By 2020, the World Trade Center Health Program had certified more than 20,600 cancer cases among people exposed to the September 11 disaster, including both rescue workers and community members who lived, worked, or attended school near Ground Zero.1Centers for Disease Control and Prevention (CDC) – MMWR. World Trade Center Health Program — United States, 2012−2020 That number has continued climbing every year as cancers with long latency periods keep surfacing. But a raw count only tells part of the story. The question of how the dust, smoke, and chemical exposures from the collapse of the Twin Towers actually cause cancer has taken researchers more than two decades to untangle, and several pieces of the puzzle are still falling into place.
How Many Cancers Have Been Officially Counted
The federal World Trade Center Health Program tracks cancer certifications among enrolled members. As of 2020, it had recorded 20,612 cancer certifications and 50,611 noncancer certifications across responders and survivors combined. The five most common certified cancer types were skin cancer, accounting for roughly 31 percent of all cancer certifications; male genital system cancers such as prostate cancer, at about 23 percent; in situ neoplasms of the skin and breast; breast cancer; and digestive system cancers.1Centers for Disease Control and Prevention (CDC) – MMWR. World Trade Center Health Program — United States, 2012−2020 The growth rate among survivors has been especially steep, with cancer certifications in that group jumping from 188 in 2013 to nearly 8,000 by 2020.
These numbers represent people enrolled in the program who have been formally certified as having a WTC-related cancer. They almost certainly undercount the true total, because not everyone exposed to the disaster enrolled, and not every cancer diagnosed in an enrollee has been attributed to the exposure. Still, the scale is striking: more than 20,000 certified cases in the first two decades, with new diagnoses still accumulating as latency periods for some cancers stretch out.
What Was in the Dust
To understand why so many cancers developed, you have to understand what people were breathing. When the towers collapsed, roughly a million tons of building material was pulverized into an enormous dust cloud that blanketed lower Manhattan. About 80 to 90 percent of the settled dust was a highly alkaline mixture of crushed concrete, gypsum, and synthetic vitreous fibers, with particles fine enough to be resuspended by foot traffic or a light breeze.2PubMed Central. Health effects of World Trade Center (WTC) Dust: An unprecedented disaster’s inadequate risk management The pH of these samples was consistently basic, meaning the dust was chemically irritating to the eyes, throat, and lungs on contact.3PubMed Central. Chemical analysis of World Trade Center fine particulate matter for use in toxicologic assessment
But the alkaline construction debris was only the bulk of the mixture. Chemical analysis of the fine particulate matter identified a long list of known or suspected carcinogens: polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls, dioxins, asbestos at 0.8 to 3 percent of the mass, and lead at concentrations between 101 and 625 micrograms per gram. Analysts found unburned or partially burned jet fuel, plastics, soot, leaded and unleaded paint, and glass fibers throughout the plume.4PubMed Central. Characterization of the dust/smoke aerosol that settled east of the World Trade Center (WTC) in lower Manhattan after the collapse of the WTC 11 September 2001 No single chemical in this mixture acts alone. The concern from the beginning was that the combination of chronic inflammation from alkaline particulate matter plus repeated low-level exposure to multiple carcinogens would drive cancer risk upward over time. That is largely what the data now show.
Which Cancers Are Elevated, and by How Much
The first comprehensive cancer study of WTC responders, covering 2001 through 2008, estimated a 15 percent increase in all cancers combined compared to what would normally be expected in the general population, based on 575 observed cases versus about 499 expected. Certain cancers stood out: thyroid cancer was more than double the expected rate, soft tissue cancers were about double, and blood and lymph system cancers were roughly a third higher than expected.5PubMed Central. Cancer Incidence in World Trade Center Rescue and Recovery Workers, 2001–2008
A longer follow-up study spanning 14 years of data found a somewhat different picture. The overall incidence of first primary cancers among responders was actually slightly below what would be expected. But specific cancer types remained clearly elevated: melanoma of the skin was about 43 percent above expected rates, prostate cancer about 19 percent above, thyroid cancer 81 percent above, and tonsil cancer 40 percent above.6JNCI: Journal of the National Cancer Institute. Cancer Incidence in World Trade Center Rescue and Recovery Workers: 14 Years of Follow-Up The fact that the overall cancer rate was not elevated while individual types were sharply elevated reflects the “healthy worker effect,” where rescue workers tend to be healthier than the general population to begin with, which masks the excess risk in specific organs.
Lung Cancer and the Dose-Response Relationship
Lung cancer took longer to emerge in the data, partly because it has a longer latency period and partly because many responders were relatively young at the time of exposure. A 2025 study in JAMA Network Open provided some of the strongest evidence yet that WTC dust exposure directly increases lung cancer risk in a dose-dependent way. Among responders classified by their level of dust and chemical exposure, those with severe exposure developed lung cancer at a rate of about 21 per 10,000 person-years, compared to roughly 4 per 10,000 person-years among those with mild exposure. After adjusting for demographics and smoking history, the severely exposed group still had nearly three times the lung cancer risk of the mildly exposed group.7JAMA Network Open. Lung Cancer Incidence After September 11, 2001, Among World Trade Center Responders
That last point matters a great deal, because it shows the elevated risk is not simply a byproduct of smoking. Smoking was more common in some exposure groups, and it did account for part of the raw difference. But even after stripping out the effect of smoking, responders who had the heaviest dust exposure and least personal protective equipment still faced a dramatically higher risk. In total, 118 responders in the study developed lung cancer over the follow-up period. The researchers characterized the finding as the first strong demonstration of a dust-dose–lung-cancer gradient in this population.
Blood Cancers and Multiple Myeloma
One of the more troubling findings involves multiple myeloma, a blood cancer that forms in plasma cells. A large study of exposed firefighters found that they had roughly twice the risk of developing the precursor condition to myeloma compared to the general population. Firefighters who did go on to develop full-blown multiple myeloma were diagnosed at younger ages and tended to have more aggressive disease than is typical.8PubMed. 9/11 Firefighters at Risk for Multiple Myeloma
The underlying data are detailed. Screening of 781 WTC-exposed firefighters found that the prevalence of the myeloma precursor condition, known as MGUS, was about 1.8 times higher than in a reference population of white males. For the light-chain subtype of MGUS, the rate was more than three times higher.9JAMA Oncology. Multiple Myeloma and Its Precursor Disease Among Firefighters Exposed to the World Trade Center Disaster A later study that included both firefighters and other rescue workers confirmed a roughly twofold elevation in overall MGUS and a more than threefold elevation in the light-chain subtype across both groups.10Blood Cancer Journal. Myeloma precursor disease (MGUS) among rescue and recovery workers exposed to the World Trade Center disaster These precursor conditions do not always progress to cancer, but they represent a clear biological signal that the immune system’s plasma cell compartment was damaged by exposure.
The Thyroid Cancer Question and Surveillance Bias
Thyroid cancer is one of the most commonly elevated cancers in WTC-exposed populations, but interpreting the numbers requires some caution. Among rescue and recovery workers enrolled in the Mount Sinai monitoring program, thyroid cancer incidence was about twice the rate in the general New York State population.11PubMed Central. Impact of healthcare services on thyroid cancer incidence among World Trade Center-exposed rescue and recovery workers That sounds alarming, and it may well be. But thyroid cancer is also one of the cancers most susceptible to what researchers call surveillance bias: the more you screen a population, the more small thyroid cancers you find, many of which would never have caused symptoms.
A study of Fire Department personnel found that their overall thyroid cancer incidence was more than double that of two comparison populations. However, when the researchers looked only at symptomatic thyroid cancers, the rate was not significantly different from the general population. It was the rate of asymptomatic thyroid cancers, the kind found through screening, that was more than threefold higher.12JAMA Internal Medicine. Evaluation of Medical Surveillance and Incidence of Post-September 11, 2001, Thyroid Cancer in World Trade Center–Exposed Firefighters and Emergency Medical Service Workers A separate study comparing WTC-exposed New York City firefighters to non-exposed firefighters in other cities found thyroid cancer rates nearly four times higher among the exposed group, though that gap narrowed and lost statistical significance after adjusting for surveillance.13PubMed Central. Post-9/11 cancer incidence in World Trade Center-exposed New York City firefighters as compared to a pooled cohort of firefighters from San Francisco, Chicago and Philadelphia (9/11/2001-2009)
This does not mean thyroid cancer risk is unaffected by WTC exposure. It means the dramatic-looking numbers are at least partly inflated by intensive medical monitoring. The true excess risk for thyroid cancer is likely real but smaller than the raw statistics suggest. Researchers continue to track this carefully because separating genuine risk from detection artifact is one of the hardest problems in WTC cancer epidemiology.
Prostate Cancer, Melanoma, and Other Specific Types
Prostate cancer has been consistently elevated in WTC responder populations, by roughly 19 to 21 percent above expected rates depending on the study and time period.6JNCI: Journal of the National Cancer Institute. Cancer Incidence in World Trade Center Rescue and Recovery Workers: 14 Years of Follow-Up Beyond the sheer number of cases, the character of the disease appears different. Among responders with the highest WTC exposure, prostate cancers were more likely to present at an advanced clinical stage, suggesting more aggressive disease.14PubMed Central. Prostate cancer characteristics in the World Trade Center cohort, 2002-2013 Molecular analysis of prostate tumors from WTC responders showed a distinct pattern of gene activity involving inflammatory pathways and DNA damage responses, with a particular type of immune cell, Th17 cells, heavily upregulated compared to non-exposed prostate cancer patients.15PubMed Central. Prostate Cancer in World Trade Center Responders Demonstrates Evidence of an Inflammatory Cascade In rats exposed to WTC dust, the same inflammatory signature showed up in prostate tissue, providing experimental evidence that inhaling the dust could trigger immune changes far from the lungs.
Melanoma risk was about 31 to 43 percent higher among WTC responders than in the general New York State population, and a temporal analysis found the elevated risk kicked in around 2004 to 2005 and persisted through at least 2015.16PubMed Central. Temporal Aspects of the Association between Exposure to the World Trade Center Disaster and Risk of Cutaneous Melanoma The mechanism behind this elevation is less clear than for cancers of the lung or blood. Some researchers suspect that PAHs and other combustion byproducts in the dust cloud could act as skin carcinogens, especially for workers who were covered in dust for extended periods with inadequate decontamination. But this remains an area where the evidence is suggestive rather than definitive.
How Dust Exposure May Drive Cancer at the Molecular Level
Beyond straightforward carcinogen exposure, researchers have begun to identify subtler biological changes in WTC-exposed populations. A review of epigenome-wide studies found a consistent pattern: WTC-exposed responders and community survivors showed increased global DNA methylation compared to unexposed individuals. This pattern was observed independently in cancer-free survivors, in survivors with breast cancer, and in responders with prostate cancer across three separate studies.17PubMed Central. World Trade Center Exposure, DNA Methylation Changes, and Cancer: A Review of Current Evidence DNA methylation is one of the ways the body switches genes on and off, and widespread changes to that system can push cells toward uncontrolled growth over time.
An earlier epigenome-wide study attempted to link specific DNA methylation sites to a composite measure of WTC exposure intensity. No individual sites survived the strict statistical correction for testing thousands of sites simultaneously, but when the researchers looked at groups of genes, several cancer-related biological pathways were enriched among the top-ranked findings for both acute and chronic exposure measures.18PubMed Central. Enhanced exposure assessment and genome-wide DNA methylation in World Trade Center disaster responders This is the kind of early-stage molecular evidence that does not prove causation on its own but fits neatly with what the cancer incidence data are showing. The picture that emerges is one of broad, low-level genomic disruption driven by chronic inflammation and repeated chemical insult, creating fertile ground for cancer to develop across multiple organ systems rather than in just one.
Which Cancers Are Officially Recognized
The federal government moved to formally recognize WTC-related cancers in stages. In 2012, rulemaking established 24 broad cancer types, covering more than 50 specific cancers, as qualifying conditions under the WTC Health Program. These include both common and rare cancers, as well as childhood cancers. Prostate cancer was added the following year based on new scientific evidence. In 2014, the rare cancer category was further refined to include cancers of the brain, pancreas, cervix, and testis, among others with incidence rates below 15 per 100,000.19PubMed Central. The World Trade Center Health Program: Petitions for adding qualifying health conditions This broad list means that most cancer types diagnosed in an enrolled member can be certified as WTC-related, which is both generous to the affected population and a reflection of how genuinely complex the exposure mixture was.
Young People Exposed on 9/11
Most of the cancer research has focused on adult responders, but a distinct group of people who were children, adolescents, or young adults at the time of exposure is now reaching the age when cancers begin to appear. A study of WTC Environmental Health Center enrollees who were between the ages of 0 and 30 on September 11 identified 269 cancer patients with 278 cancer diagnoses. About 80 percent had solid tumors and 20 percent had blood or lymph cancers. The study documented 23 different cancer types, including types that are rare for this age group. Importantly, many of these patients lacked traditional risk factors for their specific cancer, such as tobacco use.20PubMed Central. Characteristics of Cancers in Community Members Exposed to the World Trade Center Disaster at a Young Age
This cohort is of particular concern because their exposure occurred during developmental windows when tissue growth rates are high and vulnerability to environmental carcinogens may be greater. These individuals were not rescue workers who chose to enter the disaster zone; they were children living or attending school nearby. The full cancer burden in this group will not be known for decades, as many are only now entering middle age.
The Role of PTSD in Cancer Risk
A less intuitive pathway connects the psychological trauma of 9/11 to cancer. A study of WTC responders found a positive association between PTSD re-experiencing symptoms and prostate cancer incidence, the first study to identify such a link. The researchers noted that their results were consistent with rodent experiments showing a direct biological connection between stress pathways and prostate tumor development.21PubMed Central. Risk factors for incident prostate cancer in a cohort of world trade center responders Chronic psychological stress is known to elevate cortisol, suppress certain immune functions, and promote inflammation, all of which can create an environment where cancer cells are more likely to survive and grow. For a population dealing with both massive chemical exposure and severe psychological trauma, these pathways may compound each other. The science here is still developing, but it suggests that the cancer toll from 9/11 is not purely a story of inhaled chemicals. The mental health burden is biologically intertwined with the physical one.
Why the Numbers Will Keep Rising
Many cancers linked to environmental or occupational exposures take 20 to 40 years to develop. Mesothelioma from asbestos exposure, for instance, has a median latency period that stretches well past two decades. The WTC dust contained asbestos, PAHs, dioxins, and other agents whose cancer effects play out over very long timescales. The population of exposed individuals includes tens of thousands of people who were in their twenties and thirties in 2001 and are only now entering the age range where baseline cancer rates begin to climb. When the underlying rate starts rising, even a modest percentage increase from WTC exposure translates into a large absolute number of additional cases. The certification count was above 20,000 by 2020, and researchers expect that number to keep growing for at least another decade, possibly longer, as late-latency cancers surface in both responders and community survivors.