Health, Pesticide Adjuvants, and Inert Ingredients: California Case Study Illustrates Need for Data Access

Abstract

Background:

Many pesticide products are mixtures of multiple chemicals. These include active ingredients intended to kill pests, and so-called inert ingredients intended to improve the physical characteristics of the product. In addition, shortly before applying a pesticide product, applicators often mix adjuvants into the sprayer tank. Adjuvants are products designed to improve the performance or physical properties of a pesticide spray mixture. Manufacturers may use a particular chemical compound both as an inert ingredient within pesticide products and as a component of adjuvant products. Nonetheless, regulations dictate that data on use are publicly available only for the portion used in adjuvants. Adjuvants are exempt from federal registration, but are defined as pesticides in California. Based on that definition, California has identified $\mathrm{\alpha }\text{-(}p\text{-nonylphenyl}\text{)-}\mathrm{\omega }$-hydroxypoly(oxyethylene) (APNOHO) as the most widely used pesticide in the state, applied to more than $10\text{\hspace{0.17em}million}$ acres annually. That quantified use includes only agricultural acres treated with adjuvants containing APNOHO. Total APNOHO use is likely higher because manufacturers also use the chemical as an inert ingredient within pesticide products, although data on such use are shielded by regulation.

Objectives:

We use APNOHO as a case study to demonstrate that the use of adjuvants and inert ingredients is difficult to track because relevant information is not publicly available. We synthesize information that suggests widespread agricultural use of alkylphenol ethoxylates, such as APNOHO, may pose significant human and environmental health risks. We then make recommendations for future research and policy.

Methods:

We used information from California’s pesticide use reporting system and the U.S. Environmental Protection Agency (EPA) to estimate use of APNOHO. We used U.S. EPA and European databases, as well as published research, to identify human and environmental health hazards of APNOHO. We focused on research showing that APNOHO is an endocrine-disrupting chemical.

Discussion:

Within California, APNOHO is applied in more than 150 adjuvant products. Nationwide, it is used as an inert ingredient in at least 650 pesticide products. Exposure to APNOHO is associated with endocrine disruption, birth defects, and aquatic toxicity. We suggest that the case of APNOHO illustrates the larger problem of a lack of publicly available data on use and toxicity of many adjuvants and pesticide inert ingredients. We recommend that researchers and regulators include adjuvants and inert ingredients when evaluating pesticide hazards, including endocrine disruption. We also recommend regulatory requirements to identify all ingredients on product labels. https://doi.org/10.1289/EHP10634

Introduction

Pesticide products are widely used in agriculture and other arenas, including public spaces and homes. However, some chemical ingredients within these products have harmed human and environmental health.¹ Many pesticide products are mixtures of multiple ingredients. Toxicological studies often focus on the health impacts of the active ingredients: chemicals that are intentionally toxic to pests, such as damaging insects or weeds.² In addition to active ingredients, most pesticide products contain other chemicals included primarily to improve the physical characteristics of the product. Although termed inert ingredients,² some of these chemicals have the potential to pose health hazards.^3–6 In addition, shortly before applying liquid pesticide products, applicators often mix adjuvants into the sprayer tank (Table 1). Adjuvants are chemicals, or mixtures of chemicals, designed to emulsify, spread, or otherwise improve the effectiveness of a pesticide spray mixture.^7,8 Adjuvants are packaged and sold separately from pesticide products.⁷

Table 1.

Characteristics of adjuvants vs. inert ingredients. Manufacturers may use a single chemical compound in both roles: as an inert ingredient within pesticide products and as a component of adjuvant products.

Characteristics	Chemicals designed to improve performance of pesticide active ingredients
	Adjuvant products	Inert ingredients within pesticide products
Definition	Any product designed to improve the performance or physical properties of a pesticide spray mixture.8 Examples include emulsifying agents and adhesives. Adjuvants are packaged and sold separately from pesticide products. Applicators mix adjuvants into the sprayer tank shortly before applying a pesticide spray mixture.	Any ingredient within a pesticide product other than those designed to prevent, destroy, or repel a pest.2,66 Inert ingredients are added by the manufacturer during formulation of many pesticide products. Thus, they are already present when users purchase the pesticide product.
Registered, including safety review?	No federal registration is required. California requires manufacturers to register adjuvants as pesticides9 although data requirements are minimal relative to non-adjuvant pesticides.31	The U.S. Environmental Protection Agency must approve a chemical compound before a pesticide manufacturer may use it as an inert ingredient.16 For pesticide products intended for application on food or livestock feed, the inert ingredients must have a residue tolerance or exemption from tolerance.2 Minimal toxicology testing of formulated products is required.67
Identified by name on the product label?	In California, only the three most abundant functioning ingredients must be identified on adjuvant labels.68	Not identified by name. Pesticide labels must indicate the total percentage of inert ingredients.66
Agricultural use data publicly available in California?	As for other pesticides, users must report their adjuvant use monthly.31 Reported data on acres treated are publicly available, as well as aggregated statewide annual use totals.20 Most other data are considered proprietary and not publicly available.	Users report their pesticide use monthly, including the names of the products and their active ingredients.31 However, information about inert ingredients contained in the products is not publicly available.

Manufacturers may use a particular chemical compound in both roles: as an inert ingredient within pesticide products, and as a component of adjuvant products. This commentary highlights the potential health hazards of pesticide inert ingredients and adjuvants, using data available from California’s exceptionally robust system of product registration and use reporting. Adjuvants are exempt from federal registration, but in California they must be registered as pesticides.⁹

According to California’s most recent pesticide use reports, which include both active ingredients and adjuvants, the pesticide applied most widely (in terms of acres treated per year) in California is $\mathrm{\alpha }\text{-(}p\text{-nonylphenyl}\text{)-}\mathrm{\omega }$-hydroxypoly(oxyethylene),¹⁰ which we abbreviate as APNOHO. Based on its chemical structures (Figure 1), APNOHO is included in the alkylphenol ethoxylate family of nonionic surfactants.^11–14 Names for alkylphenol ethoxylates can be inconsistent. In this commentary we use California’s name, although other entities use other names. We also used California’s online chemical ingredient database to identify the Chemical Abstract Service (CAS) numbers associated with APNOHO. California identifies three CAS numbers for APNOHO: 127087-87-0, 26027-38-3, and 9016-45-9.¹¹ As an adjuvant, APNOHO is applied to more than $10\text{\hspace{0.17em}million}$ acres of agricultural land annually.¹⁰ It was the active component in more than 150 adjuvant products registered in California as of 2018.¹⁵

Figure 1.

Chemical structures of $\mathrm{\alpha }\mathrm{-(p}\text{-nonylphenyl)-}\mathrm{\omega }$-hydroxypoly(oxyethylene). Two chemical structures are given because of the multiple CAS numbers used for APNOHO.¹¹ Structures are from https://pubchem.ncbi.nlm.nih.gov/.

APNOHO is not used as an active ingredient in any pesticide products registered in California other than adjuvants.¹¹ Manufacturers do use APNOHO as an inert ingredient in a range of pesticide products.^12–14 The U.S. Environmental Protection Agency (EPA) does require limited safety data for inert ingredients.¹⁶ However, the U.S. EPA treats the identity and amounts of inert ingredients within any individual pesticide product as confidential business information,² so these data are difficult for the public to access. In response to our Freedom of Information Act requests, the U.S. EPA stated that there are at least 650 federally registered pesticide products that contain APNOHO as an inert ingredient. According to the U.S. EPA, approximately one-third of those are herbicides, one-third are insecticides, and one-third are fungicides.

Our assumption is that, if data were available regarding APNOHO present as an inert ingredient, the amount of California acreage treated with APNOHO would be even greater. We also assume that similar scenarios exist in other states; adjuvants that contain APNOHO are used in an average of 40 states (Table S1). Both for its quantifiable use as an adjuvant,¹⁰ and its substantial but less-quantifiable use as an inert ingredient, we selected APNOHO for our case study. We focused our case study within California because it has a comprehensive system for reporting data on pesticide use within agriculture, including adjuvant use.

We then evaluated whether widespread agricultural use of alkylphenol ethoxylates, such as APNOHO, may pose significant human health risks, based on their potential role in endocrine disruption. A prominent review by the Endocrine Society concluded endocrine-disrupting chemicals (EDCs) are of particular concern based on recent evidence that exposure is associated with increased susceptibility to serious diseases, such as cancer, cardiovascular disease, respiratory diseases, and diabetes.¹⁷ EDCs also pose ecological and environmental hazards. Multiple impacts of EDCs on wildlife have been documented, including declining population numbers and reduced reproductive success in birds, fish, and gastropods.¹⁸ The strongest evidence is for persistent legacy chemicals, but links with currently used chemicals are probable.¹⁸ Another recent review discussed altered sexual development in aquatic invertebrates caused by EDC exposure.¹⁹

Our goal in this commentary is to provide data suggesting that the use of adjuvants or pesticide inert ingredients that have health or environmental hazards is impossible to track adequately because of the lack of publicly available data. In addition, we use our case study to make research and policy recommendations for improving the quantity and accessibility of information.

Methods

Our review of California’s pesticide use reporting annual summaries¹⁰ alerted us to the widespread use of APNOHO. The use reporting website¹⁵ currently contains data only for 2017 and 2018; we also obtained information for 2000–2016 and for 2019 (California Department of Pesticide Regulation, personal communication).

We used the online search application California Pesticide Information Portal (CalPIP)¹⁵ to obtain the names of adjuvant products containing APNOHO used in California in 2018. We then used the names of the most commonly used adjuvant products (Table S1) as the search terms for an internet search to estimate how widely APNOHO-containing adjuvants are used in the United States.

We used the European Chemicals Agency’s chemical information database to find preliminary information about the endocrine disrupting activity of APNOHO.^21–23 We also used the U.S. EPA CompTox Chemicals Dashboard²⁴ to find information about the toxicology of APNOHO, as well as two pesticide active ingredients (methoxychlor and vinclozolin) that the Endocrine Society (in Table 1 of Gore et al.)¹⁷ had identified as well-documented EDCs. That table identifies a total of four pesticide active ingredients: atrazine, 1,1,1-trichloro-2,2-bis($p$-chlorophenyl)ethane (DDT), methoxychlor, and vinclozolin.¹⁷ To provide the most relevant benchmark for APNOHO, we chose the two active ingredients (methoxychlor and vinclozolin) for which CompTox also had reported endocrine activity for APNOHO based on the same assays.²⁴

We used ToxCast data²⁴ to evaluate the endocrine-disrupting potency of APNOHO by comparing the activity of APNOHO with the activity of the two active ingredients identified as EDCs¹⁷ (methoxychlor and vinclozolin) in assays designed to measure estrogenic, androgenic, progesterone-related, and thyroid-related activity. We used the concentration where half maximal activity occurs (${\text{AC}}_{50}$)²⁵ as the comparison metric. However, we excluded assays with an ${\text{AC}}_{50}$ greater than the cytotoxicity limit because such results can be caused by general cell stress rather than endocrine disruption.²⁵

Google Scholar was our primary source for evaluations of APNOHO that examined hazards other than endocrine disruption. We also used the U.S. EPA’s Substance Registry Services²⁶ and the National Library of Medicine’s PubChem.²⁷

Discussion

Reported Use

APNOHO is widely used as an agricultural adjuvant. Based on data provided by the California Department of Pesticide Regulation to the authors, APNOHO was applied to $10.45\text{\hspace{0.17em}million}$ acres as an adjuvant in 2019. This is 1.8 times the acreage treated with the second-most widely used chemical ($5.82\text{\hspace{0.17em}million}$ acres treated with dimethylsiloxane, another adjuvant). Acreage figures are cumulative, meaning that the acres treated in each application are summed when the same site is treated more than once in a year.²⁰

Agricultural acreage treated with APNOHO has almost doubled since 2000, when reported use was $5.35\text{\hspace{0.17em}million}$ acres (Figure 2). The amount used has more than doubled from less than 500,000 kg in 2000 to more than $1\text{\hspace{0.17em}million}$ kg in 2019. These increases occurred despite a small decrease in overall farmland in California during the same period. California farmland has decreased from $27\text{\hspace{0.17em}million}$ acres in 2002²⁸ to $24\text{\hspace{0.17em}million}$ acres in 2020.²⁹

Figure 2.

Agricultural acres treated in California with adjuvants containing $\mathrm{\alpha }\mathrm{-(}p\text{-nonylphenyl}\mathrm{)-}\mathrm{\omega }$-hydroxypoly(oxyethylene). Data provided by the California Department of Pesticide Regulation (personal communication).

In addition, people may be exposed to other alkylphenol ethoxylate adjuvants. According to data provided to the authors by the California Department of Pesticide Regulation, two other adjuvants that are chemically related to APNOHO, namely $\mathrm{\alpha }\text{-(}p\text{-nonylphenyl}\text{)-}\mathrm{\omega }$-hydroxypoly(oxyethylene)phosphate ester and $\mathrm{\alpha }\text{-(}p\text{-dodecylphenyl}\text{)-}\mathrm{\omega }$-hydroxypoly(oxyethylene), were applied to more than $2\text{\hspace{0.17em}million}$ acres in 2019.

Regulations that shield ingredient information² make it difficult to obtain information about the use of APNOHO as an inert ingredient within pesticide products. The U.S. EPA has approved all three of the relevant CAS numbers for use as inert ingredients in pesticide products, according to the U.S. EPA’s InertFinder.^12–14 Even though product-specific information on ingredients or extent of use are not publicly available, manufacturers use APNOHO as an inert ingredient in enough pesticide products (650 as compared with 150 adjuvant products) that we believe the extent of this use is significant.

We focused our case study within California because of its relatively comprehensive registration and use reporting requirements. Our analysis would likely not be possible for other U.S. states because of the lack of information available there. Unlike most U.S. states,³⁰ California requires manufacturers to register adjuvants as pesticides.⁹ Adjuvant registration requires submission of some toxicity data, although data requirements are minimal relative to nonadjuvant pesticide active ingredients.³¹ California’s system for reporting pesticide use is detailed for agricultural pesticide use, but it is much less detailed for pesticide use outside of production agriculture, requiring only monthly summaries with less information about amount of area treated.^20,32 In short, the range of pesticide chemistry and uses that we could evaluate for a potential case study was constrained by data availability. This lack of information is troubling given APNOHO’s association with human and environmental health hazards, as summarized below.

Health Hazards

European authorities have identified APNOHO as an EDC^21–23 but relevant testing has not been required by U.S. agencies. A 2021 report from the U.S. EPA’s Office of Inspector General concluded that the agency “has not implemented Section 408(p)(3)(A) of the Federal Food, Drug, and Cosmetic Act to test all pesticide chemicals for endocrine-disruption activity.”³³ In this context, “pesticide chemical” includes both active and inert ingredients. According to the report, the U.S. EPA has issued test orders for only 52 pesticide chemicals of 1,315 that should be tested.³³

The U.S. EPA’s CompTox Chemicals Dashboard²⁴ summarizes data generated by the agency’s in vitro high-throughput screening programs. The CompTox Dashboard includes assay results for one of the CAS numbers (26027-38-3) associated with APNOHO.³⁴ For that CAS number, the CompTox Dashboard reports results from 53 assays that assess EDC activity on androgen and estrogen, two common endocrine hormones.³⁴ We excluded 14 of those assays because the ${\text{AC}}_{50}$ was greater than the cytotoxicity limit. This left 39 androgen and estrogen assays that report results for APNOHO. APNOHO showed EDC activity in 10 of those assays. To provide context about the magnitude of that activity, we looked for CompTox results for the pesticide active ingredients methoxychlor³⁵ and vinclozolin³⁶ in the same 10 assays. CompTox reported EDC activity for methoxychlor and vinclozolin in 4 of the 10 assays^35,36; in 3 of the 4 (75%) APNOHO had a lower ${\text{AC}}_{50}$, suggesting APNOHO is a more potent EDC than the comparison pesticide active ingredients. Table 2 gives details about the 4 assays used for these comparisons.

Table 2.

Relative endocrine-disrupting potencies of APNOHO vs. pesticide active ingredients previously identified as endocrine-disrupting chemicals (EDCs).^34–36

Assay name	Gene name	${\text{AC}}_{50}$ ($\mathrm{\mu }\mathrm{M}$)
		APNOHO (CAS number 26027-38-3)	Pesticide active ingredients previously identified as EDCs17
			Methoxychlor	Vinclozolin
OT_AR_ARELUC_AG_1440	Androgen receptor	0.36	—	5.23
TOX21_AR_LUC_MDAKB2_Antagonist_0.5nM_R1881	Androgen receptor	2.26	—	1.07
TOX21_AR_LUC_MDAKB2_Antagonist_10nM_R1881	Androgen receptor	1.56	—	11.00
ACEA_ER_80hr	Estrogen receptor 1	0.99	6.80	—

Note: Within a given assay, a lower ${\text{AC}}_{50}$ suggests greater endocrine-disrupting potency. —, not applicable; ${\text{AC}}_{50}$, concentration at which half maximal activity occurred; APNOHO, alpha-(para-nonylphenyl)-omega-hydroxypoly(oxyethylene); CAS, Chemical Abstracts Service.

We found seven studies published since 1993 that measured endocrine-disrupting activity of chemicals with APNOHO’s CAS numbers. The two studies published prior to 2000 showed that APNOHO induced vitellogenesis, a marker of estrogenic activity, in fish.^37,38 A 2000 in vitro study showed that Activate Plus, one of the adjuvants identified as containing APNOHO,¹⁵ caused proliferation of estrogen-responsive breast cancer cells.³⁹ A 2005 study showed that exposure to R-11, another APNOHO-containing adjuvant product,¹⁵ combined with the herbicide active ingredient triclopyr, was associated with increased vitellogenesis in rainbow trout.⁴⁰ The three most recent studies found that APNOHO was antagonistic to estrogen activity⁴¹ or to the activity of estrogen, androgen, and thyroid hormone.^42,43 All three studies identified the alkylphenol ethoxylate studied as CAS number 9016-45-9,^41–43 which is one of the CAS numbers associated with APNOHO.¹⁵ Overall, our analysis of published studies and the U.S. EPA’s high-throughput screening results suggest support for the EU’s classification of APNOHO as an EDC.

In addition to endocrine disruption, APNOHO exposure has been associated with other significant health hazards. For example, a laboratory study using SPF rats found that exposure to Antarox CO 630 (CAS No. 26027-38-3) during pregnancy was associated with reduced litter size, increased embryo loss, and offspring with extra ribs and abnormal pelvises.⁴⁴ Another laboratory study found that exposure of CD (Sprague-Dawley) rats to Tergitol NP-4 (CAS No. 127087-87-0) during pregnancy was associated with cleft palate and abnormal ribs in offspring.⁴⁵ Residential proximity during pregnancy to APNOHO applications in California was associated with approximately double the adjusted odds ratio for craniosynostosis, a birth defect.⁴⁶ According to the Globally Harmonized System of Classification and Labeling of Chemicals (GHS), APNOHO is “suspected of damaging fertility or the unborn child.”⁴⁷ A surfactant with CAS No. 9016-45-9 has also been associated with cytotoxicity and genotoxicity in human neuroblastoma SK-N-SH cells in vitro.⁴⁸

We found little information about human exposure to APNOHO. Although California has more extensive pesticide monitoring programs than most states, APNOHO and other adjuvants are not included in the state’s monitoring of food (Table S2), air,⁴⁹ or groundwater.⁵⁰ A recent critical review found a small number of studies investigating environmental contamination by APNOHO: one study of mussels along the California coast and three studies of ground- or surface water.⁵¹ Other alkylphenol ethoxylates and metabolites are more frequently studied and have been found in food and human tissues.⁵¹

We did not find toxicity studies that evaluated interactive effects, such as potential synergistic toxicity, between APNOHO and other pesticides. However, we believe this is an important issue and a potential hazard. A recent systematic review examined toxicity studies contrasting the toxicity of active ingredients alone vs. the toxicity of pesticide products that contained both active and inert ingredients.⁶ The review concluded that 75% of such studies found increased toxicity when inert ingredients were present.⁶

Overall, publicly available toxicity data about APNOHO is limited. However, the in vitro studies conducted over several decades,^{37–39,41–43} together with three in vivo studies^40,44,45 and an epidemiological study,⁴⁶ indicate concerns for endocrine disruption and developmental toxicity. Current evidence is strongest in studies of laboratory animals, but it is significant for humans as well.

Environmental Hazards

APNOHO exposure also has been associated with environmental hazards in addition to the EDC activity in fish described above.^37,38,40 The GHS classifies APNOHO as “very toxic to aquatic life with long lasting effects.”^47,52 R-11, an adjuvant containing APNOHO,¹⁵ reduced Ceriodaphnia dubia population growth more than the insecticide active ingredient imidacloprid, and the combination of the two reduced population growth more than either R-11 or imidacloprid alone.⁵³ R-11 also caused extinction of experimental Daphnia pulex populations at concentrations below the environmental concentration expected from R-11 applications.⁵⁴ APNOHO’s three CAS numbers are identified as hazardous chemicals under the Toxic Release Inventory^55–57 and one is identified as hazardous under the Clean Air Act.⁵⁷ All three APNOHO CAS numbers were added to the Toxics Release Inventory because of the compounds’ high toxicity to aquatic organisms.⁵⁸ One of the CAS numbers (9016-45-9) was also identified under the Clean Air Act because of its role in creating ozone pollution.⁵⁹

Relevance to Other Pesticides

We believe that the issues we highlight with respect to APNOHO are not unique to this particular chemical but, rather, are relatively widespread. For example, California Department of Pesticide Regulation data show that there are 37 adjuvants other than APNOHO on California’s list of 100 most widely applied pesticides (measured as acres treated). At least 15 of them are also on the U.S. EPA’s list of approved inert ingredients (Table S3). Researchers seeking data about their use would face the same challenges illustrated by our case study.^2,15 Researchers studying use outside of California would face even greater limitations on data availability.

Recommendations

Given APNOHO’s widespread use, its association with health and environmental hazards, and the potential for other adjuvants and inert ingredients to have similar concerns, we suggest that this case study indicates a clear need for changes in pesticide research, policy, and regulation. We make the following recommendations for improving the quantity and accessibility of data about pesticides.

Recommendations for researchers are as follows:

1. Researchers should include widely used adjuvants and inert ingredients in studies of pesticide toxicology and epidemiological studies of pesticide exposure. Appropriate toxicological and statistical methods for chemical mixtures are being rapidly developed.⁶⁰

2. Researchers should obtain and report the names and CAS numbers of all ingredients contained in the products they study. This includes studies of pesticide products and of adjuvants. If such information is unavailable because of proprietary or trade secret claims, researchers as much as possible should use analytical techniques to identify all ingredients.

3. Few pesticide chemicals have been studied for endocrine-disrupting activity, partly because of minimal data requirements.^17,33 To help address this lack of data, researchers should include, when feasible, in vivo or in vitro testing for endocrine-disrupting activity when studying the toxicology of pesticide active ingredients, inert ingredients, and adjuvants.⁶¹

Recommendations for regulators and policymakers are as follows:

1. Pesticide registration assessments should not be based on assessing single-pesticide ingredients but, rather, assessing the combination of ingredients to which applicators, the public, and ecosystems are exposed when a pesticide product is used.

2. Regulators outside of California should consider requiring registration of adjuvants and reporting of pesticide use comparable to California’s requirements.^9,31 The availability of California data has facilitated more than 500 papers in scientific journals.⁶²

3. The identity and concentration of all ingredients should be identified on pesticide product labels and safety data sheets. It has been 25 y since the American Medical Association made this recommendation.⁶³ Implementation is overdue.

In the past, improving public access to data about emissions of toxic chemicals has prompted important health and safety improvements. For example, just 2 y after Toxic Release Inventory data were first made publicly available, the General Accounting Office found that the data had prompted the U.S. Congress to expand the regulation of toxic chemicals under the Clean Air Act, helped states to enact pollution control laws, and spurred companies to make commitments to reduce emissions of pollutants.⁶⁴ Reduction in emissions continues currently.⁶⁵ We hope that our recommendations, based on the lessons learned from this California case study, will lead to similar improvements with respect to adjuvants and pesticide inert ingredients.