Submitted:
08 May 2026
Posted:
09 May 2026
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Abstract
The use of spot-on pet parasiticides has risen substantially. Imidacloprid, a commonly used active ingredient (AI), was removed from outdoor agricultural use in 2018 due to evidenced environmental risks. Imidacloprid is an AI in certain spot-on pet parasiticides and, with its metabolites, is now a domestic contaminant. We report two studies of dust and surface contamination in >50 homes in London, UK. In study 1, a time series pre-and-post spot-on application, imidacloprid rapidly contaminates the home at concentrations far exceeding the environmental quality standards that exist. Seven days post-application, imidacloprid concentration in domestic mop water exceeded the acute toxicity maximum acceptable concentration (MAC) by 600-fold and rinsate from fabric on which animals frequently sat was almost 5000-fold the MAC. In study 2, of dust in 50 homes, the 10 homes without resident pets had the lowest imidacloprid concentration. Homes using spot-ons had much higher concentrations (38+/-17 µg/g), comparable to days 5-7 in Study 1. The imidacloprid acceptable daily intake (ADI) for humans applies to the gastric route (residues in food). Multiple routes of human contamination exist; transdermal and inhalation have no standards. There is evidence that imidacloprid is associated with cardiac, neurological and endocrine disruption in mammals, including humans. A precautionary approach is advisable, with responsive use rather than prophylactic use which maintains high levels of domestic contamination.
Keywords:
spot-on
; health risk
; neonicotinoid
; environmental pollution
; domestic contamination
; pet
; parasiticide
Context
The use of spot-on pet parasiticides has risen substantially in the UK and elsewhere in the past decades with an industry-driven shift from responsive application (treat when ectoparasites are present) to a prophylactic approach (treat frequently to prevent). Two commonly used active ingredients (AIs), imidacloprid and fipronil, are of serious environmental concern and were removed from outdoor agricultural use by 2018 due to their evidenced environmental risks (Preston-Allen et al. 2023). These continue to be identified in freshwater bodies (Richardson et al. 2026; Egli et al. 2023) and the source is now persuasively identified as household wastewater (Perkins et al. 2024). In addition to this aquatic presence, their frequent use in homes suggests they are domestic contaminants (Salthammer 2020; Salis et al. 2017; Testa et al. 2019). While some of the routes to domestic wastewater have been characterised (e.g., handwashing, washing of pet bedding, etc.) there is a substantial shortfall in the mass-balanced understanding of other routes contributing to total residue concentrations measured in municipal wastewater, especially arising from within the home.
Imidacloprid is a neonicotinoid insecticide of synthetic origin. It acts systemically and through contact and is a nicotinic acetylcholine receptor (nAChR) channel blocker. For insects, it is a neurotoxin leading to impaired olfactory function, disrupted navigation ability and reduced motor coordination (Lewis et al. 2016). Imidacloprid is increasingly being considered for its potential to pose health risks to people, with epidemiological evidence adding weight to chronic and sub-chronic toxicity studies, especially in the context of neurodevelopment in mammalian systems (DPR 2024). There are known reproductive / developmental effects, it is currently considered moderately toxic (potential liver, kidney, thyroid, heart and spleen toxicant effects) (PPDB 2026). The increasingly widespread and frequent use of this AI in domestic environments may pose particular risks to women of childbearing age, infants, and young children (NRDC, 2017). Frequent exposure to imidacloprid via pet parasiticide use during pregnancy has been associated with raised autism spectrum disorder risk in exposed children (Keil et al. 2014). Recent epidemiological evidence has revealed a connection between neonicotinoids, and especially their metabolites, such as imidacloprid and imidacloprid-olefin, and oxidative stress, inflammatory response, and endocrine disruption pathways leading to cardiac arrhythmia (Ge et al. 2026).
Once applied to pets, a proportion of the pesticide transfers from the animal through direct contact with people (e.g., stroking), indoor surfaces (e.g., contact traces on shared furniture, fabrics and floors) and through shed fur (Craig et al. 2005). Without regular removal through washing or vacuuming, the AI may accumulate on furniture, floor coverings and in household dust. Thus, human exposure can be direct and indirect and more pronounced for those who frequently encounter these surfaces, such as children who crawl on the floor (Salthammer 2020).
A full review of the published evidence in this area is being compiled and will be published in late 2026 or early 2027.
Imidacloprid in the Home
To better understand domestic exposure in the home, we report here data being prepared for peer review and publication. Two separate investigations are presented, the first is a time series following imidacloprid concentration in dust, mop water and rinsate from fabrics on which animals frequently sat (sofa and bed covers) starting from before treatment with a spot-on and extending for three-weeks after the application. The second is a snapshot of the imidacloprid concentration in the house dust of 50 homes, 40 of which had pets.
Study 1: Time Series Data
This experiment took place in the homes of three volunteers with cats (2 homes) and dogs (1 home) prepared to apply an over-the-counter, spot-on product with imidacloprid as AI to their animals. Samples of dust, mop water and the rinsate of fabric placed where animals regularly sat were taken (Table 1). Imidacloprid was identified and quantified to µg/g of dust or µg/L by using liquid extraction followed by liquid chromatography tandem mass spectrometry (LC-MS/MS). All concentrations are to be considered semi-quantitative.
Findings
Trace concentrations of imidacloprid prior to treatment in both dust and mop water was found in all three homes. Domestic contamination rose immediately post-treatment (Figure 1 L & R). The concentrations in dust attenuated more rapidly than in mop water, possibly because of the more frequent vacuum sampling procedure. The homes with cats had the highest concentrations of imidacloprid in dust samples, this was greater than 200-fold higher than previous studies of household dust on farms (Hwang et al. 2008; Navarro et al. 2023).
The UK Scientific Committee on Health, Environmental and Emerging Risks has aligned aquatic concentration environmental quality standards (EQS) with the European Union’s Water Framework Directive. These are a maximum acceptable concentration (MAC) EQS of 0.057 µg/L for acute toxicity and an annual average EQS of 0.0068 µg/L for chronic toxicity (SCHEER 2021). At seven days post application, the mean imidacloprid concentration in domestic mop water exceeded the acute toxicity MAC by a factor of approximately 600-fold (34 +/- 9.5 µg/L, Figure 1 R). Three weeks after application the exceedance had abated but remained ~300-fold this threshold (17 +/- 1.2 µg/L). The persistence of the AI in mop water (Figure 1 R) in repeated cleaning events, suggest ongoing contamination from the treated animal(s).
Rinsate water (two-hour soak in room-temperature water) from fabrics on which animals had regularly sat or slept in the two-weeks since application supports this. Concentrations on these materials were extremely high in all houses (2,800 +/- 550 µg/L in rinsate, Figure 2), but particularly so in the dog home (9,900 +/- 290 µg/L), where the extraction was made from a sofa cushion cover on which the animal regularly slept. This may reflect differences in behaviours between dogs and cats, suggesting that cats may disperse the pesticides more widely in the home as many access higher-level surfaces and display a lower level of locational fidelity than dogs.
Study 2: Snapshot Data – 50 Homes
This investigation sampled the dust in 50 volunteer urban London households to explore what more typical concentrations might be. Forty of the homes had pets and these represented a range of approaches to parasite vigilance, from prophylactic application of veterinary product, through responsive use of these, to alternative practices. The sample thus included a range of products approaches and application routes. Dust was sampled using a robotic Dyson™ vacuum, imidacloprid was again identified and quantified as µg/g of dust by using liquid extraction followed by liquid chromatography tandem mass spectrometry (LC-MS/MS).
Findings
Sixteen of the 40 homes with resident pets had used spot-on treatments within the past year, of which nine had applied them within the past month. A further 15 homes knew their animal(s) to have been treated within the past year, but were not sure what had been applied (i.e., the “N/A” category in Figure 3) and nine homes had used an oral administration route. Seven of the homes in which spot-ons had been used recollected using an imidacloprid-containing brand, two were uncertain about brand, two had fipronil as an active ingredient and three applied a selamectin-based brand.
Control homes, i.e., those without resident pets, had measurable quantities of imidacloprid, but had the lowest concentration of all categories, and not statistically distinguishable from those who used oral administrations (ANOVA, p=0.97) nor from homes which could not remember what or when they had used parasiticides (p=0.72). Those applying spot-ons had higher concentrations (38 +/- 17 µg/g, p=0.0052), comparable in magnitude on average to those detected between day five and day seven in Study 1 (Figure 3).
Both data sets indicate a low, but omnipresent, level of imidacloprid present in the dust of these London homes (range detected 0.12-4.8 µg/g of dust in “Control” and “Oral” homes). This may arise from other sources, such as visiting pets, incomplete decontamination of sampling equipment, or being tracked into the homes following footfall through contaminated outdoor areas. These are, however, dwarfed by the concentrations found in the homes of people who apply spot-on parasiticides to their domestic animals (Figure 3).
Domestic Exposure Risk
Human Health Regulatory Standards
The European Union advises an acceptable daily intake (ADI) of imidacloprid (through all routes) of 60 µg/kg body weight per day and the acute reference dose (ARfD) is 80 µg/kg body weight/day (European Commission 2022). These are based on pesticide residues consumed in food and the assumption is of entering the body via the gastric tract. There are no ADI estimates for inhalation. Also relevant are dermal penetration studies, which reveal a concentration-dependent penetration of 0.3-8.0%; occupational exposure is considered possible via dermal contact (Lewis et al. 2016).
Routes to Human Health Risk
These two studies reveal concerning concentrations of imidacloprid on surfaces and in the dust of homes which use spot-on pet parasiticide treatments. The concentrations found on fabrics and floors, and quantified via rinsate and mop water, exceed the aquatic MAC by factors of hundreds-to-thousands. Much of this will be washed down the drain with excess water through housecleaning and laundry (Perkins et al. 2024), but specific trans-dermal absorption from contact with pets themselves and the contaminated surfaces of the home remains unquantified and may constitute a substantial risk to health. A biomonitoring study of urine from a human cohort in the Republic of Ireland revealed that post-treatment application cuddling of pets was a major source of human exposure, and more than the initial treatment application itself (Wrobel et al. 2024). In this single animal-single treatment study, human imidacloprid exposure was measured as 10.6 µg/kg/day, a factor of five below the current acceptable daily intake set by the EU (for adults). However, the researchers noted that these data may not be generalisable to all domestic exposure situations, and, by extension, to other regions or countries where practices may be different. In addition, several concurrent exposure instances (eg. several pets treated simultaneously) are also possible and need further study. There is thus a lack of knowledge relating to exposures through inhalation of dust or dander or dermal contact with multiple and/or different treated pets in the same home. Children are estimated to inhale ~ 100 mg/day of household dust, with adults estimated at half this, largely through being taller (Oomen et al. 2008). Within this category, there are no specific inhalation estimates for babies, young infants or toddlers.
Summary Points
Regular use of imidacloprid-containing spot-on parasiticides widely contaminates the home at levels far in exceedance of the few environmental quality standards that exist.
- While dust inhalation alone in contaminated homes is unlikely to reach ingestion levels above the ADI, even for children of ~5 kg (~3.3% ADI estimated), this standard is for the gastric route and no standard has been established for lungs.
- Multiple routes of human contamination exist. Transdermal and airway penetration have no standards.
- There is increasing evidence that imidacloprid is associated with neurological and endocrine disruption, particularly to thyroid hormones, in mammals including humans (citations herein).
- A precautionary approach is advisable, including recommending only responsive use of imidacloprid rather than frequent prophylactic use which contributes to the maintenance of high levels of domestic contamination.
Administrative Elements
Funding
This research received no external funding but received benefit in kind from Dyson UK (see acknowledgment).
Ethics Approval
The study was conducted in accordance with the Declaration of Helsinki, and approved by the Institutional Review Board (SETREC RGIT) of Imperial College London (Approval No 7400632 20th March 2025). Informed consent was obtained from all subjects whose homes were sampled in the study. Anonymised data will be made available on publication.
Credit Statement
Conceptualisation: TC & LB. Methodology: TC, LB, XL, WC, ME, AR, MW, HRW. Formal Analysis: TC, XL, ME, AR. Resources: LB, TC. Data Curation: TC, XL, WC, ME, AR, MW. Writing Original Draft: TC, XL EG, AR. Writing review and editing: LB, AR, RP, TC. Visualisation: TC, AR, ME. Supervision: HRW, LB, TC Project Administration: TC, LB. Funding Acquisition: TC, LB.
Acknowledgments
We would like to thank Dyson UK for providing the robotic vacuum cleaners essential to dust sampling.
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Figure 1.
Imidacloprid concentration in (L) house dust and (R) mop water as a function of time in days since ‘Spot-on’ Advantage application (mean +/-SE).
Figure 1.
Imidacloprid concentration in (L) house dust and (R) mop water as a function of time in days since ‘Spot-on’ Advantage application (mean +/-SE).
Figure 2.
Imidacloprid concentration in furniture cover rinsate prior to, and two weeks after, ‘Spot-on’ Advantage application to pets in the home (mean +/-SE).
Figure 2.
Imidacloprid concentration in furniture cover rinsate prior to, and two weeks after, ‘Spot-on’ Advantage application to pets in the home (mean +/-SE).
Figure 3.
Imidacloprid concentration in 50 volunteer homes (mean +/-SE) as a function of application. Control = no resident pets, N/A = treated within the past year, but did not know what had been used, Oral = treated with an oral parasiticide, Spot-On = treated with an external parasiticide.
Figure 3.
Imidacloprid concentration in 50 volunteer homes (mean +/-SE) as a function of application. Control = no resident pets, N/A = treated within the past year, but did not know what had been used, Oral = treated with an oral parasiticide, Spot-On = treated with an external parasiticide.
Table 1.
Sampling schedule for the exploration of imidacloprid in the home as a function of time since treatment.
Table 1.
Sampling schedule for the exploration of imidacloprid in the home as a function of time since treatment.
| Day | T-1 | Treatment day | T+1 | T+3 | T+5 | T+7 | T+14 | T+21 |
| Dust | Yes | Yes | Yes | Yes | Yes | Yes | Yes | |
| Mop | Yes | Yes | Yes | Yes | Yes | |||
| Rinsate from fabric | Yes | Yes |
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