Global cancer risk from burning organic matter comes from unregulated chemicals

Date:
September 22, 2021
Source:
Massachusetts Institute of Technology
Summary:
Scientists have found that benzo(a)pyrene, traditionally measured to
gauge risk of developing cancer from exposure to polycyclic aromatic
hydrocarbons (PAHs), is a poor proxy for this type of cancer risk.



FULL STORY ========================================================================== Whenever organic matter is burned, such as in a wildfire, a power plant,
a car's exhaust, or in daily cooking, the combustion releases polycyclic aromatic hydrocarbons (PAHs) -- a class of pollutants that is known to
cause lung cancer.


========================================================================== There are more than 100 known types of PAH compounds emitted daily
into the atmosphere. Regulators, however, have historically relied on measurements of a single compound, benzo(a)pyrene, to gauge a community's
risk of developing cancer from PAH exposure. Now MIT scientists have found
that benzo(a)pyrene may be a poor indicator of this type of cancer risk.

In a modeling study appearing today in the journal GeoHealth, the team
reports that benzo(a)pyrene plays a small part -- about 11 percent --
in the global risk of developing PAH-associated cancer. Instead, 89
percent of that cancer risk comes from other PAH compounds, many of
which are not directly regulated.

Interestingly, about 17 percent of PAH-associated cancer risk comes from "degradation products" -- chemicals that are formed when emitted PAHs
react in the atmosphere. Many of these degradation products can in fact
be more toxic than the emitted PAH from which they formed.

The team hopes the results will encourage scientists and regulators to
look beyond benzo(a)pyrene, to consider a broader class of PAHs when
assessing a community's cancer risk.

"Most of the regulatory science and standards for PAHs are based on
benzo (a)pyrene levels. But that is a big blind spot that could lead
you down a very wrong path in terms of assessing whether cancer risk
is improving or not, and whether it's relatively worse in one place
than another," says study author Noelle Selin, a professor in MIT's
Institute for Data, Systems and Society, and the Department of Earth, Atmospheric and Planetary Sciences.



========================================================================== Selin's MIT co-authors include Jesse Kroll, Amy Hrdina, Ishwar Kohale,
Forest White, and Bevin Engelward, and Jamie Kelly (who is now at
University College London). Peter Ivatt and Mathew Evans at the University
of York are also co- authors.

Chemical pixels Benzo(a)pyrene has historically been the poster chemical
for PAH exposure. The compound's indicator status is largely based on
early toxicology studies. But recent research suggests the chemical may
not be the PAH representative that regulators have long relied upon.

"There has been a bit of evidence suggesting benzo(a)pyrene may not be
very important, but this was from just a few field studies," says Kelly,
a former postdoc in Selin's group and the study's lead author.

Kelly and his colleagues instead took a systematic approach to evaluate
benzo (a)pyrene's suitability as a PAH indicator. The team began by
using GEOS-Chem, a global, three-dimensional chemical transport model
that breaks the world into individual grid boxes and simulates within
each box the reactions and concentrations of chemicals in the atmosphere.



==========================================================================
They extended this model to include chemical descriptions of how
various PAH compounds, including benzo(a)pyrene, would react in
the atmosphere. The team then plugged in recent data from emissions
inventories and meteorological observations, and ran the model forward
to simulate the concentrations of various PAH chemicals around the world
over time.

Risky reactions In their simulations, the researchers started with 16 relatively well-studied PAH chemicals, including benzo(a)pyrene, and
traced the concentrations of these chemicals, plus the concentration
of their degradation products over two generations, or chemical transformations. In total, the team evaluated 48 PAH species.

They then compared these concentrations with actual concentrations of the
same chemicals, recorded by monitoring stations around the world. This comparison was close enough to show that the model's concentration
predictions were realistic.

Then within each model's grid box, the researchers related the
concentration of each PAH chemical to its associated cancer risk; to
do this, they had to develop a new method based on previous studies
in the literature to avoid double-counting risk from the different
chemicals. Finally, they overlaid population density maps to predict the
number of cancer cases globally, based on the concentration and toxicity
of a specific PAH chemical in each location.

Dividing the cancer cases by population produced the cancer risk
associated with that chemical. In this way, the team calculated the
cancer risk for each of the 48 compounds, then determined each chemical's individual contribution to the total risk.

This analysis revealed that benzo(a)pyrene had a surprisingly small contribution, of about 11 percent, to the overall risk of developing
cancer from PAH exposure globally. Eighty-nine percent of cancer risk
came from other chemicals. And 17 percent of this risk arose from
degradation products.

"We see places where you can find concentrations of benzo(a)pyrene are
lower, but the risk is higher because of these degradation products,"
Selin says.

"These products can be orders of magnitude more toxic, so the fact that
they're at tiny concentrations doesn't mean you can write them off."
When the researchers compared calculated PAH-associated cancer risks
around the world, they found significant differences depending on whether
that risk calculation was based solely on concentrations of benzo(a)pyrene
or on a region's broader mix of PAH compounds.

"If you use the old method, you would find the lifetime cancer risk is
3.5 times higher in Hong Kong versus southern India, but taking into
account the differences in PAH mixtures, you get a difference of 12
times," Kelly says.

"So, there's a big difference in the relative cancer risk between the
two places. And we think it's important to expand the group of compounds
that regulators are thinking about, beyond just a single chemical."
This research was conducted in MIT's Superfund Research Center and is
supported in part by the National Institute of Environmental Health
Sciences Superfund Basic Research Program, and the National Institutes
of Health.

========================================================================== Story Source: Materials provided by
Massachusetts_Institute_of_Technology. Original written by Jennifer
Chu. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Jamie M. Kelly, Peter D. Ivatt, Mathew J. Evans, Jesse H. Kroll,
Amy I.

H. Hrdina, Ishwar N. Kohale, Forest M. White, Bevin P. Engelward,
Noelle E. Selin. Global Cancer Risk from Unregulated Polycyclic
Aromatic Hydrocarbons. GeoHealth, 2021; DOI: 10.1029/2021GH000401 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210922121755.htm

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