Engineering

Bacteria

Biology

Endocrinology

Occupancy

Waste management

Genetics

Environmental engineering

Legionnaires' disease

Environmental science

Legionella pneumophila

Architectural engineering

Medicine

Flushing

Microorganisms

When stay-at-home orders were issued to slow the spread of COVID-19, building occupancy (and water demand) was drastically decreased in many buildings. There was concern that widespread low water demand may cause unprecedented Legionella occurrence and Legionnaires’ disease incidence. In lieu of evidenced-based guidance, many people flushed their water systems as a preventative measure, using highly variable practices. Here, we present field-scale research from a building before, during, and after periods of low occupancy, and controlled stagnation experiments. We document no change, a &gt; 4-log increase, and a &gt; 1.5-log decrease of L. pneumophila during 3- to 7-week periods of low water demand. L. pneumophila increased by &gt; 1-log after precautionary flushing prior to reoccupancy, which was repeated in controlled boiler flushing experiments. These results demonstrate that the impact of low water demand (colloquially called stagnation) is not as straight forward as is generally assumed, and that some flushing practices have potential unintended consequences. In particular, stagnation must be considered in context with other Legionella growth factors like temperature and flow profiles. Boiler flushing practices that dramatically increase the flow rate and rapidly deplete boiler temperature may mobilize Legionella present in biofilms and sediment.

Variable Legionella Response to Building Occupancy Patterns and Precautionary Flushing