Novel method to measure pollen response in exhaled air

Hay fever is an allergy to plant pollen, it can significantly limit daily life. Many people are allergic to pollen, but measuring symptoms objectively remains difficult. Until now, questionnaires that depend on personal perception and skin or blood tests have been used most often. Recently, a novel, non-invasive method has been tested in which exhaled breath is analyzed to make hay fever symptoms measurable. By exposing participants to pollen from flowering allergenic plants during walks, researchers can investigate how the body responds and which chemical compounds are released.

Walks and collection of exhaled breath

In June 2024 and 2025, volunteers with and without hay fever walked past flowering grasses in Leiden. Before and after the walk, participants recorded their nose, eye, and throat symptoms using questionnaires. After fifteen minutes of walking, the participants’ exhaled breath was collected in special air bags. The collected air samples were transported to Radboud University in Nijmegen for analysis.

Analysis of exhaled breath

The collected air was chemically analyzed at the Life Science Trace Detection Laboratory of Radboud University. For this purpose, Proton-Transfer-Reaction Time-of-Flight Mass Spectrometry (PTR-ToF-MS) was used, a highly sensitive technique for measuring volatile organic compounds. This method is already applied in research on various lung diseases, but has not previously been used for hay fever.

The analyses showed that, in hay fever patients, certain organic compounds are present in increased concentrations in the exhaled breath during exposure to grass pollen. The exact role of these compounds in hay fever reactions is not yet known, but previous studies indicate that some of them are associated with inflammation and histamine responses.

Pollen sniffer

Concentrations of allergenic pollen in the air during the walks was measured using a so-called 'pollen sniffer'. This is a portable device that draws in air, causing pollen to adhere to a thin layer of petroleum jelly. A microscopic slide was prepared from the petroleum jelly, after which the pollen grains were identified and counted under a light microscope. During the walks past flowering grasses in June 2024, grass pollen concentrations proved to be very high. This corresponded with a clear difference in the composition of exhaled breath between hay fever patients and the healthy control group.

Control walk

Another walk was organized in November 2024, this time as a control. During this period, no allergenic plants are in bloom. Analysis of the pollen sniffer samples confirmed that there was no pollen present in the air during the walk. After the control walk, hay fever patients could not be distinguished from the control group based on exhaled breath. This confirms that the differences measured during the hay fever season can be attributed to exposure to allergenic pollen in the air.

Applications and future perspectives

Our method makes it possible to quantify hay fever symptoms in a new, non-invasive, and objective way using exhaled breath. The observed differences in organic compounds can serve as biomarkers for hay fever, which is a major advantage over subjective questionnaires or invasive skin and blood tests. In addition, the method offers opportunities to study the impact of urban greenery on health and allergies. This may help in deciding different options for avenue trees and shrubs in city parks. The research is currently being expanded. Because hay fever patients may be allergic to tree pollen as well, this year the method will be applied during exposure to flowering trees.

Participate in follow-up research

About half of hay fever patients are allergic to grasses, whereas the other half mainly react to tree pollen. Therefore, a walk past flowering alders in Leiden will be organized in January. This will take place on 23 or 30 January, depending on when the alders start to bloom. Anyone aged 18 and over can participate, with or without hay fever, provided you do not smoke. The walk lasts 15 minutes, after which exhaled breath will be collected. You can register now. 

Practical information

• Activity: hay fever walk past flowering alders.
• Location: Potgieterlaan, Leiden.
• Date: Friday 23 or 30 January 2026.
• Time: 10:00 a.m. to 10:30 a.m.
• Registration form.

Text: Nemi Dorst, Hortus botanicus Leiden; Barbara Gravendeel, Hortus botanicus Leiden; Joris Meurs, Radboud University
Images: Barbara Gravendeel, Hortus botanicus Leiden (lead photo: alder in bloom); Nemi Dorst