Around the world, regulators have long relied on one compound to assess a community's lung cancer risk from a class of chemicals that we're exposed to while grilling burgers, waiting in traffic, and breathing in wood smoke from a fire.
That compound—benzo(a)pyrene, a polycyclic aromatic hydrocarbon (PAH)—however, only accounts for 11% of lung cancer risk associated with PAHs, MIT researchers found in a study published earlier this month in GeoHealth. Meanwhile, 17% of the PAH-linked cancer risk in the study came from the largely unregulated and under-studied breakdown products.
People can be exposed to PAHs in a variety of ways, from smoking to eating grilled food to breathing in tailpipe or wildfire emissions. Workers in coal plants, or those who use coal products, are considered especially at-risk to PAH exposure.
When people inhale PAH particles, the particles can travel deep into the lungs, causing cell mutations that can lead to lung cancer. Scientists are also concerned about exposure to PAHs through food and drinking water, as ingestion has been linked to birth defects and higher prevalence of developing breast, pancreatic, and colon cancers.
Experts say this study provides further evidence that both regulators and scientists need to factor in a broader range of PAH compounds when assessing a community's cancer risks — and determining what pollution reduction projects to fund.
"The big challenge in regulating air pollutants is: What are the most important sources and locations to prioritize?" Noelle Selin, director of MIT's Technology and Policy Program and a co-author of the paper, told EHN. "If you're using just a model of benzo(a) pyrene, you might not actually end up with the best answer in terms of the most beneficial reductions."
Toxic breakdown products
In the 1970s, the U.S. Environmental Protection Agency (EPA) identified 16 of the more than 10,000 PAH compounds as pollutants of concern, and since then, that group of chemicals has been widely monitored around the world. One of those—benzo(a)pyrene—is still used as the toxicity benchmark for polycyclic aromatic hydrocarbons in epidemiological studies, in large part because it's the best-studied PAH.
But in recent years, researchers have been questioning whether that narrow focus makes sense. In particular, researchers have been challenging the assumption that once PAH compounds break down in the atmosphere, they're no longer carcinogenic. "It turns out that some of the products that they can react to are even more toxic than what's initially emitted," said Selin.
As part of their work on a Superfund site in Maine, the MIT researchers examined global lung cancer risk from 16 PAH compounds and their degradation products — 48 altogether.
Once they had developed a global atmospheric model for PAH concentrations and fine-tuned it against real-world pollutant measurements, the researchers used animal studies to assess the associated lung cancer risk from different PAH compounds. They also estimated lung cancer risks based on epidemiological studies that use benzo(a)pyrene as a proxy for overall PAH cancer risk.
While they found that industrial regions in China, India, and Eastern Europe had the highest levels of lung cancer risk in both methods, animal experiments showed that changing benzo(a)pyrene emissions did not have a linear correlation with overall lung cancer risk from PAHs. For example, although simulated benzo(a)pyrene emissions were 3.5 times higher in Hong Kong than in India, the animal-based method predicted that Hong Kong residents are 12 times more likely to develop lung cancer, according to the paper.
While it's difficult to scale up the animal studies to human outcomes, that data provides researchers with a window into the "relative importance" of different PAH compounds in overall cancer risk, said Selin. The study also showed the importance of monitoring the sub-compounds that PAHs can break down into.
Toxic mixtures
Staci Simonich, an environmental toxicology professor at Oregon State University who also researches PAHs but was not involved in this study, told EHN that the new paper likely under-estimated the cancer risk as the researchers did not include the class of heavier-weight PAH compounds described in her 2011 study as a significant contributor to cancer risk. Selin said that her group had not included those and other PAH compounds due to limitations in global monitoring data—including having almost no measurements from Africa.
Both Selin and Simonich stressed the need for future studies that assess the risk of PAH mixtures, noting that the total toxicity might not always be as simple as just adding up the toxicity of the individual compounds.
"I think regulators, whether it's in air, soil or sediment, are getting the message…in Europe and the U.S. that you really have to take a much broader look at PAHs in terms of exposure and risk," said Simonich.
Banner photo: People can be exposed to PAHs in a variety of ways, from smoking to eating grilled food to breathing in tailpipe or wildfire emissions. (Credit: Rubén Bagüés/Unsplash)
