Growing cancer risks on a warming planet

In August 2017, unusually warm waters in the western Gulf of Mexico helped a sputtering tropical storm to re-form into what would become the most damaging hurricane in recorded Texas history. Hurricane Harvey dumped a record 52 inches of rain into Houston’s Cedar Bayou and flooded the heavily industrialized Houston Ship Canal along with the petrochemical facilities lining its banks and at least 13 Superfund sites.

When the floodwaters receded, the tons of chemical contaminants left behind brought a worrisome new trend into sharp relief: Climate change may be significantly increasing some communities’ exposure to carcinogens. That exposure is being felt most heavily by communities already bearing the brunt of health inequities, compounding the danger. After climate scientists calculated that nearly 30% of Harvey’s rainfall could be attributed to global warming, a 2022 study estimated that climate change was responsible for up to half of all flooded properties in Harris County, which includes Houston.¹ Low-income Latino neighborhoods, such as the ones near the ship canal, were hit the hardest.

Leticia Nogueira, PhD, MPH, scientific director of health services research at the American Cancer Society, says that the hurricane triggered an epiphany of how the same extracting processes that release greenhouse gases into the atmosphere—the burning of fossil fuels—are also releasing carcinogens into surrounding communities. “I had to witness a very acute, very extreme exposure to make the connection in my brain, but this is happening in many communities in a longer-term, lower dose all the time,” she says. “We’re just not thinking about it.”

Increasingly extreme events—hurricanes, floods, wildfires, heat waves, and droughts among them—are helping Dr Nogueira and other researchers to connect the dots between climate change and higher cancer risks and worse outcomes for patients diagnosed with malignancies. The individual and synergistic effects of climate change and natural disasters fueled by global warming, they warn, are threatening to undo decades of progress in cancer prevention, detection, and treatment. The growing threat, however, may create a new opening to shine a light on the widening health disparities in vulnerable communities as well as the global consequences of doing nothing to address a warming planet.

Parsing the environmental contributors to diseases with multifactorial causes such as skin cancer can be tricky, says Eva Rawlings Parker, MD, assistant professor of dermatology at Vanderbilt University Medical Center in Nashville, Tennessee. Even so, she concluded in a review of available data that “strong circumstantial evidence supports the hypothesis that factors related to climate change, including stratospheric ozone depletion, global warming, and ambient air pollution, have likely contributed” to the growing global incidence of skin cancer.²

Higher temperatures, for instance, may contribute to skin cancer in two separate ways. “There’s evidence to suggest that heat enhances carcinogenesis and speeds that process,” Dr Parker says. “But also, our behavior changes when it’s hot and we don less clothing; if our skin is more exposed, then we are getting more UV radiation and potentially more exposure to air pollution.”

The fine particles in wildfire smoke and pollution from fossil fuel combustion may also contribute to skin cancer. “When you have that particulate air pollution, it’s not just the particle, but there are many other noxious things stuck to the outside of it, like polycyclic aromatic hydrocarbons and heavy metals,” Dr Parker says. “Air pollution components are lipophilic, meaning they dissolve in a fatty environment, and that’s the perfect environment for our skin to absorb pollutants.”

Alternatively, people can breathe in particulate matter that migrates through the circulatory system until it reaches skin cells. Once there, the pollutants can bind cellular receptors that trigger a cascade of harmful effects and contribute to carcinogenesis and inflammation pathways. “On top of that, we see that UV light can act synergistically with those same pathways that air pollution is triggering, and you can get enhanced carcinogenesis,” Dr Parker notes. “So, UV is bad, air pollution is bad, but the combination together is even worse.”

Communities exposed to the highest levels of environmental toxins—or “environmental justice communities,” as they are sometimes known—are often among the most vulnerable to the effects of climate change. “It’s not a coincidence that those places are also hot spots of health disparities, including cancer disparities,” says Kilan Ashad-Bishop, PhD, a principal scientist at the American Cancer Society. “Those issues intersect because they are both patterned by larger systems of power, privilege, and oppression, like structural racism and classism.”

A 2022 study linked toxic air pollution to 86 extra cases of cancer every year in Louisiana. Census tracts with the highest proportions of low-income or Black residents drove that pollution–cancer association, the analysis revealed.³ The study suggested that heavily industrialized areas, including a stretch between New Orleans and Baton Rouge dubbed “Cancer Alley,” shouldered much of the excess cancer burden associated with point sources of pollution, primarily industrial facilities and power plants.

In aggregate, however, a growing understanding of how climate change heightens cancer risk and where those risks are greatest could spur social change and help to deconstruct the built systems of oppression and racism that contribute to the disparities, Dr Ashad-Bishop says. “The same way that discriminatory policies have been implemented and have patterned the disparities that we see now, we can think about how to apply this research to implement interventions.”

Identifying the communities that are at the highest risk is a good first step. “Then, the health equity-driven action is to funnel the appropriate amount of resources based on that vulnerability,” says Dr Ashad-Bishop. For an area with a serious and long-unaddressed environmental vulnerability that can increase the lifetime risk of cancer, for example, research could suggest the kind of programming best suited for moderating that risk through regular screening and early detection. “There’s a lot of room for imagination because it’s uncharted territory,” she adds.

Like other natural disasters, wildfires fueled by global warming can increase the challenge of effective intervention by creating multiple health hazards. Because the fires burn everything in their path, Dr Nogueira says, the combustion of plastics, cleaning products, construction materials, and other human-made materials can release carcinogens that linger in the soil and water. The associated air pollution, meanwhile, can widely disseminate the health risks.

In a recent study of roughly 467,000 US patients who had their non–small cell lung cancer surgically removed, Dr Nogueira and her colleagues found that those who were exposed to a wildfire during their first year of recovery fared significantly worse than those who were unexposed.⁴ The drop in overall survival rates was most pronounced among patients exposed to a wildfire in the first 3 months after their surgery.

Dr Nogueira and her coauthors concluded that the findings suggested a high need to identify medically vulnerable populations and prioritize them in climate adaptation, disaster preparedness, and emergency response efforts. “Sometimes people think, ‘Well, of course, if you’re committed to equity, you immediately are concerned about this disproportionate exposure,’” she says. “But I’ve also heard the flip side as well, that, ‘It’s a problem for those people over there. I don’t have to worry about it.’”

Climate change is challenging those latter assumptions. “The interesting thing about wildfire smoke is that it travels thousands of miles,” Dr Parker says. “Unlike a factory in Los Angeles spewing out pollution that’s probably not going to affect me directly living in Tennessee, a wildfire in California absolutely can affect air quality here because the amounts of smoke generated are so massive, and winds carry that particulate matter really extreme distances.”

The same phenomenon gave New York City the worst air quality of any city in the world in June 2023 when smoke from Canadian wildfires blanketed the region in an orange-tinted haze. “It really is spreading the exposure to a level where no neighborhood is really safe,” Dr Nogueira says. “It doesn’t care about county borders or your neighborhood gate.”