The June heat wave led to a notably higher excess mortality rate in Wallonia and Brussels than in Flanders. Experts point to a combination of factors: geographic conditions, the urban heat island effect, socioeconomic vulnerability, housing quality, and possibly air pollution.

Because no matter how you look at it, poverty has long been linked to poorer health, and it is precisely these vulnerable groups that are hardest hit by heat: heat strains the heart, and elevated ozone levels affect the respiratory system.

Regional distribution in striking

Between June 18 and July 1, 1,747 more people died in Belgium than the average for the past 5 years. According to the scientific institute and the federal research center Sciensano, these are the highest mortality figures recorded during a heat wave since analyses began in 2000.

The regional distribution is striking. In terms of excess mortality, the figure was 31% in Flanders, 61% in Brussels, and as high as 76% in Wallonia.

It is difficult to pinpoint the exact causes of this increased mortality based on the data currently available, says Sciensano, but a combination of factors is nevertheless emerging.

Heat island effect

The epicenter of the heat dome was in France, which recorded its hottest June on record – 3,8° above normal. Wallonia is closer to that epicenter, which is why it also experienced higher ozone concentrations.

Around Liège, there is an additional local effect: the rocky soil absorbs more heat, and there is less wind flow in the valleys, causing both heat and air pollution to linger longer.

Brussels is also particularly vulnerable to heat due to the urban heat island effect. Paved surfaces such as asphalt and concrete, as well as buildings, absorb heat during the day and release it at night. In densely built-up neighborhoods, this effect is amplified by the so-called canyon effect: buildings lining narrow streets trap heat longer, restrict air circulation, and prevent the city from cooling down sufficiently at night. As a result, neighborhoods with little greenery and few trees can become several degrees warmer than their surroundings.

During the heat wave, this was further exacerbated by the persistent heat dome, which kept nighttime temperatures exceptionally high. As a result, homes barely had a chance to cool down, and temperatures in many apartments remained around 35° even at night. Poorly insulated and older apartments in densely built-up neighborhoods are particularly vulnerable as a result.

Plus: Low-income renters rarely have the money for insulation, window coverings, or air conditioning – and rarely own their homes, which means renovation grants don’t reach them.

On top of that, poverty has long been linked to poorer health, and it is precisely these vulnerable groups that are hardest hit by heat: heat strains the heart, and elevated ozone levels affect the respiratory system.

Effect of LEZ in Brussels

Specifically for Brussels, there may be an additional factor that completes the picture. Air quality there has been steadily improving for 10 years. Thanks to the introduction of the Low Emission Zone (LEZ) and the renewal of the vehicle fleet, NO2 concentrations on busy thoroughfares such as Art-Law/Kunst-Wet have fallen by up to 46%. By 2024, all official monitoring stations met European standards.

But according to the urban advocacy group BRAL, that progress has stalled over the past 2 years. BRAL’s measurements at 24 additional monitoring sites show that the NO2 concentration did not decrease further between June 2024 and June 2025 and even rose slightly from 32.4 to 33.2 µg/m³ – well above the WHO threshold of 10 µg/m³ and the future European standard of 20 µg/m³ by 2030.

BRAL attributes this to the postponement of the next LEZ phase, which the Brussels Parliament pushed back by two years in October 2024. This explanation is supported by the Constitutional Court, which definitively overturned that postponement in December 2025 and ruled that the earlier timeline – between 2018 and 2023 – had, in fact, produced results.

Less car use in the city

Earlier this year, BRAL conducted a public survey in 4 Brussels neighborhoods with high levels of air pollution – Étangs Noirs/Zwarte Vijvers, Porte de Flandre/Vlaamsepoort, Toison D’Or/Guldenvlies, and the ULB-VUB campus. More than 80% of the 830 residents surveyed were concerned about air quality, according to the recently published results, with their own health and that of their children being their biggest concern.

This makes sense: These are densely built-up neighborhoods with little green space, a lot of paved surfaces, and busy traffic arteries with poor air circulation, which leads to both higher NO2 concentrations and a more pronounced heat island effect.

In fact, 67% of respondents say they are insufficiently informed about air quality and its consequences, or not informed at all. According to the respondents, the Brussels Region needs to do more (43,5%). Among the most important measures are reducing car use (53%) and taking a tougher stance on industrial pollution (32%).

BRAL points out that particulate matter (PM10 and smaller particles) and nitrogen dioxide are the 2 main air pollutants in Brussels. They have harmful effects on health, including inflammatory reactions, the development of chronic and cardiovascular diseases, and an increased risk of stroke. These pollutants come mainly from road traffic – particularly diesel engines – and from building heating systems.

Or, however you look at it, heat, air pollution, and poverty in Brussels follow strikingly similar patterns, and that makes the most vulnerable residents doubly vulnerable.