3.1. Overall detection of NNIs in PM2.5 samples from Wuhan.
Seven NNIs, namely, CLO, ACE, NIT, DIN, FLO, IMI, and THM, as well as three metabolites, 5-OH-IMI, N-ACE, and 6-CINA, were analysed in 101 PM2.5 samples. Table 1 summarizes the detection frequency (DF), detection range, geometric mean (GM), median and mean of these analytes. Among the PM2.5 samples collected, the detection frequencies of all 10 substances reached more than 60%. IMI and DIN reached 100%, followed by 5-OH-IMI (98%), N-ACE (98%), and ACE (97%), and then FLO (94.1%), NIT (88.1%), THM (77.2%), CLO (75.2%), and 6-CINA had the lowest detection rate (64.45%). In terms of the median concentration, DIN (53.3 pg/m3) and IMI (42.8 pg/m3) had higher concentrations than did the other substances. Moreover, the average concentrations of these two are also relatively high, reaching 194.1 pg/m3 and 71.6 pg/m3, respectively.
Table 1
Concentration (pg/m3) and detection frequencies of NNIs in PM.5 samples from 2019 to 2021 in Wuhan.
NNIs | DF | GM | Median | Average | Range |
CLO | 75.2% | 4.4 | 8.3 | 8.0 | <LOD-24.3 |
ACE | 97.0% | 2.8 | 3.3 | 14.9 | <LOD-1106.1 |
NIT | 88.1% | 1.7 | 2.2 | 63.9 | <LOD-5671.7 |
DIN | 100.0% | 59.0 | 53.3 | 194.1 | 7.8-4389.4 |
FLO | 94.1% | 2.9 | 2.8 | 3.5 | <LOD-10.4 |
IMI | 100.0% | 43.0 | 42.8 | 71.6 | 5.0-368.9 |
THM | 77.2% | 1.3 | 1.1 | 2.6 | <LOD-20.6 |
5-OH-IMI | 98.0% | 11.4 | 10.6 | 16.1 | <LOD-103.9 |
N-ACE | 98.0% | 5.3 | 5.6 | 6.2 | <LOD-22.8 |
6-CINA | 64.45% | 0.4 | 0.6 | 10.3 | <LOD-850.0 |
IMIeq | / | 251.3 | 256.1 | 708.0 | / |
DF: detection requency; GM: geometric mean; Range: means minimum to max-imum concentration |
Overall, there was a correlation between the frequency and concentration of substances detected in PM2.5 samples and the amount of production and use in the actual situation. For example, the high detection frequency of IMI (100%) also confirms that IMI are the most frequently used NNIs(ltd n.d.). In China, the total ACE productivity is 8000 tons per year(Shao et al. 2013). Notably, ACE was also detected at a higher frequency and concentration in this study. This finding is also consistent with the results of a national study. Wang et al.(Wang et al. 2019) collected dust from three cities, Taiyuan, Wuhan and Shenzhen. It was found that NNIs and their metabolites were widely present in dust from the three cities, with ACE and IMI having the highest detection rates. The metabolite of ACE, N-ACE, was detected about as often (98%) as ACE (97%) itself, and even the median and geometric concentrations (GM) of N-ACE were higher than those of ACE itself. In addition, 5-OH-IMI also exhibited a high detection frequency and concentration. This suggests that NNIs are readily metabolized to specific metabolites through biotransformation (demethylation, hydroxylation, and nitro reduction)(Casida 2011; Taira, Fujioka, and Aoyama 2013). Thus, NNIs and their metabolites cooccur in the environment. Different metabolic pathways result in different metabolites, leading to significant differences in toxicity among NNI metabolites(Marfo et al. 2015). The acute and chronic thresholds of 5-OH-IMI have been reported to be significantly greater than those of IMI itself(Suchail, Debrauwer, and Belzunces 2004). Thus, the high detection frequency of 5-OH-IMI in the environment should be considered. In addition, imine derivatives of IMI were more than 300 times more toxic to vertebrates based on the half maximal inhibitory concentration (IC50)(Tomizawa and Casida 2000). In-depth studies on NNI metabolites are also necessary.
3.2. The results of PM2.5 samples stratified by season
NNIs were stratified by season and analysed for variability in concentrations between seasonal stratification groups. The results of the analysis are shown in Table 2. The 101 PM2.5 samples were categorized into spring (n = 27), summer (n = 15), autumn (n = 33), and winter (n = 26) according to the time of collection. The distributions of ACE, DIN, IMI, THM, and 5-OH-IMI significantly differed among the four seasons (P < 0.05). The median concentrations of DIN remained high during the spring (53.33 pg/m3), autumn (58.33 pg/m3), and winter (76.11 pg/m3). In addition, IMI presented the highest median concentration in the summer (129.44 pg/m3), followed by spring (82.78 pg/m3). Detailed information is provided in Table S5. Moreover, its metabolite 5-OH-IMI exhibited the same trend. The seasons with a high frequency of pesticide use are usually spring and summer. This is because in spring and summer, warmer temperatures and increased humidity favour the reproduction and spread of pests. Crops also experience peak growth during these times, increasing susceptibility to pest infestation. Therefore, most of the NNIs in our study maintained high concentrations during the spring and summer months. However, our findings suggested that there are also a number of NNIs that had higher levels in the winter months, similar to NIT (3.6 pg/m3) and DIN (76.11 pg/m3). We speculate that the possible reason for this difference is that the near-surface atmosphere is relatively stable in the fall and winter months, particulate matter floating in the air tends to accumulate, and it also facilitates the formation of aerosols of organic compounds and other pollutants or their full attachment to suspended particulate matter in the atmosphere(Achar et al. 2020). Therefore, higher concentrations were maintained in the collected PM2.5 samples. In addition, the variability in the distribution of concentrations of NNIs and their metabolites in different seasons may also be controlled by chemical processes and regional sources, which will be mentioned in the following paragraphs.
Table 2
Analysis of concentration disparities of various NNIs across the four seasons (pg/m3)
| Spring(n = 27) | Summer(n = 15) | Autumn(n = 33) | Winter(n = 26) | P |
CLO | 8.33 | 7.78 | 7.78 | 8.89 | 0.169 |
(7.22, 15.00) | (6.67,9.44) | (< LOD,9.44) | (< LOD,12.92) |
ACE | 3.33 | 3.89 | 1.67 | 4.17 | < 0.001 |
(2.22,4.10) | (2.78,4.44) | (1.39,2.98) | (3.24,6.8) |
NIT | 2.22 | 1.11 | 1.90 | 3.06 | 0.917 |
(1.11,4.98) | (1.11,4.19) | (0.56,5.00) | (0.42,7.08) |
DIN | 53.33 | 22.22 | 58.33 | 76.11 | < 0.05 |
(20.56,100.00) | (20.00,28.3) | (27.78,121.67) | (43.19,135.54) |
FLO | 3.33 | 2.78 | 2.78 | 3.33 | 0.844 |
(2.22,5.00) | (2.78,3.33) | (2.78,3.33) | (2.78,4.44) |
IMI | 82.78 | 129.44 | 16.11 | 51.67 | < 0.001 |
(32.78,171.11) | (34.44,182.22) | (10.28,35.56) | (26.25,127.5) |
THM | 0.56 | 0.56 | 0.56 | 2.78 | 0.034 |
(< LOD,2.22) | (< LOD,2.22) | (< LOD,2.50) | (0.56,7.78) |
5-OH-IMI | 13.89 | 9.44 | 11.11 | 8.33 | 0.029 |
(8.89,30.77) | (8.33,12.22) | (6.67,18.61) | (6.67,19.58) |
N-ACE | 5.98 | 5.00 | 5.00 | 5.74 | 0.062 |
(4.44,8.89) | (3.33,5.56) | (3.61,6.39) | (3.75,9.17) |
6-ClNA | 0.56 | 1.11 | 0.56 | 1.39 | 0.214 |
(< LOD,1.11) | (0.56,1.67) | (< LOD,1.11) | (< LOD,3.61) |
3.3. Correlation analysis between the NNIs detected in the PM2.5 samples
Spearman correlation analysis of 10 analytes detected in PM2.5 samples revealed positive correlations between CLO and IMI, 5-OH-IMI, and N-ACE (p < 0.05) but a negative correlation between CLO and THM (p < 0.05). ACE was positively associated with NIT, DIN, IMI, THM, N-ACE, and 6-CINA (p < 0.05). A positive correlation was observed between the NIT and DIN concentrations (p < 0.01). DIN was positively associated with 5-OH-IMI and N-ACE (p < 0.05). In addition, FLO was positively correlated with IMI, 5-OH-IMI and N-ACE (p < 0.05). IMI was positively correlated with N-ACE (p < 0.001), and 5-OH-IMI was positively correlated with N-ACE (p < 0.001) (Fig. 1). Most of the 10 analytes were positively correlated, suggesting that they might share a common source of environmental contamination, and the negative correlations between several targets suggest that they may be substitutes for each other in practical applications. The average concentration of CLO (8.0 pg/m3) was greater than that of THM (2.6 pg/m3), and there was a negative correlation between CLO and THM, with a correlation coefficient of 0.2 (p < 0.05); this may be because in addition to CLO itself, THM in the environment can be transformed into CLO(N et al. 2015).
3.4. The distribution characteristics of the compositions of the 10 NNIs in the PM2.5 samples in the different seasons.
The distribution characteristics of the compositions of the 10 NNIs in PM2.5 during the different seasons are shown in Fig. 2. The distributions of NNI components in the four seasons were similar. These products were mainly composed of IMI, DIN, 5-OH-IMI and CLO. Among them, IMI was the most dominant NNI in summer and spring, and the proportions reached 71% and 47%, respectively. DIN was the most dominant NNIs in autumn and winter, with 55% and 46%, respectively. 5-OH-IMI and CLO had comparable compositional distributions in the four seasons.
Residues of NNIs and their metabolites in the environment are affected by multiple factors, such as physical and chemical properties, external environmental conditions and regions. In terms of their physicochemical properties, the water solubility of different kinds of NNIs varies. NNIs have small molecular weights and high solubility and are generally between 185 and 590000 mg/L(Dores et al. 2008). In addition, NNIs have strong water solubility. All these factors make NNIs highly mobile in various environmental media. Moreover, because NNIs are polar insecticides, their volatility is low. Therefore, NNIs are generally found at low concentrations in the atmosphere but are more abundant in soil and water environments. The water solubility of NNIs is also related to temperature. For example, the water solubility of THM was 340 mg/L, while that of FLO reached as high as 39830 mg/L at 20°C(Yi et al. 2019). We speculate that when the water solubility of NNI decreases, it may more frequently reach environmental media other than water, such as the atmosphere. Thus, the concentrations and species distributions of NNIs in atmospheric particulate matter vary under different seasonal conditions and with changes in temperature. In terms of regional distribution, a study of drinking water in eight cities across the country showed that CLO, IMI, and THM are the predominant NNIs in South China, Central China, and Southwest China, while ACE, IMI, and THM are the major NNIs in East China and Northeast China. ACE and IMI are the most significant NNIs in Northwest China and North China(Mahai et al. 2021). Different provinces and regions grow different types of crops, which may result in different types and doses of NNIs being used. During the widespread application of NNIs, only approximately 5% of the active ingredient is absorbed by crops, 90% of the active ingredient enters the soil, and the remainder is dispersed into water and the atmosphere(Giorio et al. 2021; Zhang et al. 2018). Some NNIs in soil are enriched in soil partly by adsorption, partly by desorption into farm water, and migrate to rivers, lakes and wetlands through irrigation leakage. In contrast, NNIs in the atmosphere are produced mainly by particulate matter formed during the sowing of coated seeds and during pesticide spraying. Pesticide spraying, in turn, is related to the type of crop grown. According to the website of the China Pesticide Information Network, NNIs registered in China are mainly used for rice and wheat(2022), with relatively few used for other crops, such as corn, fruit trees, and vegetables. As a result, in provinces or regions where rice and wheat are predominantly grown, environmental media tend to have higher levels of NNIs.
3.5. Inhalation exposure in different populations
Moreover, NNI in PM2.5 might be exposed to populations via inhalation and increase health risks. Based on the target concentrations obtained at the monitoring sites, the median and geometric mean of the concentrations were used to calculate the EDI for the low-exposure scenario, and the 95th percentile was used to calculate the EDI for the high-exposure scenario. The EDIs for different populations exposed to NNIs are shown in Table S6. Figure 3 shows that ACE, CLO, IMI and DIN are the main targets for population exposure. The daily exposure dose per unit body weight to NNIs in the population was much greater in the high-exposure scenario than in the low-exposure scenario. The daily exposure dose per unit body weight for children in both exposure scenarios was 1.3 times that of adult residents and 2.8 times that of adult workers. IMI and DIN had high EDIs in both exposure scenarios.
Humans are exposed to NNIs mainly through diet, drinking water, soil/dust and respiratory intake. Diet and drinking water intake may be the main routes of human exposure to NNIs. However, respiratory intake is also a route that should not be underestimated. NNIs can be sprayed directly on crop surfaces or taken up by plants after application and distributed in tissues and organs, where pollen can be dispersed into the ambient air by windborne vectors or pollinators. A test of NNIs in pollen samples collected from 35 hives in North America revealed THM in 19 samples, ACE in 11 samples, and IMI in 10 samples(Mullin et al. 2010). NNIs are commonly found in pollen, and airborne pollen can be ingested by humans through respiration, and once inhaled, it may be adsorbed in the human respiratory tract and lungs due to its high water solubility. In Hernandez et al.’s study, respiratory functions were measured and compared between 89 pesticide sprayers and 25 non-spraying control farmers in southeastern Spain. The results suggested a relationship between neonicotinoid application and lung dysfunction (lower total lung capacity, residual volume and functional residual capacity)(Hernández et al. 2008). Therefore, research on the risks to human health from respiratory exposure should be strengthened. Most of the previous studies have focused on the effects of the NNI on the respiratory health of occupational populations during agricultural activities(Hernández et al. 2008). However, in our study, infants and young children, a sensitive population, were more susceptible to health effects due to NNI exposure.
3.6. Limitations
At present, most domestic and international studies on NNIs have focused on other environmental media, such as water, soil, sediment and living organisms, and there are few reports on NNIs in atmospheric particulate matter. This study has several limitations. The study selected samples from only the outdoor atmosphere of the Wuhan urban area, and the sample size was limited, which cannot reflect the atmospheric NNI pollution in the whole city. In addition, there are many types of NNIs, and only 10 common NNIs and their metabolites were selected for the study. The concentrations of other NNIs are not yet known. More importantly, the study focused only on the outdoors, while NNIs were also detected at high concentrations in indoor dust samples(Wang et al. 2019). Finally, a comprehensive assessment of the human health effects of neonicotinoid pesticides requires the simultaneous evaluation of multiple environmental media, especially in drinking water.