Ninety-Five Percent of Chemicals, Never Tested

Roughly 350,000 synthetic chemicals are registered for commercial use globally, but fewer than 5% have undergone comprehensive toxicological assessment for chronic, low-dose exposure effects. The standard approach — two-year rodent bioassays — costs $2–6 million per chemical and takes 3–5 years, making it impossible to screen the backlog. Newer high-throughput screening methods (in vitro assays, computational toxicology) can flag acute toxicity but cannot reliably predict chronic effects like endocrine disruption, developmental neurotoxicity, or carcinogenicity from low-dose, long-duration exposure — the exposure pattern most relevant to human health.

Chemical pollution is responsible for an estimated 9 million premature deaths per year — more than malaria, HIV, and tuberculosis combined. Deaths from "modern" pollutants (industrial chemicals, pesticides, pharmaceutical wastes) are increasing, having risen 66% since 2000, while deaths from "traditional" pollutants (household air pollution, unsafe water) have decreased. The chemicals with the least toxicological data are often the ones produced in the largest volumes. PFAS compounds circulated for decades before their persistence and bioaccumulation were characterized; similar latent risks likely exist among thousands of newer compounds.

The EPA's ToxCast/Tox21 program has screened ~10,000 chemicals across ~700 in vitro assays, but these assays measure molecular-level endpoints (receptor binding, enzyme inhibition) that don't reliably predict organism-level chronic effects. QSAR (quantitative structure-activity relationship) models can predict some toxicity endpoints from molecular structure but have poor accuracy for complex endpoints like developmental toxicity (AUC typically 0.6–0.7). Organ-on-chip and organoid models offer better physiological relevance than cell-based assays but face the same challenge as all in vitro methods: no validated mapping exists from in vitro response to in vivo chronic effect at population scale. The European REACH regulation requires toxicity data for chemicals produced above 1 ton/year but compliance is low and penalties are weak.

An integrated approach combining high-throughput in vitro screening, QSAR prediction, and human biomonitoring data (exposome studies) could enable tiered screening that prioritizes the most concerning chemicals for deeper assessment. The key missing piece is a validated adverse outcome pathway (AOP) framework that connects molecular initiating events (measurable in vitro) to adverse outcomes in organisms. If even 10 well-characterized AOPs were validated for the most common toxicity endpoints, they could serve as bridges from high-throughput data to regulatory decisions.

A student team could take an existing ToxCast dataset for a specific chemical class (e.g., organophosphate flame retardants) and attempt to build a predictive model linking in vitro assay results to known in vivo outcomes from the literature, evaluating which assay combinations are most predictive. Alternatively, teams could develop visualization tools for the ToxCast/Tox21 database that help regulators identify chemicals with concerning activity profiles that lack in vivo studies. Relevant disciplines: computational biology, toxicology, data science, environmental engineering, public health.