Belgium Pollen Season Lengthens as Heat Worsens Allergies

Belgium is experiencing a longer and more intense grass pollen allergy season this year, with scientists linking the worsening symptoms to rising temperatures and climate change. The combination of an extended pollen season, repeated heatwaves, and ozone pollution is causing increased discomfort for millions of allergy sufferers across the country, according to RTBF.

A Season Out of Pattern

Grass pollen allergies, commonly known as hay fever, are the most significant pollen allergies in Belgium, affecting at least one in six people. The traditional “high season” typically concentrates in June, with pollen concentrations frequently exceeding the critical threshold of 50 grains per cubic meter of air — the level at which most allergic individuals develop symptoms.

This year, however, the pattern has shifted. “In Belgium, the grass season seems to stretch a little beyond the classic pattern,” said Nicolas Bruffaerts, head of the Aerobiology Unit at Sciensano and coordinator of the Belgian Aerobiological Surveillance Network, in an interview with RTBF. While some grass species naturally flower later in July, concentrations have remained markedly higher than last year for several weeks.

Landmark Study Confirms Synergistic Effects

The health impact is being felt across the country, and new scientific evidence now explains why. A landmark study published in June 2026 by KU Leuven and Sciensano in the journal Environment International analyzed 20 years of general practitioner data from Flanders, covering hundreds of thousands of patients and millions of medical records.

The research, led by Professor Gijs Van Pottelbergh of KU Leuven, cross-referenced medical data with daily measurements of temperature, air pollution, and pollen concentrations. The findings were striking: when high pollen concentrations coincide with extreme heat, the risk of hay fever doubles, and asthma symptoms increase by approximately 24%.

“Remarkable are the increased asthma and allergy complaints when very high temperatures or elevated ozone levels were measured during the pollen season,” Van Pottelbergh said in a university press release. “It seems that air pollution and heat cause pollen to have a greater impact on our body. Often just a few days are enough to cause a strong effect.”

Heat, Pollution, and a Growing Crisis

The recent heatwave in Belgium coincided with an ozone peak, creating conditions for what scientists describe as a dangerous synergy between pollution and allergens. As reported by Belga via VCP-BHL, the study demonstrates that multiple consequences of climate disruption reinforce each other, with measurable effects on respiratory health.

“It’s clearly a synergy between increasing allergens and heatwaves whose frequency is also rising,” Bruffaerts told RTBF. He emphasized that environmental conditions like heat and pollution appear to act on both the allergenicity of pollen and the sensitivity of patients.

Professor Raf Aerts of KU Leuven underscored the broader implications: “If we want to know the impact of climate disruption on our body, we must not only look at the individual risk factors but also the overall picture.”

Public Health Implications

The extended season means prolonged suffering for millions of Belgians, increased GP consultations, and higher medication use. The synergistic effects of heat, pollen, and pollution may also lead to more emergency visits for asthma exacerbations during heatwaves.

Bruffaerts warned that public authorities need to adapt: “Public authorities should anticipate this problem more effectively by taking into account the interactions between these different health risks, which are all impacted in one way or another by climate change.”

The researchers advocate for integrated warning systems that simultaneously monitor pollen, heat, and air pollution data to alert at-risk groups — a shift from current siloed approaches.

Practical Advice for Allergy Sufferers

For those affected, the Belgian Aerobiological Surveillance Network, coordinated by Sciensano, provides real-time pollen monitoring through its website and mobile app. Experts recommend following prescribed medical treatment, avoiding outdoor activities during peak pollen times around midday, airing out homes early in the morning or after rain, and wearing sunglasses outdoors.

“The best advice is to follow the treatment recommended by your doctor,” Bruffaerts said. “Being less in contact with allergens directly reduces the risk of allergy.”

What to Watch For

As temperatures continue to rise, scientists expect the grass pollen season to start progressively earlier each year, further lengthening the exposure period. The findings from this 20-year study open new avenues for understanding how multiple environmental stressors interact to affect human health, and raise urgent questions about how healthcare systems should adapt to a warming world.

The key question now, as Bruffaerts put it, is to better understand these mechanisms — and for policymakers to act on what the science is already making clear.