Pesticides in house dust from urban and farmworker households in California: an observational measurement study

Young children are particularly vulnerable to adverse health effects that may result from pesticide exposures. For example, in utero and/or postnatal chronic exposures to organophosphorous (OP) pesticides have been associated with poorer neurodevelopment in children [1–5], and altered fetal growth [6], and shortened gestational duration [7]. Animal studies have also shown that neonatal exposures to other contemporary-use pesticides such as pyrethroids are associated with impaired brain development [8], changes in open-field behaviors, and increased oxidative stress [9].

Pesticides have been measured in residential environments, most notably in indoor dust [10–17]. Poor housing conditions in low-income homes, such as overcrowding and housing disrepair, are associated with pest infestations and increased home pesticide use in both urban and agricultural communities [18, 19], potentially increasing pesticide residues indoors. Additionally, the presence of farmworkers in the home and/or proximity of homes to nearby fields in agricultural communities have been associated with higher indoor pesticide concentrations [13, 20].

Several studies indicate that pesticide residues persist indoors due to the lack of sunlight, rain, temperature extremes, microbial action, and other factors that facilitate degradation [15]. Semi- and non-volatile pesticides (e.g., OPs and pyrethroids) have chemical properties that increase binding affinity for particles and the tendency to adsorb onto household surfaces such as carpet or dust, also prolonging their persistence indoors [11]. For example, pyrethroid pesticides have low vapor pressures, and high octanol/water (K_ow) and water/organic carbon (K_oc) partition coefficients which facilitate partitioning into lipids and organic matter and binding to particulate matter in dust [21]. Because of this, several studies suggest that house dust is an important pathway of pesticide exposure for children [11, 15, 17, 22]. Young children are particularly vulnerable to inadvertent ingestion of pesticide-contaminated dust due to their frequent hand-to-mouth activity and contact with indoor surfaces [15].

California (CA) has intense agricultural pesticide use [23], including OP insecticides. Due to their potential health effects in children, formulations of the OP insecticides, chlorpyrifos and diazinon, were voluntarily phased out for residential uses between 2001 and 2004 [24, 25]. One study showed that this residential phase-out resulted in decreased air concentrations among low-income households in New York City [26]. However, these OPs are still used in agriculture and trends in residential contamination of these compounds have not been studied in agricultural communities, where pesticide drift and transport from fields on work clothing may impact indoor pesticide concentrations [20]. It is also widely accepted that house dust is a reservoir for environmental contaminants with concentrations remaining fairly stable [11]; however, to our knowledge, only one study [27] has documented the temporal stability of pesticides in house dust focusing on the OP pesticide chlorpyrifos. Additionally, exposure data on other contemporary-use pesticides (e.g., pyrethroids) in low-income households is limited. In this study, we characterized and compared house dust levels of agricultural and residential-use pesticides from low-income homes in an urban community (Oakland, CA) and an agricultural community (Salinas, CA). We evaluated the correlation of several semi- and non-volatile pesticide concentrations in samples collected several days apart from the same general area in the home; and examined whether house dust concentrations of chlorpyrifos and diazinon declined in Salinas, CA after the U.S. Environmental Protection Agency's (EPA) voluntary residential phase-out of these compounds. Finally, we estimated resident children's potential non-dietary ingestion exposures to these indoor contaminants to determine if exposures via this pathway exceeded current U.S. EPA recommended guidelines.

We detected several pesticides in most homes, including OP pesticides previously phased-out for residential uses, pyrethroids, and the pesticide synergist piperonyl butoxide (PBO). Biological exposure metrics for these pesticides are relatively transient and highly variable, typically reflecting recent exposures [35]. However, consistent with other studies [15, 36], we found that dust serves as a stable matrix and indicator of potential indoor exposure for some pesticides. The high correlations observed in dust concentrations from samples collected 5-8 days apart suggests that, for some pesticides, measurements in house dust may be relatively stable indicators of potential indoor exposure over this time frame. To our knowledge, this is the first study to evaluate the correlation of concentrations within homes for several pesticides over a short sampling period.

Although the detection frequency for chlorpyrifos and diazinon was higher in Salinas than Oakland, we did not observe statistically significant differences in pesticide concentrations or loadings between locations. This is notable given that >28,000 and 65,000 kgs of chlorpyrifos and diazinon, respectively, were applied for agricultural purposes in Monterey County in 2006 (Additional File 1 Table A1) and minimal applications (65 kgs and 3 kgs of chlorpyrifos and diazinon, respectively) occurred in Alameda County. Previously, we showed a significant correlation with local agricultural use and chlorpyrifos dust concentrations (but not diazinon) for homes throughout the Salinas Valley [13]. Mapping of dust concentrations and agricultural use suggests that chlorpyrifos dust concentrations are higher in the center of the Valley (south of the city of Salinas), where agricultural use is higher [13]. Farmworker homes in the present study were from the city of Salinas where the impact of drift from agricultural applications may have been lower. Additionally, our small sample size may have prevented us from observing significant differences in concentrations between locations for these OP pesticides as well as other analytes.

Malathion was not frequently detected in homes from either location; however, higher levels were observed in urban homes. This pesticide is used in agriculture and is also registered for use in home gardens, as a building perimeter treatment, as a wide-area spray for mosquitoes, and by prescription for head-lice control [37]. However, no parents reported treating their children for lice or using it themselves in their gardens. The main county uses for this OP pesticide in 2006 in the urban region were landscape maintenance and structural pest control [38]. These applications were reported more than 25 km away from the nearest study home, thus it is not readily apparent why higher levels were observed in urban homes although it should be noted that we only sampled a small number of homes.

We generally observed significantly lower house dust concentrations of chlorpyrifos and diazinon in the present study compared to levels measured in dust from homes located in the same zip codes sampled between 2000 and 2002 [10, 13], suggesting that indoor concentrations in the city of Salinas are decreasing despite continued agricultural use in the area. In New York City, air concentrations for these OP pesticides in low-income homes also significantly decreased between 2001 and 2004 [26]. The temporal declines in indoor concentrations reported here and in the New York City study may reflect the decreasing usage of these OP pesticides for home or structural applications per the U.S. EPA's residential phase-out. Nonetheless, despite declining concentrations indoors, detection of these OP pesticides, especially in Oakland where there was little agricultural or structural use, underscores their persistence indoors.

Pyrethroids were detected in house dust in several study homes. Similar to low-income urban housing units in Boston, MA [41], pyrethroids and PBO were detected in higher concentrations and used more frequently in our study homes compared to other pesticides. This finding is consistent with the fact that pyrethroid insecticide formulations for residential applications have largely replaced OP pesticide residential formulations [42, 43]. Although over 19,000 kgs of permethrin were applied in Monterey County in 2006 for agricultural purposes [44], we did not observe significant differences in permethrin concentrations (or loadings) between locations. Allethrin and cypermethrin were also widely detected in most homes. Our findings suggest that home use likely contributed to the presence of pyrethroid pesticides in house dust since pyrethroids were commonly used indoors and negligible to no agricultural applications took place at the county level (except for permethrin). It is also possible that structural pest control applications influenced indoor detection of certain pyrethroids in some homes. For example, it is estimated that ~80% of the non-agricultural cypermethrin use reported in Alameda County in 2006 was for structural pest control [38]. The presence of pyrethroids in house dust is also consistent with their physical and chemical properties, including high octanol:water partition coefficient values (log K_ow > 4.0) and low vapor pressures (Additional File 1 Table A1). To our knowledge, only two studies [10, 13] have measured pyrethroid dust concentrations in farmworker homes. Similar to the present study, permethrins were the most frequently detected pyrethroids indoors. Median cis- and trans-permethrin concentrations in our farmworker homes were higher than those observed in a previous study [10].

The detection of chlorthal-dimethyl in all Salinas farmworker homes and none of the Oakland urban homes is consistent with other Salinas Valley studies showing an association between agricultural use and house dust contamination [13] and a positive correlation between outdoor and indoor air concentrations [10]. This herbicide had relatively high agricultural use (~ 33,970 kgs) in the Salinas Valley and is not found in home-use pesticides. Chlorthal-dimethyl also has a high log K_ow value and low vapor pressure (Additional File 1 Table A1), and may be bound to particulate matter at room temperature.

Over 16,000 kgs of malathion and iprodione were used in 2006 for agricultural applications (Additional File 1 Table A1); however, they were not commonly detected in farmworker homes from the city of Salinas. For some of these pesticides, e.g., iprodione, LODs were higher than for other analytes. Other factors including physico-chemical properties, e.g., high vapor pressure and low log K_ow values (≤3), may have resulted in lower detection frequencies. These pesticides were also not frequently detected in dust samples from our previous study in the city of Salinas [10].

This study has several limitations. Location differences in pesticide dust levels have been reported previously when using loadings rather than concentrations [21]; however, our small sample size limits statistical power and may have prevented us from observing statistically significant differences between locations for concentrations and/or loadings. Additionally, although homes with insufficient sample mass were demographically similar to those with adequate sample mass, exclusion of these homes may have introduced some bias and prevented us from detecting a difference in pesticide concentrations and/or loadings between locations. We also focused on low-income homes and thus the results may not be generalizable to other populations. Although estimated intakes for select pesticides were below EPA RfDs (i.e., HQ <1.0), it should not be concluded that intakes below RfDs are "acceptable" or free of any health risks. For example, recent studies have identified mechanisms of OP pesticide toxicity that were not considered in defining current U.S. EPA RfDs (e.g., suppressed expression of serotonin transporter genes) [45]. Moreover, RfDs do not account for differences in vulnerability to pesticide toxicity due to genetic factors, such as paraoxonase (PON1) polymorphisms [46]. Additionally, our intake calculations for pesticides do not account for other exposure pathways (e.g., inhalation or diet); nor did we consider that some children could have pica or other behaviors that could increase or decrease intake. Although we surveyed participants on their usage of pesticides indoors, we were not always able to corroborate whether formulation ingredients were present at high concentrations as the pesticide containers were not always available to confirm the active ingredients. Lastly, children in the homes sampled are clearly exposed to multiple indoor contaminants and our hazard evaluation does not account for exposure to complex mixtures.

Studies of contaminants in low-income homes, including our study, have been limited in sample size and, often, selection of participants has not been random. In addition, collection methods, analytical techniques, analytes measured, and timing of data collection differ. To our knowledge, only one other study has assessed indoor dust concentrations of pyrethroids in low-income homes in an urban setting [41]. Nonetheless, the results from these studies indicate that low-income children are potentially exposed to a mixture of pesticides. Agricultural pesticide use may contribute to additional exposures to some pesticides in rural areas; historical or current residential use is also likely to contribute to ongoing exposures. Although children's non-dietary intake did not exceed U.S. EPA RfDs for select pesticides, this does not ensure that children are free of any health risks as RfDs have their own limitations, and the children may be exposed indoors via other pathways. The frequent pesticide use reported among participating households in this and previous studies of low-income homes [18, 19, 41] and high detection of several home-use pesticides in house dust suggests there is a need to educate families on the potential health impacts of pesticide use and effective integrated pest management strategies to control pests and reduce exposures to household occupants [42]. Particular at-risk populations are those living in households with poorer housing quality, where there may be greater needs for pest control [18, 19].

Additional research is needed to quantify exposures and potential health effects from these compounds, particularly frequently used pesticides such as pyrethroids. Such research should consider the complex mixture of chemicals found in indoor environments, include both environmental and biomonitoring measurements to assess cumulative exposures, and consider exposures in homes of different socioeconomic status.