Variable Legionella Response to Building Occupancy Patterns and Precautionary Flushing

Area of Science:

• Environmental microbiology
• Public health
• Building water systems

Background:

• COVID-19 stay-at-home orders drastically reduced building occupancy and water demand.
• Concerns arose regarding potential increases in Legionella bacteria and Legionnaires' disease due to low water demand.
• Variable, evidence-free flushing practices were widely adopted as a preventative measure.

Purpose of the Study:

• To investigate the impact of reduced water demand (stagnation) on Legionella pneumophila in building water systems.
• To evaluate the effectiveness and potential unintended consequences of precautionary flushing practices.
• To understand the complex interplay of stagnation, temperature, and flow on Legionella growth.

Main Methods:

• Field-scale research in a building experiencing periods of low occupancy (3-7 weeks).
• Controlled laboratory experiments simulating water stagnation and boiler flushing.
• Quantification of Legionella pneumophila (L. pneumophila) levels before, during, and after stagnation and flushing.

Main Results:

• Observed varied L. pneumophila changes during low demand: no change, a >4-log increase, and a >1.5-log decrease.
• Precautionary flushing prior to reoccupancy resulted in a >1-log increase in L. pneumophila.
• Controlled boiler flushing experiments corroborated increased L. pneumophila following rapid flow and temperature changes.

Conclusions:

• The effect of water stagnation on Legionella is complex and not universally predictive.
• Certain precautionary flushing methods can have unintended negative consequences, potentially increasing Legionella.
• Effective management requires considering stagnation alongside other factors like temperature and flow dynamics, especially in boiler systems.