Environ. Sci. Technol. 2009, 43, 5665–5670
Fipronil and its Degradates in Indoor and Outdoor Dust BARBARA J. MAHLER,* PETER C. VAN METRE, JENNIFER T. WILSON, AND MARYLYNN MUSGROVE U.S. Geological Survey, Austin, Texas 78754 STEVEN D. ZAUGG AND MARK R. BURKHARDT† U.S. Geological Survey, Denver, Colorado 80403
Received April 30, 2009. Revised manuscript received June 24, 2009. Accepted June 25, 2009.
Fipronil is a potent insecticide used for control of termites, fleas, roaches, ants, and other pests. We measured fipronil, fipronil sulfide, and desulfinyl fipronil concentrations in indoor and outdoor dust from 24 residences in Austin, Texas. At least one of these three fipronil compounds was detected in every sample. Fipronil accounted for most of the total fipronil (T-fipronil; fipronil + desulfinyl fipronil + fipronil sulfide), followed by desulfinyl fipronil and fipronil sulfide. Nineteen of 24 samples of indoor dust had T-fipronil concentrations less than 270 µg/kg; the remaining five had concentrations from 1320 to 14,200 µg/kg. All three of the residences with a dog on which a flea-control product containing fipronil was used were among the five residences with elevated fipronil concentrations. In outdoor dust, all concentrations of T-fipronil were less than 70 µg/kg with one exception (430 µg/kg). For every residence, the concentration of T-fipronil in indoor dust exceeded that in outdoor dust, and the median concentration of T-fipronil was 15 times higher indoors than outdoors.
Introduction Fipronil is a phenylpyrazole insecticide first registered for use in the United States in 1996 to control a wide range of insects, including ants, roaches, termites, and fleas. Home uses include topical pet care products, liquid termiticides, gel baits, and granular turf products. Agricultural uses of fipronil in the U.S. primarily are on maize and turf grass. Its mode of action involves interference with the mechanism by which γ-aminobutyric acid (GABA) regulates the influx of chloride into neurons, resulting in excessive neuronal activity, severe paralysis, and death. Fipronil is more toxic to insects than to mammals, apparently because GABA receptors are more sensitive in arthropods (1). Fipronil has been found to be toxic at very low concentrations (∼1 g active ingredient per hectare [gai/ha]) to several species of insects, including beetles, wasps, bees, ants, and flies (2), as well as to copepods (3), crayfish (4), and other crustaceans. Fipronil also has been found to be highly toxic to some lizards and birds and moderately toxic to rats (5). It has been classified as a Class C (possible human) carcinogen (6) on the basis of its association with an increased incidence of thyroid follicular * Corresponding author e-mail:
[email protected]. † Current affiliation: U.S. Environmental Protection Agency, Region 8 Laboratory, Golden, Colorado 80403. 10.1021/es901292a
Not subject to U.S. Copyright. Publ. 2009 Am. Chem. Soc.
Published on Web 07/09/2009
cell tumors in both male and female rats. It is suspected to be an endocrine disruptor (7, 8). Chemical transformation of fipronil can occur by several mechanisms (9). Among the major degradates are desulfinyl fipronil (from photolysis), fipronil sulfone (formed aerobically in soils), fipronil sulfide (formed anaerobically in soils and water), and desulfinyl fipronil amide (formed aerobically in soils and by hydrolysis at basic pH). Desulfinyl fipronil and fipronil sulfone have been found to be more persistent and less selective between arthropods and mammals than the parent compound (10). Although fipronil and desulfinyl fipronil show similar levels of interference with the GABA receptor in insects, the latter compound interferes with the mammalian GABA receptor 10 times more readily than does fipronil (11). There have been numerous studies of the partitioning and persistence of fipronil under laboratory conditions and outdoors (5, 12, 13). Fipronil is only sparingly soluble in water (0.0022 g/L at pH 5 and 0.0024 g/L at pH 9) and does not readily volatilize from solution (Henry’s law constant of 3.7 × 10-5 Pa m3/mol) (14). Its relatively high Koc value (803 mL/g) indicates that it tends to associate with solid-phase organic matter, and in one study in an aquatic environment, 95% of fipronil residues detected were found to be associated with sediment within 1 week (15). The amide (Koc 166 mL/g) is less hydrophobic than the parent compound, although the desulfinyl, sulfide, and sulfone degradates are more hydrophobic than fipronil (Koc values of 1290, 2719, and 4209 mL/g, respectively [(16), as cited in ref 12]). In field studies, reported half-lives for fipronil range from 0.4-0.5 months for turfed soil to 1.1-1.5 months for bare soil (17). All of the degradates have been found to be more persistent in soils than the parent compound ((16), as cited in ref 12). To our knowledge there are no published data on rates of degradation of any of these compounds inside homes. Indoor house dust can scavenge numerous hydrophobic contaminants, including those tracked in from outdoors and those with an indoor source. Contaminants detected in association with indoor dust include lead (18), PCBs (19), pesticides (20), PBDEs (21), and PAHs (22). Because house dust is not subject to the same environmental conditions as outdoor soil (e.g., sunlight, moisture, soil microbes), degradation of contaminants associated with dust is assumed to be slower indoors than outdoors (23). Exposure to house dust and associated contaminants is of particular concern for children, who spend much of their time on the floor and have frequent hand-to-mouth contact; estimates of ingestion of dust by infants and toddlers range widely from study to study, but have been estimated to be about 50 mg/d for the average child and 430 mg/d for the 95th percentile child (24). Here we report the results from a study of the occurrence of fipronil and two degradates (desulfinyl fipronil and fipronil sulfide) in indoor (household) and outdoor dust in Austin, Texas. Relations between concentrations of fipronil and several potential predictor variables, including location (indoor vs outdoor) and pet ownership, were tested statistically. Although occurrence and fate of fipronil in agricultural settings have been studied extensively (5, 12, 13), to our knowledge this is the first study to examine the occurrence of fipronil and its degradates in indoor dust and the strength of association with potential explanatory factors.
Materials and Methods Sample Collection. Dust was collected in April through July, 2008, from 24 ground-floor apartments in Austin, Texas, and VOL. 43, NO. 15, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 5665
their 23 associated parking lots (two apartments were in the same complex). The apartments were in north and south Austin, and are believed to be reasonably representative of apartments in the central Texas area; no particular subpopulation or geographic area was targeted. None of the households included children, although this was not by design. All of the apartments had central heat and airconditioning. Apartment residents answered questions covering a variety of lifestyle activities and actions that might influence contaminant concentrations, including pet ownership, pet flea treatment, other pesticide use including termite treatment, and wearing of shoes indoors. Notes were taken regarding the nature of indoor furnishings, including carpeting. Of the 24 households, 11 had one or more dogs or cats, or both: seven households had one or more dogs, and five had one or more cats; of those, two had both a dog and one or more cats. Three of the dogs and one of the cats (four separate households) were treated monthly with a flea- and tick-control product containing fipronil. Residents in six of the 24 households removed their shoes when inside. The area sampled in 18 of the 24 residences was carpeted and in six was bare floor (concrete, wood, linoleum, or tile). The survey questions and tabulated results are given in the Supporting Information. Dust was collected using a model HVS3 high-volume surface samplersthe American Society for Testing Materials (ASTM) standard method for recovering settled dust samples for chemical analysissfollowing the dust-collection methods recommended by the manufacturer (25). The indoor area sampled ranged from 1.6 to 15 m2 (median of 3.4 m2); the outdoor area sampled ranged from 2.0 to 6.5 m2 (median of 4.8 m2). After collection, the dust was weighed and the fine (
