US cancer rates among those aged 15 to 49 have increased by 10% since 2000, even as they have fallen among older people.
Gary Patti leaned in to study the rows of plastic tanks, where dozens of translucent zebrafish flickered through chemically treated water. Each tank contained a different substance - some notorious, others less well understood - all known or suspected carcinogens.
Patti’s team is watching them closely, tracking which fish develop tumours, to try to find clues to one of the most unsettling medical puzzles of our time: why are so many young people getting cancer?
The trend began with younger members of Generation X but is now most visible among millennials, who are being diagnosed in their 20s, 30s and early 40s - decades earlier than past generations. Medications taken during pregnancy, the spread of ultra-processed foods, disruptions to circadian rhythms - caused by late-night work, global travel and omnipresent screens - and the proliferation of synthetic chemicals are all under scrutiny.
Older Americans are still more likely to be diagnosed than younger ones. But cancer rates among those aged 15 to 49 have increased by 10% since 2000, even as they have fallen among older people, according to a Washington Post examination of federal data.
Young women are more affected than men. From ages 15 through 49, women have a cancer rate that is 83% higher than men in the same age range.
The rise in early-onset cancers has drawn a growing number of scientists into a shared investigation: not into the inherited traits that remain largely unchanged as a cause of cancer across generations, but into the ways modern life might be rewriting the body’s cellular fate. The new research direction examines the “exposome” - the full range of environmental exposures a person experiences throughout his or her life, even before birth - and how those exposures interact with biology.
Many researchers are focusing on a window that opened in the 1960s and ’70s and accelerated in the ’80s and beyond, when a wave of new exposures entered daily life.
Certain medications taken during pregnancy may disrupt fetal development or programming of gene activity, potentially increasing susceptibility to early-onset cancers.
Exposure to environmental chemicals - including those in microplastics that accumulate in tissues after being ingested or inhaled - can increase the risk of hormonal imbalances, genetic mutations, inflammation and other effects that contribute to early cancers.
A diet that contains large quantities of highly processed food can influence cancer risk by promoting inflammation, obesity and metabolic changes that may trigger tumorigenesis.
Disruption of circadian rhythms may impair DNA repair mechanisms and hormone, metabolic and immune regulation, heightening the risk of early-onset malignancies.
The research is sprawling and interdisciplinary, but it is beginning to align around a provocative hypothesis: shifts in everyday exposures may be accelerating biological aging, priming the body for disease earlier than expected.
“We’ve changed what we’re exposed to considerably in the past few decades,” said Patti, a professor of chemistry, genetics and medicine at the Washington University School of Medicine in St Louis.
The sheer complexity of modern life makes it difficult to pinpoint specific culprits. But advances in rapid, high-volume chemical screening, machine learning and vast population datasets have made it possible to look with unparalleled depth and detail into the human body and the world around it. These methods test thousands of variables at once, revealing some never-before-seen patterns.
Last year, researchers released findings from a 150,000-person study at the annual American Association for Cancer Research meeting that took the cancer community by surprise. They found that millennials - born between 1981 and 1996 - appear to be aging biologically faster than previous generations, based on biomarkers in blood that indicate the health of various organs. That acceleration was associated with a significantly increased risk - up to 42% - for certain cancers, especially those of the lung, gastrointestinal tract and uterus.
Much of the work in this area is in its early stages and has not proved a direct cause and effect in humans. The evidence comes from epidemiological studies, which look at patterns of disease in large populations; observational studies, which track people’s behaviours and exposures without intervening; and animal models, which are sometimes, but not always, good proxies for people. Such research is difficult to interpret and especially prone to overstatement or misreading of the data.
John Ioannidis, a professor of medicine, epidemiology and population health at Stanford University, said research that searches for correlations across large datasets is highly susceptible to producing spurious results. While he believes there is strong and growing data that there are a lot of harmful exposures in today’s environment, he emphasised, “We should not panic and think everything new we live with is toxic.”
Identifying the forces behind the rise in cancer among young people is only the first step. Confronting them and developing treatment may be an even more complex task. Microplastics drift through our bloodstreams; synthetic chemicals line our homes, our food, our clothes; and modern medicine depends on many of the same substances that may be contributing to the problem.
Researchers say the surge in cancer cases among young adults reflects a deeper trend in human health: a number of major diseases, from heart disease to Alzheimer’s disease, aren’t just being detected earlier - they’re actually starting earlier in life.
“This is not just about cancer,” said Yin Cao, an associate professor of surgery at the Washington University School of Medicine in St Louis whose team led the accelerated-aging study. “This is a universal problem across different diseases.”
Maternal medications
Modern medicine has profoundly altered the experience of pregnancy. Women giving birth in the second half of the 20th century were treated with drugs not as an exception, but as a new standard. Antidepressants, anti-nausea medications, antibiotics, hormone treatments - even in combinations, sometimes all in one trimester - heralded a new normal of active pregnancy management.
At the time, these developments were seen as progress; the pregnancy was safer and more comfortable thanks to science. However, as researchers revisit this era with new methods and by examining how events unfolded over an extended period - and with the discovery of the link between the morning sickness drug thalidomide and birth defects in the 1960s - a more complicated story has emerged.
What if a drug’s real risk may not be apparent in the days or weeks after birth, but only show up years - or possibly decades - later?
Caitlin Murphy, a professor and cancer epidemiologist at the University of Chicago, found herself wrestling with exactly this question. While combing through epidemiological data, she noticed a curious trend. The rise in cancer diagnoses tracked with birth year.
But rather than a steady increase across the board, cancer rates appeared to spike among millennials. The pattern, Murphy realised, was about a birth cohort, a group of people born during the same period.
“The rates weren’t just increasing with age - they varied dramatically by generation,” she explained.
At 37, Murphy had personal reasons to care. Her mother was diagnosed with and died of cancer in her 40s. Now, nearing that age herself, Murphy began to wonder whether the mystery of rising early-onset cancers might begin not in adolescence but in gestation.
To find out, she turned to one of the longest-running maternal-health studies in the United States - a cohort in Northern California that began collecting blood samples from pregnant women in 1959. The mid-century period, Murphy knew, was a golden age of medical intervention in pregnancy: a time when hormonal treatments, sedatives and experimental drugs were widely prescribed to expectant mothers, often with little long-term follow-up.
By linking these prenatal medical records to statewide cancer registries, Murphy determined that children whose mothers had taken bendectin, an anti-nausea drug, during pregnancy were 3.6 times more likely to develop colon cancer as adults, when all other factors were taken into consideration. Even more startling was that children of women who received a different medication to prevent miscarriage, hydroxyprogesterone caproate, had more than double the overall lifetime cancer risk. In this group, about 65% of cancers occurred before the age of 50.
Bendectin was voluntarily withdrawn from the market in 1983 amid concerns about birth defects. Follow-up testing found no link with birth defects. The Food and Drug Administration withdrew its approval for a brand name and generic hydroxyprogesterone caproate in 2023 for preventing preterm birth after a large clinical trial failed to prove the drug works.
Diet
By the 1980s and ’90s, a new kind of diet had become the norm.
Shelf-stable snacks, frozen entrées, sugary cereals and reconstituted meats filled lunch boxes, cupboards and grocery store aisles. It was a drastic change in food habits compared with generations past, which had grown up with diets made up mostly of meals cooked at home with whole foods.
Today, ultra-processed foods account for more than half of the total daily calorie intake in the United States, among other countries. Designed for flavour, convenience and shelf stability, they have been correlated with rising rates of obesity and metabolic disease - and perhaps a rise in cancer in young adults.
A 2023 study published in the BMJ found that heavy consumption of ultra-processed foods was associated with significantly elevated risks of developing several cancers, including colorectal and breast cancer - two of the fastest-rising malignancies in people under 50.
According to the Post analysis of the latest data, breast, thyroid, colon-rectum, skin and cancers of the testes are the most common diagnoses for young adults. Young people are more likely to suffer late diagnoses of some of the most common cancers.
Andrew Chan, a gastroenterologist at Massachusetts General Hospital and professor of medicine at Harvard Medical School, is co-lead of a global research initiative launched in 2024 to investigate the surge in colon cancer among young adults. In May, his team presented early findings suggesting a troubling link. Individuals under 50 who consumed the largest quantities of ultra-processed foods faced a 1.5-fold increased risk of developing early-onset colon tumours.
The association, Chan emphasised, isn’t simply about weight gain.
“Ultra-processed foods appear to have independent metabolic effects that could have negative consequences on human health,” Chan said.
Scientists are examining a variety of ways these products could cause cancer: chronic inflammation caused by additives, the disruption of gut microbiota by emulsifiers, carcinogenic compounds formed during high-heat cooking, and changes to hormones from excess sugar and carbohydrates. Even packaging might play a role, because leaching chemicals, particularly when heated, from plastics may disrupt the balance of hormones in the body.
As part of his research, Chan is preparing a clinical trial to test whether the new generation of diabetes and weight loss drugs such as Zepbound can slow molecular changes associated with cancer in younger adults. If industrial food has affected a generation’s health, he wonders, can that trajectory be altered?
Circadian rhythm
Nearly every organism on Earth, from bacteria to humans, runs on a biological rhythm shaped by the rotation of the planet. This internal clock - the circadian system - regulates functions from hormone release to cell repair, syncing the body to the 24-hour cycle of light and dark.
But over recent decades, the explosion of artificial light, erratic work schedules and 24/7 digital connectivity has fundamentally altered when and how we sleep, eat and rest. As a result, researchers, many of whom have been funded by the National Institute of General Medical Sciences, say the biological processes that rely on the rising and setting sun - like immune regulation, endocrine control and metabolic functions - may unravel.
Melatonin, a hormone produced in darkness, plays a crucial role in this system. But in today’s glowing, sleepless world, melatonin production is regularly disrupted.
Research has linked chronic circadian misalignment to higher risks of breast, colorectal, lung, liver and pancreatic cancers, all increasingly diagnosed in younger populations. And in 2007, the International Agency for Research on Cancer declared shift work that disturbs circadian rhythms a probable human carcinogen.
Katja Lamia, a professor of molecular and cellular biology at Scripps Research, found that mice with lung cancer exposed to conditions that simulate chronic jet lag developed 68% more tumours than those that got more regular sleep.
At the University of California at Irvine, Selma Masri found similar effects related to colorectal cancer. Using animal models to mimic the impact of shift work, jet lag and constant light exposure on humans, she found that circadian disruption alters the gut microbiome and intestinal barrier function, potentially making it easier for cancerous cells to spread.
“Our bodies need those dark periods for many aspects of homeostasis,” explained Masri, an associate professor of biological chemistry at the UC-Irvine School of Medicine.
Chemicals and microplastics
Patti is a biochemist by training, but his vigilance doesn’t stop at the lab.
Married with two young children, his scientific knowledge has deeply shaped his family’s lifestyle. At home, he practices what he calls “exposure remediation” - scrutinising ingredients on shampoo bottles for questionable dyes, scanning cleaning products for chemicals known to disrupt hormones, and avoiding anything scented or labelled “antibacterial” to reduce exposure to substances that might weaken the body’s natural defences against disease.
Chemical and plastic exposure today is diffuse, ambient and inescapable, unlike legacy toxins such as asbestos or lead, which tended to be more occupational or localised.
“There’s still so much we don’t understand about how these exposures interact with our bodies,” Patti said. “But we do know that small changes, especially early on, can have lasting effects.”
Chemical exposure has grown in tandem with the explosion of microplastics. By the 1980s and ’90s, entire generations chewed on plastic toys, ate food wrapped in cling film and drank from microwaved containers. Microplastics have now been found in the placenta, the lungs, and even the brain and heart.
These fragments act as sponges for environmental toxins; laboratory studies demonstrate that microplastics can damage DNA, interfere with cell division and promote chronic inflammation, a well-known mechanism in carcinogenesis. In animal models, microplastic exposure has been linked to colon and lung cancer and immune system dysregulation. An analysis of peer-reviewed studies published in December 2024 and led by University of Sydney researcher Nicholas Chartres, scientific lead of the Center to End Corporate Harm, at the University of California at San Francisco, found repeated evidence linking microplastic exposure to mechanisms indicative of cancer across multiple systems - digestive and respiratory.
The Centers for Disease Control and Prevention has estimated that 97% of Americans have some level of toxic “forever chemicals” - a group of synthetic compounds often found in plastics with negative health effects that persist in the environment and in the human body - in their blood.
It’s this hidden complexity that drives Patti’s work.
His team is focused on metabolomics - the vast, largely unmapped study of the small molecules coursing through the human body. Using high-resolution mass spectrometry and custom-built computational tools, Patti’s lab has developed a system capable of scanning a single blood sample for tens of thousands of chemicals at once.
Traditional toxicology has been reactive, testing chemicals one by one, often after problems emerge. Patti’s approach flips that model - scanning everything first and asking questions later. The goal is to find chemical signatures that appear more often in people diagnosed with early-onset cancers than in those without.
“We’re just now beginning to understand the full chemical complexity of modern life,” he said. There are estimated to be more than 100,000 synthetic chemicals on the market. Their global production has almost doubled since 2000.
Only a small fraction of these have been studied for links to cancer: The International Agency for Research on Cancer (IARC) puts this number at about 4%. But among those examined, many have been shown to have some links to the disease. A 2024 study in Environmental Health Perspectives, for example, identified 921 chemicals that could promote the development of breast cancer.
Patti’s zebrafish research explores how diet and chemical exposure interact in cancer development. Beneath the glow of the lab’s cool lights, tiny fish dart through their tanks - some fed a standard, unremarkable diet, others given tightly controlled meals. The study is still ongoing, but early data is starting to raise questions about the role of artificial sugars.
He hopes his lab’s work may one day provide access to tests that provide snapshots of a person’s environmental history written directly in the blood, offering clues not just about cancer’s origins but about how we might finally begin to prevent it.
“The data,” Patti added, “is already in us.”