Ambient pollen exposure affects allergy symptoms, lacks threshold
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Key takeaways:
- People with a pollen allergy felt a change from 0 to 50 pollen grains/m3, but were unlikely to feel it between 200 and 300 pollen/m3.
- Pollen exposure had the strongest effect 5 hours before symptom assessment.
Ambient pollen exposure increases allergy symptom severity, but a lack of exposure threshold may impact pollen warning systems, according to the European Journal of Allergy and Clinical Immunology.
“With climate change, the pollen seasons are changing,” Marloes Eeftens, PhD, assistant professor and group leader of Sensoring and Environmental Epidemiology at the Swiss Tropical and Public Health Institute, told Healio.
“Plants start to flower earlier in the year, and pollen seasons are becoming more intense. More pollen is put out over the course of a season than several decades ago and more people are now suffering from allergic rhinitis than ever before,” she said.
Eeftens further explained that as environmental epidemiologists, she and her colleagues study many ways in which the environment influences our health.
“One of the important things to derive is always a concentration-response relationship, which tells us what health effects might be expected at a certain ambient concentration,” Eeftens said.
“We know relatively a lot about the health effects of some pollutants, such as typical air pollutants produced by city traffic and burning of fossil fuels, and we have characterized the concentration-response relationship in detail,” she continued. “We always knew that pollen was related to allergic symptoms, but we really did not know how much pollen causes how many symptoms. This is what this study aimed to find out.”
Methods
A total of 396 adults aged 18 to 65 years were recruited from 2021 to 2022 (mean age, 40 years; 61.87% women) from the Basel region of Switzerland. The presence of a pollen allergy as well as asthma was determined by participant questionnaires.
Pollen allergy symptom severity was reported 16 times during the 6-week study enrollment period, including during six weekly home nurse visits and 2 weeks during which participants completed 10 daily questionnaires.
In all the 16 points of data collection, participants reported medication use in the last 24 hours and rated the severity of their allergy symptoms on a 4-point scale. Symptoms included itchy nose, sneezing, runny nose, blocked nose, itchy eyes, eye watering, coughing, shortness of breath and wheezing.
Skin prick testing was used to determine pollen sensitization to seven species — early trees (hazel and alder), later trees (birch and ash), grasses and weeds (mugwort and ragweed) — with a reaction diameter of at least 3 mm considered as a sensitization.
Daily and hourly pollen concentrations were taken from the Federal Office of Meteorology and Climatology MeteoSwiss for the Basel site, and participants were required to live within a median distance of 2.3 km from the site.
Hourly nitrogen dioxide (NO2), hourly relative humidity and concentrations of particulate matter with a diameter of 10 µm (PM10) or less were taken from the National Air Pollution Monitoring Network and measured at the Basel-Binningen station.
Participants’ hourly exposure to the seven pollen species were recorded using a new pollen exposure metric.
Individually relevant pollen exposure (IPE) was calculated by adding the pollen concentrations of the plants a participant is sensitized to for a specific hour. IPE was calculated at the time of symptom reporting and for each hour prior up to 96 hours.
Symptom severity scores (SS) were assessed in three categories: the nasal and ocular symptom score, the pulmonary score and the combined symptom and medication score (CSMS).
Results
Among the 396 participants, the nine allergy symptoms were assessed 6,416 times, with 5,990 of these assessments included in analysis. Participants without a pollen allergy were assessed 1,424 times, and the remaining 4,566 were done in participants with a pollen allergy.
Those with a pollen allergy experienced allergy symptoms more frequently and more severely than those without. The most frequently reported symptoms were nasal, then ocular and pulmonary.
Within the pollen allergic group, runny nose (3.4%), itchy eyes (4%) and cough (1%) were the symptoms that were most frequently rated as severe in each category. This group also reported mild, moderate and severe symptoms in 30%, 10.7% and 2.3% of assessments, respectively. The nonallergic group reported mild, moderate and severe symptoms in 9.3%, 1.3% and 0.2% of assessments, respectively.
A significant association was found in the allergic group between IPE and nasal and ocular symptom severity. This association was not found in the nonallergic group. Nasal and ocular SS estimates were higher considering the IPE within the last 96 hours than in 24 hours.
No threshold pollen concentration was found in the allergic group, but increases in nasal and ocular SS were evident when pollen was in the air until concentrations of about 40 pollen/m3. Above concentrations of 80 pollen/m3, increases in pollen exposures are shown by small but consistent increases in symptom scores.
Pollen exposure had the strongest effect in about 5 hours before symptom assessment took place, with the effect being the strongest in nasal and ocular SS. The most severe nasal and ocular symptoms were reported by those aged 18 to 30 years. Grass pollen exposure compared with tree pollen exposure was associated with higher nasal and ocular SS.
Patients who did not have allergy did not experience any associations between IPE and the severity of their pulmonary symptoms.
However, the association between IPE and symptom severity was statistically significant when counts were in the range of 80 to 260 pollen/m3, the researchers said, although the effect on these symptoms was small.
The significance of this association only persisted for patients aged 18 to 30 years after stratifying for age, the researchers continued.
Also, symptoms were slightly stronger among patients with both pollen allergy and asthma compared with patients who only had pollen allergy.
Pollen exposures were most relevant to symptom severity approximately 9 hours before pulmonary symptoms were assessed, the researchers said, and this relevance quickly decreased as lag time increased.
“The concentration-response relationship which we found shows us that there is no threshold below which people do not experience any symptoms,” Eeftens said. “As soon as there is only a little bit of pollen in the air to which people are allergic, they will (on average) perceive symptoms also.”
She further explained that symptoms become more severe as the concentration increases up to 50 pollen grains/m3, which she called a typical in-season day. But, as pollen levels increase beyond that, symptoms still increase a bit, but not so much anymore, indicating that allergic suffering has reached its peak. As a result, people with allergies can perceive a change from 0 to 50 pollen grains/m3, but they are unlikely to be able to tell the difference between 200 and 300 pollen/m3.
“The absence of a threshold indicates that there is no pollen level (other than 0 pollen/m3) that ensures that allergic people will be free of pollen-related symptoms,” Eeftens said. “Even low levels of pollen may cause symptoms, so it is important that pollen warning services (such as MeteoSwiss and the aha! Swiss Allergy Centre) do not use the color green for any other level than 0.”
She advised that in the future, these services may also consider that symptoms could linger and remain elevated for several days after high exposures and that the pollen count from the day prior and after a high level may still be relevant.
Also, she said, these results may help people understand their symptoms and when they experience them.
“Many countries are making quick progress with real-time pollen monitoring systems, which can be effectively used for allergy management,” she said. “For example, if you see that today’s pollen levels are high, you may decide to take medication before going out, or take the tram instead of walking.”
Study impact, future research
The results of this study can inform public policy, Eeftens said.
“Many of the trees we have planted in our cities are highly allergenic: birches and ash trees are very popular urban trees,” Eeftens said. “Hazel trees (often bushes) are typically not planted, but highly prevalent in cities. Many cities are now making an effort to increase tree cover to combat climate change and increase biodiversity as well.”
Eeftens also said there is a great opportunity for urban planners to start choosing tree species that are less allergenic or do not spread their pollen by wind at all but by insects instead, which increases even more biodiversity.
She further explained that future research on this topic will investigate whether pollen has other effects than the well-known itching and sneezing such as breathing difficulties or poor sleep, or issues with concentration.
“We have measured several of these other health parameters in our study, and have recently also shown that allergic people experience small increases in blood pressure on high-pollen days,” Eeftens said. “Our blood pressure always fluctuates, also in response to stress, diet and other factors, but all these risk factors may add up. And if blood pressure becomes too high, this is a risk factor for a myriad of cardiovascular diseases.”