The World Trade Center (WTC) Health Program (WTCHP) is a long-term, federally funded health program that provides medical monitoring and treatment for specific WTC exposure-related medical conditions included in the aerodigestive, cancer, and mental health categories to enrolled members directly affected by the September 11, 2001 terrorist attacks on the WTC in New York City (NYC) [1], [2]. In 2020, a total of 76,543 responders were enrolled in the WTCHP, of which 43,503 had at least one certified WTC-related health condition [1].

Studies of 9/11 responders by different entities, including the Fire Department of the City of New York (FDNY), the General Responder Cohort (GRC) at the Icahn School of Medicine at Mount Sinai, and the WTC Health Registry (WTCHR) at the New York City Department of Health and Mental Hygiene, have reported similar long-term health effects associated with a high level of WTC exposure. This includes increased risk of respiratory diseases, gastroesophageal acid reflux (GERD), cancer (e.g., prostate cancer), cardiovascular conditions, and mental health disorders (e.g., post-traumatic stress disorder (PTSD), and depression) among 9/11 responders [3], [4], [5], [6], [7], [8], [9], [10].

Collaborative studies using a de-duplicated combined cohort of 69,100 9/11 responders from FDNY, GRC, and WTCHR [11] have suggested the potential benefit of longer cancer survival [12] and lower all-cause mortality risk among those who were members of the WTCHP at FDNY or GRC compared to 9/11 responders who were not members of FDNY or GRC [13]. However, these prior studies [12], [13] were unable to adjust for comorbidities because the data on comorbidities were not collected using consistent questions or methods across the three cohorts, preventing the data from being harmonized for analysis. As a result, it remains unclear whether the observed mortality benefits are attributable to WTCHP membership itself or to unmeasured differences in baseline health status.

In this study, we addressed this gap by comparing mortality risk between WTCHP members and non-members, using additional data from the WTCHR. The WTCHR, one of the three contributing cohorts to the combined cohort of 9/11 responders, offers a unique advantage in having consistently collected comorbidity information across periodic follow-up surveys. This cohort also includes a comparison group of 9/11 responders who were not members of FDNY or GRC cohorts, allowing for comparisons in mortality risk over time, and enhancing internal validity and interpretability.