Nitrogen fertilizers, widely used in agriculture, not only boost the quantity of pollen produced by grasses, they also raise the allergenicity of the pollen. And that’s That bad news for allergy sufferers.
Researchers at Katholieke Universiteit, Leuven, Belgium, compared pollen production from fertilizer-treated versus untreated Belgian grasslands and found a six-fold increase in pollen loads from the fertilized grasses.
Moreover, when they tested pollen for its ability to activate basophils from seasonal allergic rhinitis patients, they found a 5-fold increase associated with the nitrogen-fertilized pollen. Pollen specific IgE titers showed a 1.3-fold elevation of in response to the pollen from fertilized fields.
More Nitrogen, More Pollen
“To the best of our knowledge, this study is the first to establish a clear relationship between ecosystem nitrogen enrichment and pollen allergenicity. Our findings show that agriculturally fertilized grasslands have a higher pollen abundance than their unfertilized semi-natural counterparts,” writes Robin Daelemans, a KU Leuven ecologist, who headed the study (Daelemans R, et al. Lancet. April 2025).
“Moreover, this pollen exhibited significantly higher intrinsic allergenic potential, as evidenced by increased basophil sensitivity and elevated specific IgE levels in patients who were allergic to grass pollen,” report Daelemans and colleagues. “Together with atmospheric nitrogen pollution, ubiquitous ecosystem nitrogen enrichment can substantially worsen pollen allergies,”
Nitrogen boosts plant growth and reproductive capacity, and for most plants, nitrogen content in the soil and the air is a growth-limiting variable. That’s the main reason why farmers use nitrogen-rich fertilizers. So it’s not entirely surprising that fertilized fields will yield more pollen than wild fields with lower nitrogen stores.
“This study is the first to establish a clear relationship between ecosystem nitrogen enrichment and pollen allergenicity.”
–Robin Daelemans et al. KU Leuven
Allergists, immunologists, and allergy patients have long suspected that increases in atmospheric nitrogen (from fossil fuel emissions) and use of nitrogen fertilizers can drive up pollen counts–and with them, allergy severity—especially in agriculturally-intense regions. Atmospheric nitrogen and deployment of fertilizers have both increased steadily across the globe in recent decades. So has the prevalence of pollen allergies. Epidemiological studies do point to associations between these trends.
But the specific effects of nitrogen abundance on composition of airborne pollen loads are unknown, and impact of this on allergy severity is a new field of study.
An Ecosystem Study
Dr. Daelemans, of the Division of Ecology, Evolution and Biodiversity Conservation, KU Leuven led a multi-center team to undertake the first systematic study of these questions. The project was funded by the Belgian Science Policy Office.
The investigators collected pollen from 50 paired semi-natural grassland fields in the Hageland region of Belgium—25 enriched with nitrogen fertilizers, and 25 non-enriched.
The fertilized and unfertilized sample areas included three subtypes of grassland–Fen Meadows, Grass Heaths, and Hay Meadows—which differ in their average yearly moisture levels and soil richness. Fen and Heath environments tend to be nutrient-poor, while Hay Meadows naturally have higher soil nutrient content.
Daelemans and colleagues analyzed grassland pollen abundance, quantified the pollen species composition via DNA sequencing, and assessed pollen allergenicity using basophil activation tests and specific IgE measurements.
They obtained the basophils and serum from a cross-sectional sample of adults—13 women and 7 men–who were allergic to grass pollen (n=20). The researchers confirmed the allergies via skin prick-testing for grass pollen. The participants—who were all over age 18 and had a mean age of 40 years, had not received any form of allergen desensitization treatment for at least two years prior to study enrollment.
“Each patient served as their own internal control, as we compared reactivity toward pollen from unfertilized versus fertilized grasslands, normalized for protein content.”
Striking Differences
The differences were striking. Fertilized fields produced pollen at a mean density of 3.6 mg/m2, versus 0.6 mg/m2 in the unfertilized areas. Pollen from the fertilized grasslands showed a 5.1-fold greater capacity to activate the patient’s basophils. IgE titers showed a 1.3-fold increase in response to the fertilized versus unfertilized pollen (3.63 kUA/L vs 2.81 kUA/L).
Because their assessment of allergenic potential was normalized by protein content, the increased allergenicity they saw cannot be explained simply by the observed increases in pollen production alone.
Immunoblot analysis showed that patients exhibited stronger responses to specific allergens, and reacted to a wider range of proteins in the fertilized compared with the unfertilized pollen samples.
The authors note that because their assessment of allergenic potential was normalized by protein content, the increased allergenicity they saw cannot be explained simply by the observed increases in pollen production alone. Some other factors are in play.
They suggest that nitrogen fertilizers not only increase total plant growth—and therefore total pollen production—they likely also alter the mix of plant species in a treated ecosystem, potentially amplifying the growth of more allergenic grasses.
Daelemans’ group did find that pollen from the fertilized grassland plots showed a higher abundance of Alopecurus pratensis (Meadow Foxtail), which produces Group 1 and Group 5 allergens. It is also possible that development in a nitrogen-supplemented ecosystem changes the biochemical properties of a plant’s pollen, raising it’s allergenic potential.
All that said, the simplicity of this initial study limits the ability to draw strong conclusions about the relative roles of specific grass species to overall pollen allergenicity, and the plant-specific effects of nitrogen fertilization.
Similarly, Daeleman’s study was not designed to assess actual symptom burden in response to direct (intranasal) exposure to the various sorts of pollen. So, their data do not permit a definitive conclusion that exposure to pollen from nitrogen-treated fields will cause more frequent or more severe symptoms.
Public Health Implications
But given the five-fold increase in granulocyte activation that they report, there is reason to suspect that it would. Daelemans and his co-authors certainly believe they’re on to something.
“These findings extend the understanding of nitrogen pollution’s impact on public health, revealing that nitrogen enrichment exacerbates pollen allergy severity,” the authors write.
“To mitigate these health impacts, nitrogen emission regulations should not only consider the direct ecological consequences of nitrogen pollution (biodiversity loss) but also its indirect health impacts (eg, allergy disability-adjusted life years). Future research should prioritize evaluating the effects of nitrogen pollution on the disease burden across entire populations on a larger spatial scale, assessing the health-care costs and benefits of various nitrogen reduction scenarios within regional and national policy frameworks.”
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