Update: outbreak of legionnaires disease in Norway traced to air scrubber.

Both approaches indicated the same source, an air scrubber in a lignin production plant. The scrubber cleans particles in the air used in the production process by exposing it to a strong counterflow of water. The water in the scrubber has a high organic content and is circulated by a pump. A continuous input of fresh water helps to keep the dry-matter level constant and replace water lost as aerosol. The scrubber operates at 40°C and expels more than 4 cubic metres of water/hour as aerosol, with an airflow of 60 000 cubic metres/hour and velocity of about 20 metres/second. The tank of the scrubber was routinely cleaned with high-pressure hot water every 3-4 weeks, but no disinfection was used. The pump and pipes had not been manually cleaned. The scrubber has been closed and there are no risks to tourists visiting the area or to other parts of Norway. A risk assessment of air scrubbers regarding conditions facilitating legionella growth (such as temperature and biofilm formation) must be done when investigating outbreaks of legionellosis.