Regulators need to look at a broader range of combustion products
Elizabeth Gribkoff for the Environmental Health News
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.
