The International Agency for Research on Cancer (IARC) has recently announced that it will be evaluating three plasticizers between June 9-16 of 2026.[1]  The additives to be evaluated include butyl benzyl phthalate (BBP), dibutyl phthalate (DBP), and diisononyl phthalate (DINP). For DINP and DBP, it is the first time being evaluated by IARC. Manufacturers, distributors, and consumers of these plasticizers should take note and prepare for IARC’s review. The ubiquity of these plasticizers in consumer products combined with recent regulatory findings on carcinogenicity, specifically for DINP, will pose a unique challenge in litigation that will likely follow the release of IARC’s evaluation.

What is IARC?

IARC is a branch of the World Health Organization (WHO) based in Lyon, France. Mandated by WHO to investigate potential causes of cancer, IARC conducts hazard evaluations of suspected carcinogens multiple times a year. The results of these evaluations are published in IARC “Monographs” — lengthy summary publications that discuss the existing literature and then apply a cancer classification based upon the Monograph Working Group evaluation. Under current IARC guidelines, it is impossible to classify any evaluated substance as “not carcinogenic” — the best the guidelines allow is “insufficient evidence to deem carcinogenic.”[2] Additionally, IARC Monograph Working Groups — with few exceptions — are only allowed to consider as part of their review published data in the peer-reviewed literature regarding substances or exposures they evaluate. In the case of regulated substances, where many safety studies are submitted to regulators but are not placed in the peer-reviewed literature, this can create a situation where IARC Working Groups only review a subset of the available data on a given compound or exposure, potentially leading to erroneous conclusions based on incomplete data sets.

IARC and Litigation

IARC evaluations have been an issue in litigation for decades, starting first with the earlier iterations of asbestos cases in the 1980s and 1990s. In recent years, IARC evaluations of the chemical aspartame (found in Diet Coke) and perfluorinated chemicals have garnered widespread attention.[3] Earlier IARC evaluations of the active ingredient in the pesticide Roundup (glyphosate), the active ingredient in the pharmaceutical drug Actos (pioglitazone), polychlorinated biphenyls (PCBs), and benzene either spurred or rekindled personal injury litigation surrounding those compounds.

In addition to inciting personal injury litigation, an IARC classification of “carcinogenic” (Group 1) or “probably carcinogenic” (Group 2A) results in automatic listing as carcinogenic under California’s Proposition 65 law. Under Proposition 65, bounties are available to private citizens or organizations who bring lawsuits claiming products contain levels of listed carcinogens that exceed the state’s safe harbor level. These Proposition 65 lawsuits have been brought alleging undisclosed carcinogens in products like coffee and French fries.[4] 

Some courts have expressed a healthy skepticism of IARC classifications.[5] Despite that skepticism, IARC classifications continue to spur litigation time and time again, and most courts still allow the findings of IARC Working Groups to provide bases for claims against corporate defendants in a wide variety of cases.

Is IARC Independent?

IARC claims to maintain a strict conflict of interest policy, but unfortunately, this has been applied inconsistently to the makeup of their Working Groups in the past. For instance, IARC has previously allowed expert witnesses in litigation for plaintiffs to serve on working groups examining exposures about which they’re testifying while excluding from participation individuals with ties to companies producing the exposures.[6] The makeup of the Monograph 142 Working Group is not finalized, though the time for nomination of Monograph Working Group members closes on August 10, 2025.

Given IARC’s disparate treatment of conflicts of interest in the past, it is possible that Working Group 142 will lack a balance of viewpoints, leading to classifications that do not reflect scientific consensus. Over the past decade, IARC classifications for a multitude of chemicals have been criticized by regulatory authorities as out-of-step with the scientific data.[7]

Current Classifications of BBP, DBP, and DINP

Overall, much of the evidence supportive of a carcinogenic hazard from plasticizers is derived from animal and mechanistic studies. Though these types of studies are useful for hazard assessment, animal models “fall far short of being able to predict human responses.”[8]

BBP

IARC has previously evaluated BBP in 2000 and concluded that the evidence of the carcinogenicity of BBP in humans was inadequate whereas the evidence of carcinogenicity in experimental animals was limited therefore resulting in a Group 3 classification of “Not classifiable as to its carcinogenicity to humans.”[9]

In 2008, the National Academy of Sciences’ National Research Council issued a final risk assessment on BBP reporting that hepatic cancer is “one of the best described carcinogenic effects of phthalates” in animal models.[10] According to the EPA, production volume in 2015 for BBP was between 10 and 50 million pounds.[11] BBP is often used as a plasticizer in artificial leather, tarps, automobile trim, traffic cones, vinyl gloves, adhesives, and weather stripping.[12]

DBP

The EPA has prioritized DBP as a High Priority Substance in December 2019, but the risk evaluation is still ongoing.[13] According to 2016 data Chemical Data Reporting, the production volume of DBP is between 1 million and 10 million lbs/year.[14] DBP is used in adhesives, sealants, paints, furniture, lubricants, textiles, leather, shower curtains, food wraps, and chemiluminescent light sticks.[15]

ATSDR issued a Toxicological Profile for DBP in 1990 concluding that DBP has a “relatively low toxicity” that requires “large amounts” to cause injury.[16] In addition, ATSDR was not aware of any studies reporting any adverse effects on humans through exposure to DBP.[17] Lastly, ATSDR reported that levels of DBP required to cause toxic effects in animals are “about 10,000 times higher than the levels of [DBP] found in air, food, or water.”[18]

DINP

California’s Office of Environmental Health Hazard Assessment concluded in 2013 that DINP is a carcinogen based exclusively on animal and mechanistic data, and the compound was placed on the Proposition 65 list of carcinogens.[19] According to the EPA, DINP reached a production volume of between 100M and 250M pounds in 2015.[20] It also has the highest production volume of all phthalates.[21] DINP has been used in both PVC products such as vinyl flooring, wire and cable insulation, stationery, fabrics, gloves, toys, garden hoses, artificial leather, footwear, and roofing materials as well as non-PVC products such as inks/pigments, paints, lacquers, adhesives, and sealants.[22]

In January of 2025, the EPA released its final risk evaluation for DINP concluding that DINP “presents and (sic) unreasonable risk of injury to human health.”[23] Furthermore, EPA concluded that DINP can cause developmental toxicity and cancer “at higher rates of exposure.”[24] The recent regulatory findings for DINP increase the likelihood of an IARC finding of carcinogenicity.

How to Prepare for a Monograph Review

Interested parties can prepare for Monograph 142 in a few ways:

1. Consider what data may not be available but could be published in the peer-reviewed literature without negatively affecting business operations. The more data that can be made available in the literature, the more data IARC—under its guidelines—will be able to consider in its review. This is especially important in areas where little data supportive of safety may be publicly available, like mechanistic data or internal manufacturing worker monitoring studies. When it comes to mechanistic data, much of the data in the published literature is of low quality due to design issues (non-OECD, product of non-GLP lab, etc) but if it is the only data a Working Group has for use, it will be used.  Greater availability of data supportive of product safety may help scientists at IARC avoid mistakes in classification, though even then, IARC may disregard certain information.[25]  
2. Send observer(s) to the Working Group meeting in Lyon, France. Though individuals with IARC-designated conflicts of interest (i.e. consulting with industry) may not participate in Working Groups, they may observe much of the meeting. Sending observers may help interested parties gain valuable insight into the thought process of the Working Group and the evidence they found most compelling to their ultimate classification.
3. Start educating the public and relevant regulatory and political bodies about what goes into an IARC classification. Specifically, the public and relevant bodies should be aware that IARC often relies upon incomplete data as the policies under which Working Groups operate do not allow for the review of data not in the peer-reviewed literature or publicly released by regulatory agencies. Thus, in many cases, IARC Working Groups do not consider many of the regulatory guideline studies necessary for product registration in the US and elsewhere. Additionally, interested parties should be aware than an IARC evaluation is a “closed” analysis – no peer review or public comment is permitted and no transcript is kept of the meeting or its deliberations. Shrouded in secrecy, exposing flaws in IARC’s process can be difficult after-the-fact. The public and relevant bodies should be informed that an IARC evaluation is a hazard assessment – in other words, it is a determination that a substance might be carcinogenic at some dose, but it does not address whether the dose people are exposed to in their daily lives could cause cancer.
4. Prepare for litigation – both of the Proposition 65 and personal injury variety. Involve in-house and potentially outside counsel in developing a strategy to deal with regulatory fallout in addition to potential litigation and customer concerns about a change in classification. During this preparation, be aware that many of the efforts outlined in the three bullets above may become subject to discovery.

By virtue of its association with the United Nations and by the participation of many individuals associated with impressive institutions, IARC Working Group classifications may be held in high regard by jurors and/or members of the general public. Early planning and a concerted, concentrated effort led by interested parties is key to ensuring sound science is employed in evaluations by IARC and other agencies reacting to IARC’s evaluation.