The problem related to a sampling frequency too low for Legionella.

Legionella bacteria multiply exponentially under favorable conditions: stagnant water, temperatures between 25°C and 45°C, and organic deposits in the system. An annual measurement provides only a snapshot and may not reflect a complete growth cycle.

This is not just a theory. Our practical experience shows that a facility can still be considered safe against Legionella in October, but reach dangerous levels as early as six months later.

How often do we recommend follow-up?

Case study: October 2025 vs. April 2026

The anonymized results below perfectly illustrate the importance of six-monthly monitoring. In October 2025, no sampling point revealed alarming values. Legionella was present, but it was the less dangerous variant and in small quantities. Six months later, a significant exceedance was found in Room 19. This time, it was a more dangerous variant, namely Legionella pneumophila, one of the strains most frequently associated with pneumonia. Although the most dangerous variant has not yet been detected, this indicates an environment conducive to its proliferation. Frankly, given our current experience in preventing Legionnaires' disease, we would not use showers with such levels ourselves. To avoid exponential growth and even more dangerous situations, it is time to undertake a thorough flush. Currently, there is still a real possibility that a complete flush could reduce levels to below 200 CFU/L, although this always depends on the situation and remains to be confirmed. This risk is not detected during annual sampling, with all the consequences that this entails.

What are the results of the six-monthly monitoring?

Decision

Annual sampling gives a false sense of security. Twelve months is far too long a period to understand the growth dynamics of Legionella. The results above illustrate how a site that appeared safe in October already showed a significant overgrowth by April.

A six-monthly sampling schedule represents a good compromise: frequent enough to identify risks in a timely manner and cost-effective enough to be structurally integrated into your management plan. Of course, the sampling frequency can vary depending on site conditions.