Whenever organic material is burned, such as in a forest fire, power plant, car exhaust, or in the daily kitchen, the combustion releases polycyclic aromatic hydrocarbons (PAHs) – a class of pollutants known to cause lung cancer.
There are over 100 known types of PAH compounds released into the atmosphere daily. Regulators, however, have historically relied on measurements of a single compound, benzo (a) pyrene, to assess a community’s risk of developing cancer following exposure to PAHs. Now, scientists at MIT have found that benzo (a) pyrene may be a poor indicator of this type of cancer risk.
In a modeling study published today in the journal GeoHealth, the team reports that benzo (a) pyrene plays a small role – around 11% – in the overall risk of developing cancer associated with PAH. Instead, 89% of that cancer risk comes from other PAH compounds, many of which are not directly regulated.
Interestingly, about 17% of the cancer risks associated with PAHs come from âbreakdown productsâ, chemicals that are formed when emitted PAHs react in the atmosphere. Many of these degradation products may in fact be more toxic than the emitted PAHs from which they were 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.
âMost of the science and regulatory standards for PAHs are based on the levels of benzo (a) pyrene. But it’s a big blind spot that could lead you down a very bad track in terms of assessing whether or not your cancer risk is improving or not, and whether it’s relatively worse in one place than in another â, says study author Noelle Selin, a professor at MIT’s Institute for Data, Systems, and Society and the Department of Earth, Atmospheric, and Planetary Sciences.
Co-authors of Selin at MIT 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 of the University of York are also co-authors.
Benzo (a) pyrene has always been the leading chemical for exposure to PAHs. The indicator status of the compound is largely based on early toxicological studies. But recent research suggests the chemical may not be the representative of PAHs that regulators have long relied on.
“There has been some evidence to suggest that benzo (a) pyrene may not be very important, but it only came from a few field studies,” says Kelly, former postdoctoral fellow in Selin’s group and author principal of the study.
Instead, Kelly and her colleagues took a systematic approach to assess the suitability of benzo (a) pyrene as an indicator of PAH. The team began by using GEOS-Chem, a three-dimensional global chemical transport model that divides the world into individual grid boxes and in each box simulates 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 connected recent data from emission inventories and meteorological observations, and advanced the model to simulate the concentrations of various PAH chemicals around the world over time.
In their simulations, the researchers started with 16 relatively well-studied PAH chemicals, including benzo (a) pyrene, and plotted the concentrations of these chemicals, as well as the concentration of their degradation products over two generations, or transformations. chemical. 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, inside the grid for each model, the researchers linked 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 the risk of double counting different chemicals. Finally, they overlaid population density maps to predict the number of cancer cases around the world, based on the concentration and toxicity of a specific PAH chemical in each location.
Dividing cancer cases by population produced the cancer risk associated with this chemical. That 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 low contribution of around 11% to the overall risk of developing cancer from exposure to PAHs worldwide. Eighty-nine percent of cancer risks came from other chemicals. And 17 percent of that risk came from degradation products.
âWe’re seeing places where you can find lower levels of benzo (a) pyrene, but the risk is higher because of these degradation products,â says Selin. “These products can be orders of magnitude more toxic, so just because they’re in minute concentrations doesn’t mean you can write them off.”
When the researchers compared the calculated cancer risks associated with PAHs around the world, they found significant differences depending on whether this risk calculation was based solely on benzo (a) pyrene concentrations or on a larger mixture of PAH compounds. of a region.
“If you use the old method, you will find that the lifetime cancer risk is 3.5 times higher in Hong Kong than in South India, but taking into account the differences in PAH mixtures, you get a 12-fold difference, âsays Kelly. âSo there is a big difference in the relative risk of cancer between the two places. And we think it’s important to expand the group of compounds that regulators think of, beyond just one chemical. “
This research was conducted at MIT’s Superfund Research Center and is funded in part by the Superfund Basic Research Program of the National Institute of Environmental Health Sciences and the National Institutes of Health.