A fresh spell of high temperatures has pushed Belgium’s authorities to switch on the warning phase of the national ozone and heat plan, and Brussels sits squarely in the path of the alert.

The Interregional Environment Agency, known as IRCEL-Celine, triggers this phase when the forecast points to sustained heat. Its rule of thumb kicks in when the cumulative gap between predicted daily highs over five days and a 25C threshold climbs past 17C, a signal that the body’s ability to cope starts to fray.

Once the phase opens, the response widens quickly. Health services issue daily warnings on temperature and ozone levels, general practitioners and care homes step up monitoring of frail residents, and social teams reach out to elderly people living alone and to those sleeping rough. Officials repeat the familiar advice to drink water often and to avoid strenuous activity during the hottest hours.

Brussels carries an extra burden that rural areas escape. Dense streets, stone and asphalt trap heat and release it slowly, turning the city into an urban heat island. The European Environment Agency recorded a striking gap on 24 June, when the centre of Brussels ran almost 23C warmer than the nearby Sonian Forest, a reminder that greenery cools and concrete does not.

The stakes are not abstract. Heat kills quietly, mostly among the old, the ill and the isolated, and the danger builds over consecutive hot nights when homes never fully cool. That is why the plan leans so heavily on monitoring rather than dramatic closures.

The Royal Meteorological Institute sets a high bar before it calls an official heatwave. The maximum temperature at its Ukkel station must reach 25C for at least five days in a row, including three days at 30C or above. The current spell may or may not clear that mark, but the warning phase does not wait for the label.

Climate scientists warn that such spells will arrive more often and last longer as the region warms, which turns each activation into a rehearsal for a hotter future. Emergency planners increasingly treat extreme heat as a public-health threat on a par with flooding, and they press cities to redesign streets, shade bus stops and protect workers who cannot escape the sun.

City services have also been expanding longer-term defences, from planting street trees to opening cooling spaces and drinking fountains, though such measures take years to blunt the worst of a heat island. For now, residents are being asked to check on neighbours who live alone and to treat the warning as a prompt, not background noise. The City of Brussels has published guidance on its official site, and local reporting tracks the forecast.