France heatwave sparks health alerts and cyanobacteria warnings

A prolonged heatwave across France has led municipal authorities in Lyon to issue heightened vigilance levels, adjust public service hours and provide free access to cooled public spaces. In the 2025 summer, Lyon recorded nearly 1,500 alerts for vulnerable residents and 200 home visits, with orange or red alerts warning of dehydration, heat stroke and exacerbated chronic illnesses.

At the same time, rising temperatures have triggered massive growth of cyanobacteria in lakes, ponds and rivers throughout several French departments, including Mayenne, Creuse, Vienne and the Landes. The blooms have forced the closure of multiple swimming sites, prompted health agencies to warn against water contact, and have been linked to dozens of dog deaths and 95 recorded human intoxication cases since 2006. The French agency ANSES advises avoiding recreational water activities, supervising children, and keeping pets on leashes in affected areas.