BACKGROUND: The prevalence of allergy to aeroallergens is rising, driven by both environmental and lifestyle changes. However, the role of ubiquitous nitrogen enrichment in exacerbating pollen allergy remains unclear. This study aimed to investigate the impact of nitrogen on pollen allergenicity by connecting the resulting ecological changes with allergic outcomes.

METHODS: We conducted a cross-sectional paired comparison study, examining differences between nitrogen-enriched (fertilised) and non-enriched common semi-natural grasslands in Belgium. Pollen from paired grasslands (n=50, enriched [n=25] vs non-enriched [n=25]) based on their common geography, were sampled following a standardised protocol. We analysed grassland pollen abundance, quantified pollen species composition via DNA sequencing, and assessed pollen allergenicity using basophil activation testing and specific IgE measurements in a cross-sectional sample of adults who were allergic to grass pollen (n=20). Basophil activation test outcome measures included area under the dose-response curve, maximal reactivity (CD63), and effective concentration eliciting 50% basophil activation.

FINDINGS: Nitrogen-enriched grasslands produced significantly more pollen, with a 6·2-fold increase compared with their unfertilised counterparts (3·6 mg/mvs 0·6 mg/m). When normalised to protein content, pollen from these enriched grasslands showed increased allergenic potential, with 5·1 times higher basophil activation test sensitivity and a 1·3-fold increase in specific IgE titres compared with their unfertilised counterparts (geometric mean fertilised 3·63 kU/L vs unfertilised 2·81 kU/L).

INTERPRETATION: Nitrogen enrichment substantially increased pollen abundance and allergenicity, indicating a heightened allergy burden in nitrogen-rich environments. These findings underscore the need for policies addressing nitrogen pollution to mitigate its public health impacts.

FUNDING: Belgian Science Policy Office.