55 °C). Hot water refers to the water circulating in domestic hot water systems.