Exposure Level of Neonicotinoid Insecticides in the Food Chain and the Evaluation of Their Human Health Impact and Environmental Risk: An Overview

: Neonicotinoid insecticides (neonics) were the most rapidly growing class of insecticides over the past few decades, and are used mainly for vegetables, fruits, and grains. Although neonics exhibit lower toxicity in mammals and humans compared to traditional insecticides, increasing numbers of studies are demonstrating that neonics may accumulate in the food chain and environmental media. Long-term exposure to neonics may raise potential risks to animals and even to humans. The present report reviews the development, application, and prohibition of neonics in the farmland ecosystem, and summarizes the exposure level and harmful e ﬀ ects of these insecticides in the food chain. In addition, the present review analyzes and summarizes the evaluation of the human health impact and environmental risk of the neonics, and overviews the unresolved problems and future research directions in this ﬁeld. The aim of the present report was to review the exposure level, potential toxicity, human health impact, and environmental risk assessment of neonics in various media in order to provide reliable technical support for strengthening the environmental and food safety supervision and green pesticide designing.


Introduction
Neonicotinoid insecticides (neonics) are a novel class of insecticides that act selectively on the nicotinic acetylcholine receptors (nAChRs) in the central nervous system of insects [1]. Since the introduction of the first neonic, named imidacloprid, by the Bayer Corporation in the 1980s, seven major neonics have reached markets worldwide: imidacloprid, thiacloprid, clothianidin, thiamethoxam, acetamiprid, nitenpyram, and dinotefuran [2]. Representing the fourth kind of insecticide, neonics are gradually replacing several conventional insecticides currently in the market, such as the chlorinated hydrocarbons, organophosphorus insecticides, carbamates, and synthetic pyrethroids; owing to their advantages, such as low dosage requirements, rapid effects, and high activity, the neonics have become widely used insecticides throughout the world [3].
Neonics are commonly used for foliage spraying in field systems and for soil and seed treatment during crop planting to protect seedlings from early-season root and leaf pests [4]. Neonics have been registered in 120 countries and are used for over 140 crops, such as rice, wheat, corn, cotton, peanut, and soybean. In addition, neonics exert good control of sucking pests in the atmosphere, e.g., aphids and However, with the widespread application of neonics worldwide, these are often detected in foods, wildflowers, and certain crops without even applying pesticides. Studies have demonstrated that neonics may have strong diffusivity and may, therefore, have spread throughout the ecosystem, threatening the safety of several non-target organisms, such as bees [32]. In consideration of the negative impacts of neonics on non-target organisms (particularly on bees) and human health, at the end of April 2018, the EU voted to ban the use of neonics. The ban, which came into force at the end of 2018, included a complete ban on imidacloprid, clothianidin, and thiamethoxam [33]. In September 2018, France became the first country among the EU nations to ban neonics for the protection of bee populations; according to national regulations, five neonics (clothianidin, imidacloprid, thiamethoxam, thiacloprid, and acetamiprid) were banned from sale and use [34]. Subsequently, the However, with the widespread application of neonics worldwide, these are often detected in foods, wildflowers, and certain crops without even applying pesticides. Studies have demonstrated that neonics may have strong diffusivity and may, therefore, have spread throughout the ecosystem, threatening the safety of several non-target organisms, such as bees [32]. In consideration of the negative impacts of neonics on non-target organisms (particularly on bees) and human health, at the end of April 2018, the EU voted to ban the use of neonics. The ban, which came into force at the end of 2018, included a complete ban on imidacloprid, clothianidin, and thiamethoxam [33]. In September 2018, France became the first country among the EU nations to ban neonics for the protection of bee populations; according to national regulations, five neonics (clothianidin, imidacloprid, thiamethoxam, thiacloprid, and acetamiprid) were banned from sale and use [34]. Subsequently, the pesticide regulatory agencies in the United States, Canada, and other countries expressed their concern regarding Sustainability 2020, 12, 7523 5 of 26 the impact of the existing neonics on pollinators, and stated that a comprehensive risk assessment of these insecticides would continue to be conducted [35].
In view of the above-stated problems, our research group has designed a few green substitutes for neonics in previous studies with the objective of screening out the neonics with multi-directional selective toxicity effects, i.e., to ensure the insecticidal effect while also maintaining low toxicity to non-target organisms, crops, and humans [1,[36][37][38]. However, the current studies have certain limitations, such as that these studies designed neonics only from the perspective of source treatment, and did not provide a way to effectively block the adverse conduction effect of the registered neonics in the food chain or for the treatment of the contaminated soil and other media. It is expected that future studies would deepen the understanding of the transmission of neonics in the food chain, and would fundamentally achieve reasonable prediction and control of the source, process, and end-behavior of these insecticides as well as effective remediation of the contaminated soil.

Analysis of the Similarities and Differences in Toxicity Mechanisms among Traditional Neonics, Novel Neonics, and Other Insecticides
Since the 1980s, three generations of traditional neonics and three novel neonics have been developed. Comparative studies have revealed that the action mechanism of each generation of neonics is similar-serving as the activator of nAChRs, thereby affecting the central nervous system of insects, causing their death [20]. In addition, no cross-resistance was reported between any of the generations of neonics, although the insecticidal effects and the target pests were different. The first-generation neonics are used mainly to control sucking pests, such as aphids and leaf-hoppers, and exhibit high efficiency and low toxicity. The second-generation neonics are superior to the first generation in terms of activity, safety, and insecticidal spectrum. Owing to their unique structure, the third-generation neonics exhibit excellent performance in terms of high efficiency and harmlessness to crops. In addition, they exhibit a higher insecticidal effect on sanitary pests such as houseflies, cockroaches, and termites [39]. On the other hand, the novel neonics, in addition to the above-stated advantages, exhibit high solubility in water, the reason for their widespread use, as well as high efficacy and functionality for arthropod pests. Therefore, the novel neonics were commercialized and are now gradually replacing the traditional neonics [23].
Neonics are the fourth major group of synthetic insecticides after the organophosphorus insecticides, carbamates, and synthetic pyrethroids. In comparison to traditional organic insecticides, neonics exhibit better environmental safety, lower toxicity to mammals, milder odor, non-irritant quality, lower vapor pressure of the effective components, lower air pollution index, lower toxicity, and lesser impact on aquatic organisms [40]. Besides, owing to their unique mechanism of action, neonics are capable of demonstrating better efficacy in crop treatment compared to conventional highly toxic insecticides; in addition, there is no cross-resistance, which allows effective control of pests that are resistant to the previous generations of insecticides [41]. Although neonics have certain advantages over other insecticides, their widespread use in recent years has made neonics, particularly the ones with broad-spectrum and high residues, a potential factor endangering the ecological security [42].

Exposure Level and Potential Hazards of Neonics in Producers
Owing to the good inhaling ability of neonics, approximately 2-20% may be absorbed by plants and may accumulate in the roots, stems, leaves, flowers, and fruits of the plants [42,43]. It was reported that neonics have been distributed and accumulated widely in the global environment [44], contaminating various environmental substrates, such as drinking water, vegetables, fruits, and even milk [27], and exerting toxic effects on humans and other species [45,46].
In 2012, Hu et al. reported imidacloprid and acetamiprid as the neonics with high detection rates in tea, with concentrations ranging from 10 to 212 µg/kg and 10 to 808 µg/kg, respectively [47], Sustainability 2020, 12, 7523 6 of 26 which was consistent with the neonic detection in tea reported by Liu et al. in 2010 [48], although the enrichment degree in the former study was higher than that in the latter (imidacloprid 2.8-6.3 µg/kg and acetamiprid 0.2-10 µg/kg). These results confirmed the above-stated view that with the increasing use of neonics, their residues in various products are also increasing [16]. In 2014, Chen et al. analyzed the common fruits, vegetables, and honey in the market, and demonstrated that over 90% of the samples had at least one neonic, while over half of the samples contained two or more novel nicotine insecticides [49]. In 2016, Tan et al. reported detecting seven neonics in 49 types of vegetables and 24 types of fruits collected from the Beijing market, with concentrations ranging from 0.01 to 126 ng/g, and with imidacloprid, acetamiprid, and thiamethoxam being the ones with the highest detection rates [50]. In 2017, Wood and Goulson reported that the concentrations of imidacloprid in two rape nectar samples were 52 and 70 ng/mL, respectively, which was more than five times the expected maximum concentration in imidacloprid-treated rape nectar [51]. In 2020, Wintermantel et al. investigated the crops grown in those regions of western France that had banned neonics for five years, and reported detecting three restricted neonics in rape nectar, while imidacloprid was detected in all study years. In particular, in 2014 and 2016, the detection rate of imidacloprid in nectar samples was greater than 60%, with certain samples exhibiting concentrations as high as 70 ng/mL [52]. Furthermore, studies have revealed that wild flowers close to fields were polluted with neonics [52,53], with their concentration distribution exceeding even that of the crops treated with neonics [54].
According to the above-stated survey results, although certain regions, such as China, the EU, and France, have explicitly banned the use of neonics such as imidacloprid and thiamethoxam [55][56][57], the persistence of neonics in the environment has not allowed complete elimination of the adverse effects of neonics in the ecosystem even after the implementation of the ban [32,58,59]. Neonics may be detected in various products even after several years of cessation of application [60,61]. Therefore, the studies of neonics also have some limitations and disadvantages. It is necessary to explore the transmission process of neonics in the food chain, the area on which future research should focus in order to further understand the mechanism of migration and transformation and to block the adverse transmission.

Exposure Level and Potential Hazards of Neonics in Bees
Although neonics are capable of effectively controlling pests in farmland systems, the application of these insecticides may cause harm to beneficial arthropods, including bees [62]. According to a global report on neonic content in honey published by Science, at least one neonic was detected in 75% of the honey samples, and a combination of at least two neonics was detected in 45% of the honey samples [63]. Bees, as the most important pollinators, play an important role in the pollination of crops, urban horticulture, and protection of endangered plants [64]. When bees forage in a flowering zone, buffer zone, cover crops, or catch crops, they may come into contact with pesticides floating from the treated land and may directly eat the polluted nectar and pollen, which ultimately affects the foraging, learning, and memory ability of the worker bees [54,65]. In addition, neonics may increase mortality in bees through the reduction of their homing ability and the reproduction success rate of bumblebees and solitary bees [66]. Therefore, it is crucial to control the enrichment of neonics in non-target organisms such as bees and to prevent the "accidental injury" they cause to the non-target organisms and crops.
In 2005, Decourtye et al. utilized the proboscis extension response (PER) method to train bees to become capable of associating odor stimulation with sucrose, and observed that after consuming LD 50/20 (48 h) sugar water of dinotefuran, there was a decrease in the western bees' ability of tongue extension response, as well as in their learning ability [67]. In 2008, Hassani et al. studied the effects of sub-lethal doses of neonics on bee behavior under laboratory conditions. The results demonstrated that when the bees were fed with 1 µg acetamiprid, each bee's sensitivity to the sucrose solution increased. If there was greater than 0.1 µg acetamiprid in the body of the bee for a long time, its olfactory memory would be damaged [68]. In 2012, Henry et al. developed a colony simulation model to monitor the bees' collection behavior for one month and observed that the normal collection time of western bees was 300 s, while after the ingestion of sub-lethal doses of imidacloprid and thiamethoxam, there was a loss of localization ability, the homing time was increased, and the number of round trips per day was also reduced [69]. In the same year, Yang et al. processed bees in the hive, recorded their pupa, pupate, emergence rate, and other parameters, and observed that when the western bee larvae ingested 24-8000 ng of imidacloprid, the number covering the larva would be reduced. After the ingestion of 0.4 ng of imidacloprid, the emergence rate of bees remained unaltered, although damage to their olfactory organs in adulthood was caused [70]. In 2014, Tan et al. experimentally observed that the number of bees collected by Chinese bees decreased by 23%, and the collection quality decreased by 46% after the ingestion of sugar water containing 34 ng/µL of imidacloprid [71]. In 2015, Rundlöf et al. demonstrated through field experiments that the content of neonics in rape plants caused serious damage to reproduction in bumblebees and monophones [72]. In 2017, McArt et al. reported that neonics could induce chronic sub-lethal effects in various plant pollinators (including bees) [73], such as a reduction in the adaptability of single bees [74], "colony collapse disorder", damage to bees' pollination service, foraging ability [75], and growth rate [76], damage to bees' health [54], and gene damage [77]. In 2020, Wintermantel et al. reported the evaluation of the risk of neonics in eight fields, stating that the mortality rate in pollen pickers due to acute toxicity was 50% [32].
It may be inferred from the above-stated research results that although the adverse effects of neonics on non-target organisms, such as bees, were recognized long ago, these problems have not been effectively resolved to date. Moreover, several scholars have reported that neonics, in addition to their highly toxic effect on bees, exert negative effects on mice at the same nutritional level as bees [78].

Exposure Level and Potential Hazards of Neonics in Rats
Several studies have reported that neonics, such as thiacloprid and acetamiprid, are able to reduce embryo quality in mice and also exert negative effects on the reproductive health, metabolism, and genes in rats [78].
In regard to reproductive toxicity, a study conducted in 2011 on female rats observed significant pathomorphological changes in the follicles, antral follicles, and atretic follicles along with ovarian weight decrease in the group treated with imidacloprid at a dietary concentration of 20 mg/kg/day. However, no significant changes were observed in the groups receiving dietary concentrations of 5 and 10 mg/kg/day [79]. In 2012, Bal et al. treated developing and adult male rats with low doses (0.5, 2, and 8 mg/kg/day) of imidacloprid via oral gavage for three months. The results revealed that the weights of the epididymis, the sperm parameters, and testosterone levels were lower in all the imidacloprid-treated developing rats, while apoptosis and fragmentation of seminal DNA were observed to occur mainly in the two higher-dose groups. In addition, the authors observed a dose-dependent relationship between the seminiferous tubule apoptosis and imidacloprid consumption [80][81][82][83]. Although the two above-stated studies had different research perspectives, the results of both of these studies presented similar findings, suggesting that even though low doses of imidacloprid did not exert a significant adverse effect on the pathological morphology of the follicles, in general, such insecticides nonetheless damaged the reproductive tissues in rats. In 2013, similar methods were utilized by Bal, who treated developing and adult rats with clothianidin (90 days of oral exposure) at the doses of 2, 8, and 32 mg/kg/day. The results demonstrated that clothianidin exerted detrimental effects on the reproductive organ systems, and that further severe effects are likely to be observed at higher dose levels in developing rats [81]. In 2015, Hirano et al. once again demonstrated that the effect of clothianidin on the reproductive organs of rats was more severe under environmental pressure [84]. In 2018, Han et al. conducted a study on female rats exposed to neonics, and reported that in the experimental group treated with imidacloprid at a dietary concentration of 20 mg/kg/day, there were visible pathomorphological changes in the follicles, anal follicles, and atretic follicles, and the ovarian weight was reduced, which was in complete agreement with the findings reported by Kapoor [27]. In hepatotoxicity, toxin metabolism and elimination occur in the liver. The liver is also a principal target organ for damage caused by neonics. Currently, the researchers are focused mainly on the hepatotoxicity and hepatocarcinogenesis of imidacloprid, thiamethoxam, and their metabolites in rats. In 2010, Bhardwaj et al. reported detecting imidacloprid in female rats with three oral concentrations (5, 10, and 20 mg/kg/day) during a longer exposure time (90 days), and also that the mild pathological changes in the liver were observed only in the high-dose imidacloprid treatment (20 mg/kg/day) group [85]. Toor et al. evaluated the hepatotoxicity of neonics by measuring liver histological changes and enzyme activity in female albino rats exposed to imidacloprid at concentrations of 45 and 9 mg/kg/day for four weeks. In the higher-dose group, the observed histological changes included degeneration of hepatocytes, dilations of the central vein, and an increase in levels of liver enzymes. However, no significant changes were observed in the rats treated with low doses of imidacloprid [86]. In 2014, Vohra et al. treated female albino rats with imidacloprid at the doses of 10 and 20 mg/kg/day for 60 days, and observed that the liver histopathology in the high-dose group was altered, exhibiting dilation and congestion of the central vein and degeneration of hepatocytes, while the transaminases were not greatly elevated [87]. However, in the same year, Kapoor et al. reported that the level of serum transaminases in adult female rats was increased at the dose of 20 mg/kg/day [88]. Furthermore, Arfat et al. conducted an in vivo study in mice. Upon injecting imidacloprid at the concentrations of 5, 10, and 15 mg/kg/day into adult male albino mice for 15 days, it was observed that the levels of serum glutamate oxaloacetic transaminase, glutamic pyruvate kinase, alkaline phosphatase, and total bilirubin, as well as damage to liver tissue, had increased at the dose of 15 mg/kg/day [89].
In regard to genotoxicity, several studies assessing the genotoxicity of neonics were conducted by measuring chromosome aberrations (CAs) and sister chromatid exchanges (SCEs), and by performing the comet assay and the micronucleus test (MN). For instance, in 2012, Calderon et al. compared the genotoxic effects of thiacloprid, clothianidin, and imidacloprid on human peripheral blood lymphocytes using alkaline comet and trypan blue dye exclusion assays, the results of which revealed that imidacloprid exhibited the strongest genotoxicity [90]. In 2014, Kocaman et al. confirmed the genotoxicity of thiacloprid in human peripheral blood lymphocytes, including decreased nuclear division index, proliferation index, and mitotic index at all concentrations (75,150, and 300 mg/L), using SCE, CAs, and MN tests [91]. In 2015, Galdikova et al. treated bovine peripheral lymphocytes with thiacloprid at the concentrations of 30, 60, 120, 240, and 480 mg/mL, and reported that the frequency of DNA damage and unstable chromosome aberrations significantly increased at the concentrations ranging from 120 to 480 mg/mL [92]. In 2016, Kataria et al. treated female mice with imidacloprid at the doses of 37.5, 75.0, and 112.5 mg/kg for 24 h, and observed that female mice in the middle-and high-dose groups exhibited significant mitotic inhibition, while micronucleus formation was detected only in the high-dose group ( Figure 2) [93].
A comparison of the degree of damage caused to different organs of rats by different doses of neonics reveals that both high and low doses of neonics could cause damage to the reproductive system of rats. On the other hand, in the genetic system of rats, high doses of neonics would cause more obvious damage, while the low doses of neonics cause lesser damage. In the liver system, only high doses of neonics would cause damage. Therefore, it is speculated that the reproductive system of rats is the most sensitive to neonics, followed by the genetic system and the liver system, respectively.
lymphocytes with thiacloprid at the concentrations of 30, 60, 120, 240, and 480 mg/mL, and reported that the frequency of DNA damage and unstable chromosome aberrations significantly increased at the concentrations ranging from 120 to 480 mg/mL [92]. In 2016, Kataria et al. treated female mice with imidacloprid at the doses of 37.5, 75.0, and 112.5 mg/kg for 24 h, and observed that female mice in the middle-and high-dose groups exhibited significant mitotic inhibition, while micronucleus formation was detected only in the high-dose group (Figure 2) [93].

Exposure Level and Potential Hazards of Neonics in Secondary Consumers
In addition to toxic effects on lower animals, neonics possess characteristics of difficult volatilization, high water solubility, and long degradation period in the soil, which enable contact with reptiles through a variety of ways, such as inhalation, food intake, and skin contact [94]. Therefore, neonics are a great threat to the reptiles living in the soil [30]. In 2010, the International Union for Conservation of Nature (IUCN) classified 28% of the reptiles as critically endangered (CR), endangered (EN), or vulnerable (VU) species [95]. As the largest family of reptiles, lizards account for 70% of the reptile species, and are highly sensitive to pollutants. Therefore, application of neonics is considered one of the important reasons for the decline in the numbers of lizards [96].
In 2013, Bicho et al. conducted field studies and observed that the thyroid system of lizards living in agricultural areas could be disrupted, which ultimately affected their reproductive function [97]. In 2014, Park et al. developed nine novel Argus microsatellites in the Korean Peninsula using biotin enrichment technology in order to understand the population's genetic structure and define the protection unit of Mongolian racerunner (a small lizard). Using Bayesian structure analysis, the authors observed that the habitat of lizards highly overlapped with the pesticide application area, and their population size was severely decreasing [98]. In 2017, Zhu et al. reported that long-term exposure to endocrine disruptors may affect the growth and development of animals. Lizards grow and develop with the process of molting and regeneration, and are exposed to endocrine disruptors for a long duration. Therefore, growth inhibition may affect the molting process of lizards, leading to a decline in the motor function and an increase in the infection rate, thereby affecting individual survival [99]. In 2020, Wang et al. evaluated the damage caused to the thyroid system of lizards upon exposure to furandione, thiamethoxam, and imidacloprid for 28 days, and confirmed that continuous exposure to neonics directly affected the thyroid system of lizards. In particular, exposure to furandione affected the intake and utilization of thyroid iodine in lizards, resulting in thyroid insufficiency, and subsequently in thyroid epithelial hyperplasia and increase in follicular volume. Thiamethoxam could activate thyroid function in lizards, significantly increase plasma T3 and T4 concentrations, and promote the binding of T3 and thyroid hormone receptor, while imidacloprid could inhibit the secretion of thyroid hormone in lizards, resulting in the down-regulation of thyroid hormone receptor and the related phase II metabolizing enzyme genes [35].
The above-stated research results confirm that the neonics have adverse effects on the thyroid and endocrine systems of the reptiles. When lizards and other reptiles ingest or absorb these insecticides, they affect the balance of normal hormone function, ultimately affecting the growth and development of these animals. This is a limitation and disadvantage of neonics. Therefore, in future research related to this field, in addition to the toxic effect exerted by neonics on bees and other beneficial insects, their effect on hormones should be included in the research scope, with the objective of minimizing the harm caused by the neonics to non-target animals.

Exposure Level and Potential Hazards of Neonics in Top-Level Consumers
Certain scholars believe that, in addition to the impact on non-target insects and reptiles, neonics affect other non-target invertebrate taxa, as well as the wild animals that rely on these invertebrate taxa for food [100][101][102]. The health and sustainability of aquatic invertebrate communities depend, to a large extent, on the abiotic and biological characteristics of the aquatic system, particularly in wetlands [103].
To date, only a few studies have evaluated the relationship between the response of aquatic communities and exposure to neonics [104]. For instance, in 2016, Anderson and Sánchez-Bayo evaluated the potential threat of neonics to aquatic invertebrates in laboratory and meso research, and observed that acute, chronic, and sub-lethal toxicity in various taxa were widespread [105,106]. In 2018, Evelsizer and Skopec constructed a model that could simultaneously monitor the community of aquatic invertebrates, the concentration of neonics, and other relevant environmental attributes. The authors reported that the attenuation of the community correlated positively with the concentration of neonics [107]. The above-stated studies determined the impact of neonics on aquatic invertebrates in the laboratory and meso research only, and did not evaluate the relationship between the neonics and the invertebrate community under field conditions [108]. In 2020, Schepker et al. evaluated the levels of six neonics, nutrients, and physical properties of water in the samples of aquatic invertebrates and surface water collected from 26 wetlands in Nebraska. It was observed that although the concentration of neonics was lower than the benchmark concentration recommended as per government regulations, there was a significant negative correlation between the concentration of neonics and the biomass of the aquatic invertebrates [109].
The above-stated research results strongly suggest damage caused by neonics to fruits, vegetables, crops, and certain animals. It may be inferred that most of the organisms at each trophic level of the food chain are likely to be affected by these insecticides, which would lead to the instability of the food chain and even of the whole ecosystem. Although these results have established a good foundation for future research on neonics, the research and development of green pesticides, the blocking of pollutant transmission, and the remediation of contaminated soil continue to be in the initial stages, which have some limitations.

Assessment of the Human Health Impact and Environmental Risk of Neonics
In recent years, neonics have been continuously detected in the environment. Neonics enter the food chain through drinking water and food intake, causing great safety risks to human health [110]. In the present report, exposure of the human body and multi-environment media (water, soil, and atmosphere) to neonics was reviewed, and their impact on the human health and the environmental risks of these insecticides were comprehensively evaluated.

Distribution Characteristics of Neonics in the Human Body
In comparison to traditional insecticides, neonics exhibit lower toxicity to mammals and humans. However, an increasing number of studies demonstrate that exposure to neonics causes a potential risk to mammals, including humans. In recent years, certain scientists have begun exploring different methods for the determination of neonics and their metabolites in human urine, serum, hair, and other biological samples.
In 2014, Ueyama et al. successfully detected seven neonics in 52 Japanese urine samples without a history of occupational spraying, among which thiamethoxam, dinotefuran, imidacloprid, and clothianidin exhibited the detection rates of 100%, 100%, 96%, and 96%, respectively, while the detection levels of furandione and imidacloprid were 2.27 and 1.54 mg/L, respectively [111]. The next year, Ueyama et al. also reported detecting seven neonics in urine samples of 95 adult Japanese women aged 45-75 in the period between 1994 and 2011. The authors observed that during this period, the detection rate and the total concentration of neonics in the urine of Japanese women had increased significantly. For instance, imidacloprid was detected in urine only in the year 1994, while the detection results for 2011 revealed that the detection rate of thiamethoxam and dinotefuran were greater than 70%. At the same time, the geometric mean concentration of neonics in the urine maintained a steady increase, which indicated that the number of neonics consumed by the human body had increased during this period [112]. In 2016, Osaka et al. studied the exposure characteristics of seven neonics in urine of 223 three-year-old children (118 boys and 105 girls) in Japan, and observed that the average and maximum concentrations of these neonics were 4.7 and 370.2 nmol/g creatinine, respectively, while the concentrations of certain neonics in urine of the children were generally higher in summer than in winter [113]. In 2019, Tao et al., using imidacloprid as an example, studied and compared the detection levels of neonics in urine of rural and urban residents, and observed that the detection rate of neonics in urine of both rural and urban residents was 100%, with the detection concentration of imidacloprid in urine of urban residents determined to be 0.91-2.68 ng/mL; the imidacloprid detection concentration in rural residents was 0.02-8.91 ng/mL, which was approximately 1.47 times the content of imidacloprid in urine of the urban residents [114].
Moreover, several studies have reported the detection of a variety of neonics in human samples other than human urine. For instance, in 2013, Kavvalakis et al. compared the exposure levels of imidacloprid in the hair of urban and rural populations, and observed that the detection rate of imidacloprid in the hair of the rural residents engaged in agricultural processing was higher than that in the hair of the urban residents. The median concentration and the highest concentration were 0.03 and 27 ng/mg, respectively [115]. In 2014, Tadashi et al. were the first ones to propose and establish an analytical method for the simultaneous determination of eight neonics and three acetamiprid-specific metabolites; the detection limit and the quantitative limit of these 11 compounds in serum were measured using liquid chromatography-tandem mass spectrometry. The results demonstrated that the detection limit and the quantitative limit of these 11 compounds in human serum were 0.1-0.2 and 0.5-10 ng/mL, respectively, while the recovery was 80.9-101.8% [116].
The above-stated research results suggest that the levels of neonics are generally higher in the human body samples of rural residents compared to urban residents, which is mainly due to the extensive use of neonics in agricultural production, combined with a long period of validity, slow degradation, and widespread presence of neonics in the agricultural environment. As a consequence, the populations residing close to agricultural areas are generally more exposed to pesticides compared to those residing far away from agricultural areas [117]. More importantly, increased attention should be paid to the health risks caused due to pesticide exposure [118].

Assessment of the Impact of Neonics on Human Health
In the last 20 years, the number of people exposed to neonics in daily life has been increasing steadily, and long-term exposure to neonics is expected to have an adverse effect on the human body [119]. Therefore, there should be awareness regarding the increased human exposure to neonics and the associated potential health impacts.
Recent studies have demonstrated that exposure to neonics, such as imidacloprid, exerts adverse effects on human health. In 2014, Wei et al. investigated whether exposure to neonics during early pregnancy would cause anencephaly, spina bifida, and cleft lip and palate in pregnant women.
The subjects belonged to San Joaquin Valley, California, and included 73 anencephalic children, 123 children with spina bifida, and 394 those with cleft lip and palate. The results of the correlation analysis conducted in that study revealed that the use of agricultural pesticides such as imidacloprid close to the residences of pregnant women during pregnancy was positively correlated with the probability of occurrence of fetal teratogenic mutation [120]. In the same year, Keil et al. used the Bayesian logic model to evaluate the relationship between autism spectrum disorders and the use of imidacloprid. The analysis dataset contained complete information regarding 262 normal development controls and 407 autistic children. Sensitivity window analysis conducted in that study revealed that, in comparison to normal development controls, the autistic children exhibited higher levels of imidacloprid exposure during pregnancy. Therefore, a certain correlation existed between imidacloprid exposure and autism spectrum disorders [121]. Furthermore, Koureas et al. investigated the oxidative damage to DNA in different populations in Greece, and studied its correlation with pesticides and other potential risk factors; the study included 80 insecticide sprayers, 85 rural residents, and 121 residents of the city of La SAR. It was observed that the frequency of neonics was associated with the 80 pesticide sprayers in terms of the oxidative damage to whole blood DNA [122]. In 2015, Marfo et al. studied 35 symptomatic patients with unknown causes and 50 asymptomatic patients, and proposed that a correlation existed between the concentration of neonics in the urine and the increased prevalence of neurological symptoms, including amnesia, finger tremor, headache, etc. [123]. In 2017, Seltenrich et al. indicated that long-term exposure to neonics would result in adverse development or neurological outcomes, including tetralogy of Fallot, congenital anencephaly, autism spectrum disorder, memory loss, and finger tremor [124]. In 2018, Mesnage et al. reported that neonics could cause adverse reproductive, developmental, and physiological effects, including decreased sperm production and function, reduced pregnancy rate, increased fetal mortality, still-birth rate, and preterm birth rate, as well as weight loss and lipid accumulation in the offspring [125].
Although it has been recognized that exposure of neonics exerts adverse effects on human health, there nonetheless exist certain limitations in the current research related to this field, and it is difficult to provide a comprehensive speculative conclusion. Therefore, for the neonics already developed and registered, further investigation is required to predict their impact on human health theoretically. In addition, future research and development related to neonics should focus on reducing their accumulation and transmission in agricultural products in order to reduce their exposure to humans via food intake.

Distribution Characteristics of Neonics in Environmental Media
Just~5% of the active components of neonics are capable of being absorbed by crops, and most of these components are diffused into a wider range of environmental media and may even enter the water environment through migration and transformation [27]. Moreover, the low molecular weight and high water solubility of the neonics increase the problem of environmental pollution. Neonics exposed to the environment mainly originate from human activities, such as agricultural planting, and the neonics entering the environment complete the redistribution among the environmental media via complex migration and transformation paths.

Distribution Characteristics of Neonics in Water
Owing to the high solubility of neonics, they may conveniently pollute the water through surface runoff, drainage, and dry and wet deposition, thereby causing harm to aquatic organisms. In 2013, Morrissey et al. comprehensively summarized the pollution levels of neonics in global surface water during the period between 1998 and 2013. It was estimated that the geometric average concentration of neonics in surface water was 0.13 µg/L, while the peak geometric average value was 0.63 µg/L (n = 27) [44]. Since then, several studies have focused on investigating pollution levels of neonics in the broader aquatic environment, including wetlands, farmland, rivers, and sediments. For instance, in 2014, the concentration of both thiamethoxam and clothianidin detected in the grassland wetland of Saskatchewan (central and eastern Canada) was 17.5 ng/g dw [126]. In 2015, Schepker et al. collected surface water samples from 26 wetlands (12 wetland buffer zones and 14 wetland non-buffer zones) in Nebraska, and evaluated the samples for six neonics. The results revealed that more than 73% of the water samples in the non-buffer zones of the wetland exhibited the presence of neonics, among which imidacloprid accounted for 15%. Although the content of the neonics in the water samples from the wetland buffer zones was significantly lower, the neonics were nonetheless prevalent [109]. In 2016, Sadaria et al. detected neonics in a sewage treatment plant, with a detection concentration of 0.002-0.11 µg/L, and observed that imidacloprid, acetamiprid, and clothianidin would exist in the sewage treatment plant for a long time and were difficult to remove, which might be due to the difficulty of the traditional sewage treatment methods in removing such pollutants, resulting in the pollution of surface water, drinking water sources, and underground water once again. In addition, the study reported detecting high concentrations of imidacloprid and acetamiprid in sediment, indicating that neonics in sediment may not be conveniently degraded and/or removed [127]. In 2019, Yi et al. reported detecting neonics in 14 sampling points of surface water and sediment in the Guangzhou section of Pearl River. The reported detection concentration of neonics in surface water samples was 92.6-321 ng/L, with imidacloprid, clothianidin, and acetamiprid, the most obvious ones, exhibiting the detection rate of 100% [30]. A comparative study detected an average neonic concentration of 12 ng/L in the rivers around Osaka in Japan [128] and 118 ng/L around Sydney in Australia [44]. It may be observed that compared to other regions, the Guangzhou section of the Pearl River exhibited relatively high levels of neonics, even higher than those detected in the agricultural area of a grassland wetland in Canada [129]. The main causes for the above-stated phenomena could be related to the runoff from the agricultural regions and grasslands containing neonics and the hydraulic transmission of effluents from sewage treatment plants [30]. In addition to surface water, the sediments of nine sampling points in the Guangzhou section of the Pearl River presented neonics content in the range of 0.40-2.59 ng/g dw. However, imidacloprid, which presented a high detection rate in the surface water, was not detected in any of the sediment samples, which might be related to its physical and chemical properties or its dissolution and dilution effects caused due to the frequent water exchange among tidal river networks [130].
In addition to the above-stated studies, Table 1 provides a detection summary of neonics in the water of certain other regions since 2013. It may be observed through a comparative analysis that the concentration of neonics has not decreased in recent years, and in certain countries (such as The Netherlands), the concentration of neonics in water has rather increased. Moreover, the concentrations of neonics in the samples from different sources were considerably different, which might be related to pesticide dosages, human activities, and environmental conditions. For instance, certain farmlands in Canada and wetlands in Texas in the United States presented high detection concentrations of neonics (the latter region presented a concentration as high as 225 µg/L), while the other surface water samples presented a relatively low detection content of neonics.

Distribution Characteristics of Neonics in Soil
In addition to causing water pollution, neonics also pollute the soil. One way that the neonics enter the soil is through the planted seeds that contain the neonics. The other way is when the neonics are applied to the plants in the farmland through absorption or stem injection, and these plants containing the pesticide residues decompose and release these residues into the soil [146]. Therefore, for environmental risk assessment, it is necessary to understand the pollution status of neonics in the soil.
In 2014, Dankyi utilized liquid chromatography tandem mass spectrometry (LC-MS/MS) and electrospray ionization (ESI) techniques to assess the neonic content in 52 soil samples obtained from cocoa cropland, and reported that imidacloprid was detected in approximately 50% of the samples, with its concentration ranging from 4.3 to 251.4 µg/kg; in addition, clothianidin was detected in approximately 10% of the samples, with concentrations in the range of 9.8-23 µg/kg [147]. In 2015, Botías et al. analyzed soil planted with rape and wheat in the UK, and observed that imidacloprid, clothianidin, thiamethoxam, and thiacloprid were the main neonics distributed in the soil, with contents in the ranges 7-7.90, 4.10-28.60, 4-9.75, and 1-0.22 ng/g, respectively [52]. In the same year, in the United States, De et al. continuously planted soil samples with corn and soybean throughout the year, and finally reported that the main neonic in the soil was clothianidin, with a content of 2.00-12.20 ng/g [134]. In 2016, in Canada, Limayrios et al. detected clothianidin and thiamethoxam as the main neonics in the soil planted with corn, with contents of 3.45 and 0.91 ng/g, respectively [148]. In the same year, Xu et al. successively planted soil samples with rape and corn, and reported that the contents of clothianidin in these soil samples were 5.70 and 7.00 ng/g, respectively [149]. Furthermore, Abdel-Ghany et al. developed a novel high-performance liquid chromatography coupled to tandem mass spectrometry (HPLC-MS/MS) method for the simultaneous determination of eight neonics and their two major metabolites in the soil of Qaluybiya, Egypt. According to the results, all eight neonics, as well as their two main metabolites, were detected in the soil samples of this area. The range of the detection concentration was 12.5-50 ng/mL for acetamiprid, 2.5-100 ng/mL for imidacloprid, 12.5-90 ng/mL for dinotefuran, 6.25-100 ng/mL for thiamethoxam, 3.5-100 ng/mL for clothianidin, 1.5-60 ng/mL for flonicamid, 1-30 ng/mL for furandione, 2.5-50 ng/mL for thiacloprid, 6-100 ng/mL for 6-chloronicotinic acid, and 25-100 ng/mL for 1-methyl-3-nitrguanidine [150]. In 2018, Zhou et al. determined the residues of nine neonics in the soil of parks (n = 35) and residential areas (n = 33) in Tianjin, China, using the rapid pre-treatment technology of LC-MS/MS. The total concentrations of the nine neonics detected in the parks and residential areas were 0.27-230.76 and 0.23-132.66 ng/g, respectively, with imidacloprid exhibiting the highest detection rate, followed by acetamiprid and thiamethoxam, respectively [151] The above-stated results suggest that the types and residues of neonics in the soil were at a high level, which may eventually increase the possibility of human exposure to neonics.

Distribution Characteristics of Neonics in the Atmosphere
The atmosphere is an important medium for the spread and diffusion of neonics. Human activities during agricultural production, such as pesticide spraying and seed coating, discharge neonics into the atmosphere. These pollutants then enter the surrounding environment with the movement of the atmosphere, producing a far-reaching impact [152].
Since neonics are difficult to volatilize, they may exist in the atmosphere, mainly in the form of particles. Pneumatic seeders were used widely in 2003, resulting in the generation of numerous dust particles containing insecticides [153]. The release of these particles containing pesticides into the atmosphere would cause pollution of the field vegetation, which could ultimately lead to the exposure of non-target organisms to sub-lethal doses of pesticides [154]. In 2013, Girolami et al. demonstrated that abrasive particles could cause serious harm to the insects flying over the field during sowing, bees, and other pollinators in particular [155]. In 2015, Xue et al. reported that the residues of thiamethoxam and clothianidin in the dust accounted for 0.01-0.40% of the actual application amount, of which 92% originated from the coated seeds after the pesticide treatment, and the total concentration of neonics in the atmosphere was 0.10 µg·m −3 [156]. In 2016, Botías et al. demonstrated that the pollen and nectar of wild plants growing in the vicinity of seed-treated crops were critically polluted with neonics, which caused long-term exposure of pollinators to these substances [157]. Moreover, with the transport and deposition of dust particles in the atmosphere, neonics would also pollute the soil and the water [158]. (Figure 3).

Distribution Characteristics of Neonics in the Atmosphere
The atmosphere is an important medium for the spread and diffusion of neonics. Human activities during agricultural production, such as pesticide spraying and seed coating, discharge neonics into the atmosphere. These pollutants then enter the surrounding environment with the movement of the atmosphere, producing a far-reaching impact [152].
Since neonics are difficult to volatilize, they may exist in the atmosphere, mainly in the form of particles. Pneumatic seeders were used widely in 2003, resulting in the generation of numerous dust particles containing insecticides [153]. The release of these particles containing pesticides into the atmosphere would cause pollution of the field vegetation, which could ultimately lead to the exposure of non-target organisms to sub-lethal doses of pesticides [154]. In 2013, Girolami et al. demonstrated that abrasive particles could cause serious harm to the insects flying over the field during sowing, bees, and other pollinators in particular [155]. In 2015, Xue et al. reported that the residues of thiamethoxam and clothianidin in the dust accounted for 0.01-0.40% of the actual application amount, of which 92% originated from the coated seeds after the pesticide treatment, and the total concentration of neonics in the atmosphere was 0.10 µg·m −3 [156]. In 2016, Botías et al. demonstrated that the pollen and nectar of wild plants growing in the vicinity of seed-treated crops were critically polluted with neonics, which caused long-term exposure of pollinators to these substances [157]. Moreover, with the transport and deposition of dust particles in the atmosphere, neonics would also pollute the soil and the water [158].
The above-stated research results suggest that with the large-scale application of neonics, their detection concentration in the environment is continually increasing. Neonics are being detected in all media of the environment, and a large number of neonic residues in the environment would produce a greater adverse impact.

Environmental Risk Assessment of Neonics
As stated earlier, a large number of active pharmaceutical ingredients of neonics either remain in the soil and the atmosphere during the application process or enter the water environment through surface runoff, leaching, dry and wet deposition, or other ways, resulting in an extensive amount of residues in various media of the environment and critical pollution [23]. In addition, the residues of neonics in the The above-stated research results suggest that with the large-scale application of neonics, their detection concentration in the environment is continually increasing. Neonics are being detected in all media of the environment, and a large number of neonic residues in the environment would produce a greater adverse impact.

Environmental Risk Assessment of Neonics
As stated earlier, a large number of active pharmaceutical ingredients of neonics either remain in the soil and the atmosphere during the application process or enter the water environment through surface runoff, leaching, dry and wet deposition, or other ways, resulting in an extensive amount of residues in various media of the environment and critical pollution [23]. In addition, the residues of neonics in the environment serve as a threat to non-target organisms, including algae, zooplanktons [159], fish [160], earthworms [161], bees [162], insectivorous birds [72], and even humans [163].
In the previous studies conducted by our research group, certain green pesticide substitutes based on the bioconcentration factor (BCF), octanol-water partition coefficient (K ow ), and lethal concentration of 50% (LC 50 ) were designed in order to reduce the negative impacts of neonics on the environment. At the same time, multi-directional-selection toxic effect evaluation and human health risk assessment were also performed in those studies to ensure the efficacy of the insecticides while maintaining low toxicity to the non-target organisms, crops, and humans, so as to design a series of green substitutes of neonics for improving the ecological security and human health at the theoretical level [1,37,38]. However, the studies mentioned in the present report only discussed the bioaccumulation capacity of the neonics and did not consider the other characteristics, such as biodegradability [164].
Currently, the detection of neonics in various environmental media is becoming increasingly frequent, and their concentrations are also on the rise. However, the research works conducted on photodegradation, bioaccumulation, and persistence of neonics, which would result in critical adverse effects on the whole environment and the ecosystem, are scarce. Therefore, in order to achieve the sustainable development of agriculture and for the benefit of society as a whole, it is of significance to pay attention to and study the environmental risks associated with neonics.

Conclusions
Neonics are used widely throughout the world, and the ecological problems caused by the application of neonics have attracted great attention globally. QuEChERS (Quick, Easy, Cheap, Efficient, Rugged and Safe), LC-MS/MS, and other detection methods have revealed the presence of a large number of neonics in the water, soil, atmosphere, and food chain, which is expected to have an adverse impact on the environment. The existing evidence provided by the epidemiological studies, both in vivo and in vitro, revealed that neonics have the potential to cause serious damage to non-target animals and humans within the ecosystem. Although these results have established a solid foundation for future studies to be conducted in relation to neonics, the research and development of green pesticides, the blocking of pollutant transmission, and the remediation of contaminated soil continue to be in the initial stages. In addition, even though the levels of pesticide residues in most foods are relatively low, the joint toxic effects produced as a result of the simultaneous presence of a number of residues of a variety of pesticides are not clearly understood so far. Therefore, it is important that the understanding of the transmission pathways of neonics in the food chain is deepened in order to fundamentally achieve reasonable prediction and control of the source, process, and end behavior of these insecticides in the future. Furthermore, effective remediation of contaminated soil and the problem of joint toxicity caused by the simultaneous presence of the residues of a variety of pesticides also deserve attention. The present report provides a large-scale theoretical reference for prospective research related to neonics, as well as reliable technical support for strengthening the environmental and food safety supervision and green pesticide design.

Conflicts of Interest:
The authors declare no conflict of interest.