Methods and standards of pollen monitoring—significance of pollen measurements at different altitudes

The measurement of pollen concentrations has been performed according to various protocols in recent decades. In all approaches the following questions were of importance: the positioning of the measuring device (whether at roof or nose level), the reliability as well as the comparability of measurements. Current methods for pollen measurements and previous studies on measurement heights are presented and compared. The most widely used device is the Hirst pollen and spore trap. Its data is widely used for pollen information, pollen prediction models, and clinical studies. The position of the trap at roof level remains the current standard as the pollen concentrations correlate best with “real world” situation and regional symptom data. The guideline for the detection of pollen and spores for allergy networks (EN 16868:2019-09) in Europe was an important step towards standardization. Modern, automated measurement methods are still in experimental stage in terms of validation and comparability. Harmonization of data from different measurement methods is a future challenge.


Aerobiology and pollen measurements for allergology
The scientific field of aerobiology, which includes measurements of pollen in the air, is a rather young scientific field which-in its present form-has only existed since the second half of the 20th century [1]. Although measurements of organic and inorganic particulate matter in the ambient air has been performed for a long time, the connection between pollen and allergy has been known only since the end of the 19th century [1]. The subsequent development of measurement methods, techniques, and procedures for the detection and analysis of biological particles in the air has led to the still existing problem of universal standardization [2]. This problem repeatedly leads to questions about which measurement method is the most reliable and how the measurement device should be positioned to obtain optimal results. The location of pollen monitoring devices on rooftops or at human level is frequently discussed. The purpose of this article is to provide brief insight into this discussion and to present an overview of methods used today and also to review the current scientific knowledge of pollen monitoring at different measurement heights.

Methods of pollen measurements
The first pollen measurements for the detection of allergic symptoms were conducted at the end of the 19th century using gravimetric methods. This basic approach uses a coated microscopical slide where pollen accumulates over a defined period of time [2].
The currently most common method of pollen measurement worldwide is the volumetric method. Most technical procedures of this approach were developed in the middle/end of the 20th century and  produce an air flow with a defined flow rate. Based on this flow rate, concentrations can be calculated that correspond to the respiratory volume of humans and therefore allow a direct comparison. There are a variety of different types of volumetric devices. Probably the most common is the automatic volumetric pollen and spore trap according to Hirst ([3]; Fig. 1), which is mainly used in Europe. Moreover, rotating arm impactors (Rotorod Sampler; [4]) are the measurement standard mainly used in the USA. These conventional methods of pollen detection usually require trained personnel for evaluation and cannot provide data in real time. Hence, different approaches have been made to obtain faster and more detailed results using image recognition techniques, fluorescence or laser detection by automatization of pollen measurements [5]. However, research in this area is still in an early stage of development and the calibration and validation of different methods is complex.

Advantages and disadvantages of different measurement methods
Many methods produce a variety of results that are not always comparable, depending on the experimental setup and measuring device. However, none of these methods has a sole claim to validity. Pollen measurements are point measurements and, thus, represent only an approximation of the real-world situation. Sources of error such as differences in the measurements depending on the location [6], errors in the measurements of the flow rate [7] or identification errors (human error) [8] can additionally affect these point measurements. It is often postulated that this would make measurement methods of the previous century obsolete and inferior to those of automatic pollen detection. However, the sources of error only shift to another level such as the software, the hard-ware or the automated detection methods themselves [9].
The gravimetric method of pollen measurements is outdated and rarely used nowadays. The only benefit from this method is that it is inexpensive and does not require electricity. However, this approach does not have good temporal and spatial resolution and is not sufficient for modern research anymore. In scientific studies gravimetric traps are still used to collect supporting data. For clinical studies or prediction models, as well as for pollen forecasts, this method is outdated.
Volumetric measurement methods currently represent the state-of-the-art measurement method worldwide. With time series dating back approximately 50 years [10] and standardized procedures (e.g., in Europe; EN 16868:2019-09: Ambient air-Sampling and analysis of airborne pollen grains and fungal spores for networks related to allergy-Volumetric Hirst method.), they are routinely used for clinical studies, forecast models, and pollen information. The major advantage of the volumetric method is a constant flow rate that allows pollen concentrations to be correlated with human respiratory volume. Temporal resolution ranges from days to hours, depending on the analysis. A volumetric pollen monitoring station can cover an entire region within a radius of about 100 km, depending on topography and location. Another advantage is the price, which is low compared to modern automatic systems. The disadvantages of this approach are (1) trained personnel is required to perform analyses, (2) the results have no real-time character and (3) can therefore only be collected or published retrospectively [5].
The current automated methods for pollen measurements were developed in the late 2000s and were improved in the last decade. Currently, there are only two countries in Europe where fully automated evaluation methods for pollen forecasts and prediction models are used and tested: Germany [11] and Switzerland [12]. The advantage of these methods is the near real-time data acquisition. Data can be reproduced in near real-time and trained personnel is not needed to analyze pollen and spores [5]. However, the disadvantages of these systems still outweigh the advantages. On the one hand, the acquisition and operation of such automatic pollen measuring stations turns out to be very maintenance-and cost-intensive; on the other hand, there are still problems with detection, calibration, and validation as these systems are only operational for a short time and show differences in the method of data collection themselves [9].

The "roof trap": "state-of-the-art measurements" in Europe
Area-wide and comparable pollen measurements in Europe and thus the first pollen information networks for the allergic population started in the 1970s [1]. Since then, volumetric pollen traps of the Hirst design became popular in Europe [3] and consequently the unofficial standard of European pollen measurement networks. In the 1980s, the database of the European Aeroallergen Network (EAN; https:// ean.polleninfo.eu/Ean/) was programmed and this standard was further consolidated by continuous implementation of European measurement networks. Nevertheless, it took more than 40 years to establish a standardized European guideline for the detection of pollen and spores for allergy networks based on numerous publications and validations (EN 16868:2019-09). The basis of this European standard is a publication that summarizes the state-of-the-art knowledge of European experts in the field of aerobiology. It includes qualitative and quantitative investigation aspects of pollen measurement and analyzing methods to make them comparable [8]. The current 400-500 active monitoring stations in the European pollen database (Fig. 2) also work according to this standard, which ensures the comparability of European pollen measurement data for pollen information, forecast models, and clinical studies. A section in the aforementioned publication also describes the ideal location of a volumetric trap based on the Hirst design. The measurement station should be positioned on the flat rooftop at a height of about 10-20 meters, without the direct influence of neighboring buildings or other obstacles that can cause turbulence [8]. Measurement at ground level is not recommended in the European standard.

Measurements at different heights-does it make sense to measure at ground level?
The exact location of a pollen monitoring station is primarily a matter of the research question. Should the measurements have a local, regional, or transregional character? The European standards for forecasting, pollen information, and clinical studies recommend measurements at rooftop level covering a regional area to record pollen concentrations in the best possible spatial and temporal resolution [13]. However, some medical professionals argue that people primarily move at ground level, raising doubts about whether measurements taken on rooftops truly reflect the actual conditions experienced by individuals suffering from pollen allergies.
This question was extensively addressed in the 1980s and 1990s. Several scientific studies conducted parallel measurements at rooftop and ground level, including weather data, to assess differences in pollen concentrations at different measurement heights [14][15][16][17][18][19]. These studies mostly showed significant differences while comparing concentrations of weeds (especially mugwort, ragweed, and nettles) as well as grasses, while pollen concentrations of early flowering trees exhibited fewer significant differences. Some results in these studies were consistently reproducible, but some individual results and studies contradicted each other. Recent studies indicate that concentrations in urban areas are more comparable to those of rooftop traps [20] and are even significantly lower when a personal pollen sampler is carried [21]. The latest work in this field conducted comparative measurements at ground and rooftop level over two years, comparing all relevant pollen types and, for the first time, incorporating crowdsourced symptom data [22]. The statistical results in this study describe a generally significant positive correlation for the pollen concentrations of all collected aeroallergens between rooftop and ground level traps (Fig. 3). In addition, not only the daily pollen concentrations but also the differences in the seasonal trends were measured. The study showed a high correlation and a great similarity, especially in the early flowering trees: birch, alder, oak, beech, and hornbeam, both in the general course of the season and in the pollen concentrations, which had already been demonstrated in the studies from the 1980s and 1990s [22]. The prevailing hypothesis for this outcome is that early flowering trees produce and release a greater number of pollen grains, leading to a more homogeneous mixture and minimal differences in pollen concentrations between rooftop and ground level [14,15]. The correlation for grasses is less significant and shows higher concentrations in the ground level trap. Due to the low height of grasses, pollen concentrations strongly increase at ground level and decrease with increasing altitude [14,15]. However, this applies only locally and when grasses grow in close proximity to the measurement device. Nonetheless, the grass pollen season's trends were found to be highly similar at both measurement levels according to the study [22]. Two different evaluation methods using crowd-sourced symptom data showed the high- est similarities with the aeroallergens birch, grasses, and ragweed from the rooftop level. The rooftop pollen measurement station was also identified as the most significant parameter, both concerning pollen measurements and crowd-sourced symptom data. This outcome confirms that not only pollen concentrations but also crowd-sourced symptom data have a regional character, making them compatible with the current European guideline for pollen monitoring [22]. Therefore, the fundamental conclusion from scientific works over the past 40 years was also validated in this recent study. In summary, pollen measurements at ground level have a local character and are therefore unsuitable for pollen information, forecasting models, and clinical studies [13].

Outlook
Technical advancements are progressing in the field of pollen measurements. However, many obstacles for the suitable long-term use of automated systems still have to be overcome, especially in the area of validation and comparability. It is evident that future measurements will continue to be "only" of regional character, in order to ensure the best possible pollen information and data situation on a regional and transregional level after using all available resources (financial, personnel, and organizational). The standardization of pollen measurements in Europe aims to enhance the comparability of current measurement and analyzing methods and was a crucial initial step in the right direction. Automated measurements offer opportunities for further improvement in realtime pollen information and pollen forecasting. In addition, the utilization of crowd-sourced symptom data holds significant potential to enhance prediction accuracy and allergen avoidance, especially as it is known that allergenicity is not only determined by airborne pollen concentrations [23]. The primary task for the future will be to harmonize all these techniques to approach the "real" situation during the pollen season more precise and provide pollen allergy sufferers with the best possible information to ensure allergen avoidance. While comparative measurements at ground level continue to yield valuable scientific insights, they are unlikely to become a permanent component of a pollen information network due to their local character. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.