In 2011, a team of environmental health experts affiliated with CHE created the Toxicant and Disease Database (TDD). The database was designed to address a common challenge in environmental health: while research on links between chemical exposures and disease is extensive, reviewing and evaluating that evidence, and translating it into clear, trustworthy summaries, is an incredibly time- and resource-intensive task.
CHE’s constituents – including scientists, physicians, policymakers, advocates, and interested individuals – needed a clear, reliable, easy-to-use resource that could translate complex science into usable information.
The database filled this gap by organizing toxicant-disease associations into easy-to-read lists and categorizing them by the strength of the evidence, based on expert judgment. This approach added clarity while reflecting nuance in the literature, but it also came with a limitation. Because the expert review process is so intensive, it is difficult to keep pace with the rapidly expanding research base. This tension between rigor and timeliness motivated our recent updates to the TDD, which are described below.
Linking to the Comparative Toxicogenomics Database
To help keep the TDD current, we added links to the Comparative Toxicogenomics Database (CTD). The CTD is a public resource that curates published literature on chemicals, diseases, genes, and biological processes. It’s mainly used by researchers, but non-scientists, including community organizations and advocates, also use it to explore environmental contributors to disease.
The CTD’s strength is its scale and timeliness. Research queries are automated, and curators review new studies monthly, capturing statistically significant findings from human, animal, and in vitro studies. This makes it possible to see a lot of up-to-date research quickly, something nearly impossible to do through expert review alone.
Importantly, the CTD sorts through existing research and curates evidence by collecting studies with statistically significant results. However, it does not apply causal inference or assess strength or quality of evidence. That’s still a unique value of the Toxicant and Disease Database: expert judgement is applied to synthesize and categorize information using toxicologic and epidemiologic reasoning.
Linking to CTD gives users a way to access newer data when our listings haven’t been updated yet, while preserving the added clarity and interpretation that comes from expert review. The CTD is an incredible resource, and we hope our linking to it brings more visibility to it from CHE’s audience.
Walking through an example: Bisphenol A
To illustrate how the TDD and CTD complement each other, let’s look at bisphenol A (BPA) as an example. This same process can be followed for any chemical or disease listed in the database.
Step 1: Starting in the TDD
In the TDD, the bisphenol A page shows a list of diseases for which there is good evidence of a connection (e.g altered neurodevelopment) and diseases for which evidence is more limited (e.g abnormal sperm), based on the research available at the time the page was created. Further down on the page, it includes references and notes on evidence quality.
Step 2: Linking out to the CTD
From the TDD, the bisphenol A page now also includes a link to CTD’s curated results. The image below shows both the original information and the link to the CTD that you’ll find on the page.
We link only to the CTD’s curated results, so users see associations that are directly reported in research studies. The image below shows what you will see when you click the link that takes you from TDD’s bisphenol A page to the curated results on the CTD page.
From here, users can navigate to other pages to see what phenotypes and genes bisphenol A is associated with. The tab "Exposure Studies” specifically lists references where human population exposure of the chemical is studied. Additionally, users can click on the number in the References column for each specific disease to see all the references for that chemical-disease relationship.
In addition to curated results, the CTD also provides inferred relationships (information on associations , rather than ones predicted through gene network analyses); we have not linked to those, but they can be found readily on the CTD site if needed.
For example, in the row showing “prostatic neoplasms,” you can click on the “References” to see a list of references on the relationship between bisphenol A and prostatic neoplasms. (NOTE: when navigating to a new page, for example the list of references, you might need to reset the filter at the top to only show curated associations. We recommend looking at curated relationships only, unless you are specifically interested in pathways inferred through gene network analysis.)
What this adds for users
By walking through this example, we see how CHE’s Toxicant and Disease Database and the Comparative Toxicogenomics Database complement each other:
- TDD provides synthesis and expert interpretation: It organizes the literature, evaluates evidence strength, and communicates it in a way that is easy to understand. However, it is based on older studies and currently does not cover vast quantities of new research.
- CTD provides breadth and timeliness: It shows the studies on associations between chemicals and diseases and related gene/phenotype information, updated monthly. It does not include expert review of the strength or quality of the evidence for any chemical-disease association.
This allows users to start with a concise, reliable summary in the TDD and, if desired, dive into the up-to-date, curated CTD evidence for more detail or to fill in gaps missing in the TDD.
The Toxicant & Disease Database in context
While doing the research for this update, we learned a lot about the landscape of environmental health databases and tools, and the tradeoffs that must be considered when deciding on an approach to create and maintain one. Environmental health studies vary widely in quality, methods, and relevance, and it takes time and resources to sort through what’s biologically plausible, consistent, and rigorous. It can also be a major challenge to keep up with the consistent stream of new research.
As a result, there is substantial variation in the methodology behind existing tools, and thus they each provide their own unique contribution.
Another important resource to be aware of is the Pharos database, a project of Habitable (formerly Healthy Building Network). Pharos relies on authoritative lists such as California’s Proposition 65 or IARC classifications to classify and organize information on chemical health hazards. This differs from the CTD’s automatic curation of statistically significant studies, and TDD’s independent literature review and synthesis. This approach is valuable and rigorous, but tends to stay at the level of broad disease categories rather than specific conditions, and can inherit and reinforce any biases or limitations present in the original reviews conducted by authoritative bodies.
Throughout this process we have been impressed by the quantity and quality of other databases and resources. We have also discovered that the TDD is unique in the independent expert review, synthesis, and inference conducted in its creation. This adds immense value and sets the TDD apart from other existing resources, but it also takes a lot of time. CHE doesn’t currently have the capacity to consistently update the TDD to keep up with new studies as the literature continues to grow. While we’re actively exploring ways to support more thorough and ongoing updates in the future, linking to CTD helps fill that gap for now by giving users a place to find more recent research when they want to explore further.
In short, this update is about making the TDD a richer and more useful resource. By linking our expert-reviewed summaries to the Comparative Toxicogenomics Database, we’re combining clear summaries of older research with access to automatically updated and curated new literature.
We encourage you to explore the updates and share feedback on how to continue to improve this resource.
Note: CHE’s Toxicant and Disease Database was originally created by Dr. Sarah Janssen, Dr. Gina Solomon, and Dr. Ted Schettler.