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The environmental issue of green algae in Brittany has been on the public agenda since the 1990s, but its emergence as a health issue is more recent (in the 2000s). The creation of expert and lay knowledge plays a crucial role in this process. This article investigates the strategies employed by local environmental groups to ensure their experiential knowledge is recognised by the broader scientific community, from which they were, until recently, excluded. We demonstrate that it is only when this knowledge is supported by tools partly derived from experience that this counter-expertise will transform the current warning and risk management systems. We begin by analysing how the official metrological system reinforces a narrow understanding of the risks addressed by public authorities, framing green algae as a circumstantial occupational hazard, manageable with relatively inexpensive methods. We then explore the unintended consequences of measurement tools that are reigniting the debate on health risks.