Impact of Prenatal Exposure to Piperonyl Butoxide and Permethrin on 36-Month Neurodevelopment

While we agree with Peterson et al. that augmenting epidemiologic results with toxicology and risk assessment is important, these authors fundamentally misrepresented the objective and the conclusion of our recent study, "Impact of Prenatal Exposure to Piperonyl Butoxide and Permethrin on 36-month Neurodevelopment". In this prospective birth cohort study, our objective was to examine the association between prenatal exposure to piperonyl butoxide (PBO) and permethrin resulting from residential use of pyrethroid insecticides and child neurodevelopment at 36 months of age. It was neither to complete a toxicological risk assessment nor to establish a causal link between PBO or permethrin and child neurodevelopment. To our knowledge, ours is the first epidemiologic study to suggest an association between prenatal PBO exposure and adverse developmental sequelae. Due to limitations discussed in the paper, we clearly state that these results are preliminary but warrant further research.

In light of recent evidence suggesting that the use of pyrethroids insecticides for residential pest control is increasing, we believe the current study is timely and progressive. The U.S. EPA decision to phase out and ultimately eliminate the uses of organophosphorus insecticides including chlorpyrifos and diazinon 1, 2 from residential use because of their potential for causing toxicity in humans, especially children, has led to the gradual replacement with another class of insecticides, namely pyrethroids. Epidemiologic studies and sales/marketing data demonstrate that the use of pyrethroid insecticides for residential pest control is increasing throughout the US in response the 2000-2001 ban on residential use of OP insecticides.3-7 PBO is the most common pyrethroid synergist and is found in mixtures of pyrethroids prepared for residential application thus exposure to PBO is likely increasing. As PBO is rarely applied without the pyrethroid mixture, it acts as a good indicator of pyrethroid use. Our original interest in PBO was as a proxy for pyrethroid use, however the recent results suggest it may have an impact on neurodevelopment. The US. EPA 2005 Health Effects Division (HED) and Reregistration Eligibility Decision (RED) documents for PBO suggest PBO alone has very little systemic toxicity. However, these documents discuss PBO toxicity based on Hazard Identification Assessment Review Committee (HIARC) selected end points for assessing occupational and residential risks for PBO including changes in maternal body weight gain and changes in F1 and F2 pup weight gain. The documents do not discuss any studies with endpoints related to cognitive outcomes, thus, contrary to Dr. Peterson's assertions, have no bearing on our findings. Further, with respect to the EPA risk characterization of PBO, the NOEAL level is based on effects that do not account for the potential of PBO to make the developing brain more vulnerable to additional chemical insult and thus may not be appropriate. The only human study to examine the toxicity of PBO in humans administered a single oral dose to 8 adult male volunteers and subsequent examined microsomal inhibition. This study is not generalizable to the potential impact of a low level, chronic exposure to PBO during pregnancy on neurodevelopment. In summary, none of the documents discussed by Peterson et al invalidate the results of our study. Rather, Peterson's comments highlight the need for additional appropriately designed epidemiologic and experimental studies on the potential neurodevelopmental toxicity of PBO and permethrin.

We would also like to note that while an employee of Dow AgroSciences LLC (the sole manufacturer of chlorpyrifos), Dr. Peterson coauthored an article with a similar tone claiming that "the risk of children experiencing an adverse health effect from exposure to chlorpyrifos through both nondietary and dietary sources is negligible"8. One year later, the US EPA initiated the withdrawal of all residential use of chlorpyrifos because their risk assessment showed that risks to children from residential use were unacceptable high.9 With this prior experience, we encourage Dr. Peterson to take a more precautionary approach to children's environmental health. We stand by our conclusion that while our findings are preliminary, they are clearly of public health concern and further research is warranted.

1. 2000. U.S. EPA;Chlorpyrifos Revised Risk Assessment and Agreement with Registrants(Washington, D.C.). 2. 2001. U.S. EPA;Diazinon Revised Risk Assessment and Agreement with Registrants (Washington, D.C). 3. Bekarian N, Payne-Sturges D, Edmondson S, Chism B, Woodruff TJ. Use of point-of-sale data to track usage patterns of residential pesticides: methodology development. Environ Health. 2006;5:15. 4. Whyatt RM, Barr DB, Camann DE, Kinney PL, Barr JR, Andrews HF, et al. Contemporary-use pesticides in personal air samples during pregnancy and blood samples at delivery among urban minority mothers and newborns. Environ Health Perspect. 2003;111(5):749-756. 5. Whyatt RM, Garfinkel R, Hoepner LA, Holmes D, Borjas M, Williams MK, et al. Within- and between-home variability in indoor-air insecticide levels during pregnancy among an inner-city cohort from New York City. Environ Health Perspect. 2007;115(3):383-389. 6. Williams MK, Rundle A, Holmes D, Reyes M, Hoepner LA, Barr DB, et al. Changes in pest infestation levels, self-reported pesticide use, and permethrin exposure during pregnancy after the 2000-2001 U.S. Environmental Protection Agency restriction of organophosphates. Environ Health Perspect. 2008;116(12):1681-1688. 7. Horton MK, Jacobson JB, McKelvey W, Holmes D, Shepard P, Rundle A, et al. Characterization of residential pest control products used in inner city communities in New York City. Journal of Environmental Epidemiology and Exposure Science. 2010;in revision. 8. Gibson JE, Peterson RK, Shurdut BA. Human exposure and risk from indoor use of chlorpyrifos. Environ Health Perspect. 1998;106(6):303-306. 9. 2000. U.S. EPA;Washington, D.C.(Human Health Risk Assessment: Chlorpyrifos):U.S. Environmental Protection Agency

Conflict of Interest:

None declared

To the Editor,

I am writing in response to an article published in Pediatrics 127 (3): doi: 10.1542/peds.2010-0133 by M.K. Horton et al entitled “Impact of prenatal exposure to piperonyl butoxide and permethrin on 36- month neurodevelopment.” The authors claim to demonstrate a statistically significant association between piperonyl butoxide (PBO) in the personal airspace of pregnant mothers and Mental Development Index (MDI) scores of postnatal children at 36 months. Although the authors were admirably forthright about certain shortfalls in methodology, there are further discrepancies in study design and interpretation of results that seriously compromise the validity of inferences derived from the statistical correlations claimed.

As the authors tacitly admitted, there are a number of confounders such as mother’s education levels for which they do not fully account. Indeed, there are further unknown/unevaluated confounders based on there being no concurrent control group. For example, their primary covariable is the concentration of PBO in the pregnant women’s personal airspace. PBO was not measured in either maternal serum or cord blood. There could be some other substance (randomly selected) in the blood which would have achieved the same result.

The study design, in and of itself, is suboptimal. The best design in this instance would have been a randomized concurrent control, but that was not feasible in this case. A concurrent control based on propensity scoring or an historical control based on propensity scoring would have provided far more meaningful results.

Furthermore, PBO toxicology and risk characterization of the subject’s exposures seem to have been largely ignored, yet may completely undermine the interpretation of the study results. Review of the 2006 USEPA Reregistration Eligibility Documents (REDs) for PBO would have challenged the authors with regard to toxic endpoints and validity of routes of exposure. For instance, in the 2006 PBO RED, the USEPA states that “Neurotoxic effects of PBO are not evident from the clinical signs reported in developmental, reproductive, and chronic studies submitted to the Agency.” This is extensively documented and directly contradicts the authors’ study rationale and conclusions. Furthermore even the most conservative estimates of mother/fetal exposures utilized in this study fall several orders of magnitude below established EPA reference doses for PBO. Assuming that inhalation within the mother’s personal airspace translates to a coupled 1:1 exposure for the fetus strains credulity and is not sufficiently discussed in the published work. At a minimum, this study does not adequately make the case for postnatal cognitive deficiency attributable to exposure to permethrin and, by extension, PBO.

Since its inception, the Environmental Protection Agency (EPA) has regulated synergized pyrethroids through enforcement of standards instituted by the Federal Insecticide, Fungicide, and Rodenticide Act. This legislation mandated documentation of extensive testing for public health insecticides according to EPA guidelines prior to their registration and use. These data requirements are among the most stringent in the federal government and are met through research by established scientists in federal, state and private institutions. This process costs a registrant several million dollars per product, but ensures that the public health insecticides available for mosquito control do not represent health or environmental risks when used as directed. Mosquito control programs utilize synergized pyrethroids, when warranted, as part of integrated mosquito management systems specifically designed to protect human health, particularly those most vulnerable to mosquito-borne disease - mothers and their children.

Conflict of Interest:

The AMCA represents the interests of the entire vector control profession including public health officials, academicians and pesticide manufacturers/distributors

Using epidemiological techniques, Horton et al. (1) show a statistically significant association between piperonyl butoxide (PBO) in the personal airspace of pregnant mothers and Mental Development Index (MDI) scores of postnatal children at 36 months. Although the authors should be commended for pointing out the potential statistical and other limitations of their methods in the discussion section of their paper, there are other aspects which need to be considered to put their findings into a more comprehensive perspective. These aspects include current understandings based on toxicology, exposure, and risk characterization. When considering the total weight of evidence, we believe it is unlikely that PBO is responsible for the effects reported in their study.

Toxicology

The study demonstrates a negative association between exposure to the non-neurotoxic compound PBO and a neurotoxic endpoint. The study, however, does not demonstrate a dose-response effect between PBO in personal airspace and 36-month neurodevelopment. This demonstration is crucial given that the statistically significant effects that were observed are modest, as indicated by the authors themselves. In their discussion section, the authors’ presentation of what is known about the toxicology of PBO is incomplete. The authors do not differentiate dose-dependent effects or routes of exposure in their review of the toxicity of PBO and its potential to cause the effects they measured in their study. More important, the authors do not cite the 2006 USEPA Reregistration Eligibility Decision Document (RED) and associated Health Effects Division (HED) documents, which provide a comprehensive discussion of the toxicology of PBO (2). The authors state that, “…concern for health effects could arise from its potential to interfere with P450 activity and alter the metabolism of other xenobiotic compounds….” and use a 1976 paper as their citation. However, the authors do not mention that more recent studies conclude that PBO is not effective as a synergist in mammals and microsomal enzyme inhibition in mammals is transient and only occurs at high doses (2). The 2005 HED review also states that “…one study in humans reports no inhibition of microsomal enzymes (measured as antipyrine half- life in blood) at a dose of 0.7 mg/kg.” (3). This dose level is orders of magnitude greater than conservative estimates of exposure to humans (see below). In its RED, the USEPA (2) states that “Neurotoxic effects of PBO are not evident from the clinical signs reported in developmental, reproductive, and chronic studies submitted to the Agency,” yet the paper’s authors do not cite or discuss this.

Exposure

The authors assume, and state several times in their paper, that the PBO in personal airspace samples relates directly to prenatal exposure in the children. However, the authors do not demonstrate this, nor do they marshal evidence from the literature to support this assumption. The authors assert that children more highly exposed to PBO in personal airspace samples of the pregnant mothers led to lower MDI scores, but do not discuss the relationship between exposure of the mothers and their fetuses. How much of the PBO in the personal airspace samples is actually inhaled by the mothers? What fraction of PBO enters the lungs, enters the mother’s blood stream, and enters the fetus? What are the temporal and spatial relationships between residential applications of insecticides containing PBO and maternal exposure in this study? We recognize, of course, that these questions were not research objectives of the authors’ study, but there is information in the literature which could have been brought to bear on these questions.

Risk Characterization

A worst-case estimate of risk is informative and augments the epidemiological data. Let’s assume: 1) all mothers were exposed to PBO at the 95th percentile (7.94 ng/m3); 2) all of the PBO collected in the air samplers was bioavailable; 3) all mothers never leave the air that contains 7.94 ng PBO/m3; 4) the breathing rate is the standard USEPA value of 13.78 m3/day; and, mothers weigh 65 kg (143 lb) (4). The USEPA reference dose, also known as the acceptable daily intake, for subchronic exposures to PBO is 0.89 mg/kg body weight/day and is based on a two generation reproduction study in rat dams and pups with a no-observed- adverse-effect-level (NOAEL) of 89 mg/kg/day 2. The reference dose is set at 1% of the NOAEL to account for possible intra- and inter-species variation. Both the acute and subchronic reference doses for PBO are based on developmental/reproductive studies, which are directly relevant to the Horton et al. (1) study.

Given these assumptions, the estimate of exposure for the pregnant women would be less than 0.000002 mg/kg body weight/day. The estimated exposure would be 0.0002% of the reference dose, or <1/100,000th of the threshold. That exposure would be <1/10,000,000th of the NOAEL. Using reasonable worst-case assumptions, the USEPA 2 estimates that the aggregate exposure of PBO for adult women from all sources and routes is approximately 0.006 mg/kg body weight, making an addition of 0.000002 mg/kg negligible. If we assume that all of the PBO that enters the mother’s body enters the fetus and the fetus weighs 0.45 kg (1 lb), the estimated exposure would be 0.00024 mg/kg body weight/day, or 0.0027% of the reference dose. For PBO to possibly produce the neurodevelopmental impairments observed in the paper, it would need to be at least 3,600 times more toxic than current studies indicate. If this was so, it would be perhaps the most important toxicological discovery of the past 100 years.

In summary, the study does not demonstrate an association between permethrin exposure and 36-month neurodevelopment, even though permethrin, and not PBO, is a neurotoxin. Yet, it does demonstrate a negative association between PBO and a neurotoxic endpoint, the MDI. The study does not relate air concentrations of PBO to body burdens of PBO, does not discuss the toxicology of PBO in a comprehensive manner, nor does it discuss fundamental issues that are evident from simple, conservative risk estimates. Modest statistical associations from the study in addition to other aspects presented here when considered in toto raise serious questions as to the likelihood that PBO could be a causative factor in impairment of 36-month neurodevelopment of children.

References

1. Horton MK, Rundle A, Camann DE, Barr DB, Rauh VA, Whyatt RM. Impact of prenatal exposure to piperonyl butoxide and permethrin on 36- month neurodevelopment. Pediatrics. 2011:peds.2010-0133.

2. USEPA. Reregistration eligibility decision for piperonyl butoxide (PBO). Washington D.C.: U.S. Environmental Protection Agency; 2006 14 June 2006. Report No.: Case No. 2525.

3. USEPA. Memorandum from B. Daiss, Health Effects Division, to C. Rodia, Special Review and Registration Division. Piperonyl butoxide HED revised risk assessment for reregistration eligibility document (RED). Washington D.C.: U.S. Environmental Protection Agency; 2005. Report No.: PC Code No 067501; DP Barcode No. 313103.

4. Portier K, Tolson JK, Roberts SM. Body weight distributions for risk assessment. Risk Anal. 2007;27(1):11-26.

Conflict of Interest:

None declared