If you're planning to revel in some gardening this weekend, be sure to wash your hands after. New research finds that activities involving exposure to compost may increase a person's risk of Legionnaires' disease.

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Inhaling or ingesting compost may raise the risk of Legionnaires' disease.

Legionnaires' disease is a form of pneumonia most commonly caused by the bacterium Legionella pneumophila, which is found in lakes, streams, and other freshwater terrains.

Legionnaires' disease is contracted when people inhale small water droplets contaminated with L. pneumophila. This may occur through showering using a water system in which the bacterium has grown and multiplied, for example.

While the majority of people do not become ill from Legionella bacteria, there are some groups that are more susceptible. These include older adults, smokers, and people with a chronic lung disease.

One lesser known cause of Legionnaires' disease is a bacterium called Legionella longbeachae.

First isolated in 1980 from a patient in Long Beach, CA, L. longbeachae is found in compost and potting soil. Studies have suggested that inhalation and ingestion of these products may cause Legionnaires' disease.

For this latest study, co-author Prof. Patricia Priest, of the University of Otago in New Zealand, and colleagues set out to determine the key risk factors for infection with L. longbeachae.

The researchers recently reported their findings in Emerging Infectious Diseases, a journal of the Centers for Disease Control and Prevention (CDC).

Gardeners should be cautious

The study involved 31 adults who had been hospitalized as a result of L. longbeachae-related Legionnaires' disease, alongside 172 controls.

Over two summers, participants completed questionnaires detailing their demographics, smoking status, pre-existing health conditions, and any activities that might have exposed them to compost or potting mix, such as gardening.

The study suggests that gardening is a significant risk factor for Legionnaires' disease; almost all patients with the condition reported gardening in the 3 weeks prior to becoming ill, which involved coming into contact with purchased compost products.

Washing hands immediately after coming into contact with compost products was associated with a lower risk of Legionnaires' disease, though wearing masks or gloves did not appear to help.

Other risk factors for Legionnaires' disease included smoking and a diagnosis of chronic obstructive pulmonary disease.

Based on their results, the researchers say that gardeners should be cautious when handling compost products.

"We recommend gardeners avoid breathing in compost or potting mix, by opening bags away from the face and keeping it close to the ground when moving it around. Also, always wash compost/potting mix off hands before putting them near the face."

Prof. Patricia Priest

"Smokers and people with chronic obstructive pulmonary disease should be particularly careful to follow these safety precautions when gardening," adds Prof. Priest.

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