Comparison with other European studies
So far, it is difficult to gain a conclusive overview of airborne pesticides in Europe. The few recent data on airborne pesticides focus on different ranges of pesticides and use different collection methods.
Most of these studies excluded glyphosate; only the French study by Marliere et al. [32] measured this pesticide. This comprehensive study collected airborne pesticides using Partisol™ Sequential Air Samplers for particulate matter and analysed them for 74 pesticides and one metabolite (AMPA) [32]. We detected glyphosate in all passive air samplers and filter mats, not surprisingly as it is the most widely used plant-protection product in Germany [38]. In France, glyphosate was detected in over 80% of samples, which were collected at a flow rate of 30 m3/h over 48 h. The findings of that study suggest that glyphosate is generally present in ambient air.
Altogether, 42 substances were detected in the air of mainland France. Glyphosate, lindane, metolachlor, pendimethalin, and triallate had a median above zero. Prosulfocarb and folpet had the highest air concentrations. Samples of semi-volatile substances were collected over 7 days with a flow rate of 24 m3/d. This rate increased to 720 m3/d for glyphosate, glufosinate, and AMPA, which were collected over 48 h.
The general sampling rates in the French study for semi-volatile substances is very low compared to the Swedish data [14, 33], which was derived from active sampling at a rate of 400 m3/d over 1 week but did not measure glyphosate. A total of 116 substances were analysed, the majority in the glass fibre and PUF partition of the sampling cartridge. Altogether, 45 substances were detected in the PUF at the Hallahus sampling station in the Swedish study [33], which is a comparable collection medium to our passive air sampler. However, six substances were only found under the quantification limit. Chlorpyrifos, prosulfocarb, terbuthylazine, and triallate are currently used pesticides that were most frequently detected. The highest concentrations were observed for prosulfocarb. Pendimethalin was detected in nearly 40% of all samples; similar to chlorpyrifos, pendimethalin is not approved for use in Sweden.
In Italy, Estellano et al. [34] assessed the occurrence and seasonal variations of 10 current-use pesticides (chlorpyrifos, chlorpyrifos methyl, malathion, terbufos, diazinon, disulfoton, dacthal, trifluralin, pendimethalin, and chlorothalonil). All 10 pesticides were detected. Chlorpyrifos in both forms was identified as the most frequent pesticide with the highest concentrations. Chlorothalonil, which we detected at almost all sites and with very high levels, was the least detected pesticide in the Italian study. Pendimethalin was surprisingly more commonly detected in urban rather than rural settings.
An earlier extensive study in France used a Partisol 2000 low-volume sampler [35]. Between 2006 and 2008, samples were collected and analysed for 56 currently used pesticides, of which 41 were detected. The herbicides trifluralin, acetochlor, and pendimethalin and the fungicide chlorothalonil were detected at a frequency of 52–78%. This corresponds to our findings in Germany, where trifluralin and acetochlor are no longer approved but pendimethalin and chlorothalonil were widely detected.
Degrendele et al. [36] assessed 27 currently used pesticides and 10 persistent organic pollutants in the Czech Republic using a high-volume air sampler exposed for 1 week and an air volume around 600 m3/d as well as a Cascade impactor air sampler. The emphasis of the study was the determination of gas–particle partitioning and particle size distribution in current-use pesticides and persistent organic pollutants. Isoproturon, metazachlor, chlorpyrifos, terbuthylazine, S-metolachlor, and fenpropimorph were detected in more than 65% of samples. Chlorpyrifos, metazachlor, acetochlor, isoproturon, and S-metolachlor were identified as substances with a total combined maximum of (gas particulate phase) concentrations exceeding 100 pg/m3. The presence of terbuthylazine varied by season. The substances our study identifies as widespread, such as chlorothalonil, pendimethalin, dimethenamid, and prosulfocarb, were not analysed here. However, the occurrence of chlorothalonil, chlorpyrifos, and pendimethalin in European air and worldwide had been established in 2012 by Koblizkova et al. [18].
In comparison, the present study covers the widest range of analysed substances. With 109 substances detected, the range of pesticides known to be airborne is extended significantly. Pesticides detected with higher frequencies, such as chlorothalonil, metolachlor, pendimethalin, terbuthylazine, and prosulfocarb, were also frequently identified in other studies. The metabolite prothioconazole-desthio was assessed in the Sweden study but not discussed [14]. Dimethenamid was included in the France study but was only rarely detected (frequency < 5%). Overall, we detected pendimethalin and prosulfocarb in over four fifths of the samples and at high levels. The medium-distance transport of these pesticides is clearly illustrated by Kreuger and Lindström [14], who found pendimethalin in nearly 40% of all samples even though this pesticide is not approved for use in Sweden and probably originated in neighbouring countries such as Denmark, Germany, and Poland.
In general, substances that were not detected in the earlier studies had a lower detection frequency in our data set. These pesticides contribute to the overall pesticide load in the air but may have potential ecotoxicological effects and still result in high air concentrations because of their seasonal application. Their relevance must be assessed further.
However, agriculture is country-specific, with unique environmental conditions in soil and climate. This results a distinctive variety of cultivars, which in turn determine pesticide application. In a comparison with the Swedish PUF data from Hallahus (2017), we found 24 substances that were detected in both countries. Twenty one substances were detected in Sweden but not Germany and 43 substances were detected in Germany but not Sweden. Nine substances detected in Germany were targeted but not found in the Sweden data set. The findings were similar for glass fibre filters and filter mats (Additional File 15).
Pendimethalin and chlorpyrifos are not approved for use in Sweden but were detected at low levels. For chlorpyrifos, this was true for Germany as well. Because of varying environmental conditions, levels of airborne pesticides may vary between areas and countries and also between years. The growing interest in this topic renders is likely that more insights into these variations will be possible in the future.
Comparability to other passive sampler data and concentration estimates for pendimethalin and chlorothalonil
Pendimethalin and chlorothalonil are among the most commonly detected pesticides in German air. For economic reasons, our findings give the total values measured in three PUF disks over 7 months. To compare the data to passive air sampler data, the pooled quantity must be divided by three. We used the calculation given by Estellano et al. [34] to estimate air concentrations of pendimethalin, chlorothalonil, and chlorpyrifos-ethyl (Additional File 16).
Pendimethalin has been detected in air samples worldwide [18]. The more recent data compiled by Estellano et al. [34] in Italy used passive air sampling to measure nine current-use pesticides, including pendimethalin and chlorothalonil. The detection frequency of pendimethalin was below 25% of samples. Surprisingly, higher concentrations were found at urban sites (maximum: 1500 pg/m3 in spring). During other sampled seasons, concentrations did not exceed 280 pg/m3. We compared these values with data from the Czech Republic in 2012 [18] and Canada in 2008 [15] in 2010 [16]. Higher values were detected in France in 2010 [35] using Partisol 2000 low-volume samplers, with a maximum of 117,330 pg/sample and an average 1840 pg/m3; the frequency of detection was 66%.
The German results identify this pesticide more often (frequency: 89.8%) but at lower levels comparable to those recorded in France in 2010. The maximum for pendimethalin was 4796 pg/m3 and was by no means a singular value. Of the seven values that exceeded 1000 ng/sample, an average concentration of 2405 pg/m3 was measured. The German average over all 49 sites, including five sites, where the pesticide was not detected, is 543 pg/m3. These findings may reflect the wide use of pendimethalin in Germany today. Since this study offers only one value over 7 months, it must be expected that the concentrations of pendimethalin are significantly higher during times when the pesticide is applied (spring and autumn). Pendimethalin and prosulfocarb were of special interest in the present study because of their effects on organic farming [11].
Chlorothalonil was widely detected (frequency: 95.9%) but was at the end of its approval period in Germany (May 2020). Estellano et al. [34] detected chlorothalonil in fewer than 25% of samples and during only two seasons, with a peak measurement of 40 pg/m3. In Germany, chlorothalonil concentrations reached an average of 775 pg/m3, even when two samples without detection are included. The highest concentration was 3030 pg/m3, and the average of seven samples that exceeded 1000 ng/sample was 2291 pg/m3. These numbers are in line with data reported by Koblizkova et al. [18], where a 2500 pg/m3 concentration was recorded in Paris and 340 pg/m3 in Kosetice (Czech Republic). However, Coscollà et al. [35] reported considerably higher levels (maximum: 107,000 pg/m3) with an average concentration of 12,150 pg/m3, but at a lower detection frequency of 58%.
Abundant data are available for chlorpyrifos in passive air samplers, which is not approved for use in Germany even though it was detected along the western and eastern borders. The highest value corresponded to 280 pg/m3. Recent data from Chile suggest an air concentration range between 444 and 14,624 pg/m3 when the pesticide is actively applied [37].
Temporal variations in pesticide levels
Our data represent sums over the entire measurement period and do not reflect temporal patterns, so whether the airborne concentrations were relatively constant over the collection period or differed by season or event is unknown. The temporal pattern of exposure was not an aim of this study, but it is highly relevant for a toxicological assessment.
Earlier studies had already shown that temporal variations are detectable using passive air samplers [34, 36].
Sweden is the first European country to conduct long-term active sampling of airborne pesticides at rural sites surrounded by forests and located more than 1 km away from treated fields [14]; weekly data are available from the University of Uppsala [33]. Kreuger and Lindström [14] showed that most of the pesticides and their related substances were captured in the glass fibre filter and the first PUF disk, while only 4% of the total pesticide content was found in the AmberLite resin and second PUF disk. Additional File 17 lists the number of substances detected in 2017 at the Hallahus site, the southernmost measuring station of the Swedish network. The weekly data for the glass fibre filter and the PUF disk are listed separately. Figure 8 shows a graphic summary of the data. Ten to 35 substances of the 101 in the testing protocol were detected in the PUF disk. The glass fibre filters were analysed for 115 substances, adding six to 42 substances to the findings. Figure 8 shows the number of substances detected in the PUF and glass fibre filter.
The Swedish data display a clear seasonal variation, with the number of substances detected decreasing in autumn (Additional File 17). A weekly maximum of 59 substances was detected twice in the first half of the year. In autumn (October 30, 2017), only 17 substances remained. The median over the measurement period was 36.5 for all substances in the glass fibre filter and PUF disk. Prosulfocarb and γ-HCH were detected consistently throughout the year [14]. Chlorpyrifos, propyzamide, and triallate were also detected during much of the year. In areas of intensive agriculture in Germany, a continuous presence of pesticide mixtures in ambient air is, therefore, highly likely and should be the subject of further investigation.
Comparing Swedish data with findings from passive air samplers and filter mats
The two sampling methods in our study are likely to reflect airborne pesticides and their related products from differing origins. The PUF disk in the passive air sampler is designed to sample volatile and semi-volatile substances and exclude particles [30], while filter mats capture dust and sometimes pollen. Therefore, filter mats capture a different range of substances. Twenty-nine substances in filter mats were not detected in the passive air samplers, while 44 substances were detected only in the passive air samplers and not in filter mats. However, while we assess these methods separately, the Swedish study [14, 33] measured substances in glass fibre and PUF disks simultaneously. The number of substances detected in the Swedish study [14, 33] peaked at 59. In one example, 37 substances were detected in the glass fibre filter and 32 in the PUF disk. The number of substances detected by both methods, in this case 10, was subtracted from the total to remove duplicates. We detected 36 substances in filter mats and 33 in PUF disks, much lower than the 59 detected in Sweden in glass fibre filters and PUF disks at a peak period.
Our data for Germany are, therefore, likely to underestimate the pesticide load at any one point. Combining filter and PUF disk measurements in a passive air sampler may increase these numbers significantly.
For future passive air sampler measurements, it may be worthwhile to analyse the PEF not only for glyphosate but for the 500 pesticides and related substances assessed in filter mats so as to broaden the detectable range of substances.
Medium- and long-range transport
In our statistical analysis, we defined distances from a potential source for all measured sites so as to separate effects from spray drift, which will only occur in proximity to a field from airborne pesticides that travelled a longer distance in the air. Long-range transport in this context is set when a source is more than 1000 m away from the measurement site.
The FOCUS report [38] took a broader view and defined short-range transport (SRT) as 0,001–1 km, medium-range transport (MRT) as 1–1000 km, and long-range transport (LRT) as > 1000 km from the point of application. Kreuger and Lindström [14] detected many substances that did not originate in Sweden, as they are not authorised for use there, suggesting medium-range transport according to the FOCUS criteria. In Germany, which is centrally located in Europe, a similar phenomenon is likely. We detected chlorpyrifos at several eastern and western sites close to the borders [13], although the use of chlorpyrifos is not permitted in Germany.
Additional File 18 contains the European Food Safety Authority (EFSA) conclusions on the potential for volatilisation and long-range transport of the most widely detected pesticides in this study (glyphosate, metolachlor, pendimethalin, terbuthylazine, and prosulfocarb). Medium- and long-range transport are not distinguished in the EFSA reports. The vapour pressure of these substances classifies all of them as having low volatility [13], with the exception of metolachlor, which is of medium volatility. EFSA maintains that a substantial loss of active substance to the air is not to be expected; a loss to the air over the short range is only considered for pendimethalin. The potential for medium- or long-range transport is disregarded entirely. In view of our findings, the assumptions incorporated into the approval processes concerning the release of pesticides to the air are not adequate. There is a need to revise current EFSA estimates of pesticide releases to include estimates for medium- and long-range air transport. This is particularly relevant for the renewal of the approval for glyphosate.
Our findings indicate that a certain degree of long-range transport between continents is also likely. Current-use pesticides can be expected to travel far from their application sites and be present in any air mass passing over land. Bearing in mind that their environmental fate depends on their chemical properties, a worldwide distribution, as observed already for POPs, nevertheless must be considered possible.