Article: Booth, S.; Hiu, J.; Alojado, Z.; Lam, V.; Cheung, W.; Zeller, D.; Steyn, D.; Pauly, D. “Global deposition of airborne dioxin.” 2013. Marine Pollution Bulletin, 75, 182-186. doi:10.1016/j.marpolbul.2013.07.041
Using atmospheric and oceanic circulation data, researchers at the University of British Columbia recently presented a global model simulating how dioxins, one of the most toxic groups of pollutants, travel from source regions and is deposited around the globe. Their findings suggest that oceans are impacted more drastically than previously thought.
What are dioxins?
Dioxins are an extremely toxic group of pollutants. The UN has recommended that we minimize daily intake of the compounds by humans to the lowest levels possible, because subtle health effects can occur even at 2-6 picograms (that’s 2-6 trillionths of a gram, 2 to 6×10-12). As unintentional byproducts of combustion, these compounds are not produced intentionally for any kind of industrial use – they are released mainly as a result of other processes.
Dioxins are very stable organochlorine molecules. They can persist for long periods of time after release, as natural processes do not break them down effectively. The structure of the dioxin compound, shown below, allows it to effectively accumulate in persist in the fat of living creatures, a process known as bioaccumulation. Dioxin exposure has been linked to several diseases in humans and animals. Most notably the compounds are associated with cancer, reproductive and developmental abnormalities, and hormone disruption.
While dioxin release has been regulated under the Stockholm Convention for Persistent Organic Pollutants and efforts are underway to minimize production, their high toxicity and persistence in the environment are still cause for concern. The World Health Organization has stated that 90% of human exposure to dioxins occurs through food we eat. This study aimed to use what is known about dioxin sources and transport to determine what areas of the globe are most heavily impacted by dioxin deposition, and whether any of these areas are important in global food production.
The researchers used available data on dioxin emissions to model a year of atmospheric dispersion, deposition from air to land or water, and transport from land to marine reservoirs via waterways. They used a model based on country gross domestic product (GDP) to estimate how these emissions were apportioned among countries without emissions data.
To estimate how dioxins are transported from source regions, the researchers accounted for diffusion (the tendency of molecules to become evenly distributed over time) as well as the transport of air masses via weekly averaged wind patterns. To model deposition, they estimated distance traveled before deposition, taking into account the grasshopper effect, or global distillation, a model of atmospheric transport of persistent pollutants which states that persistent compounds can deposit and then revolatilize, or return to the air, and undergo further transport, thereby hopping around the globe in patterns that depend on temperature changes and wind patterns, and eventually accumulating in cold, remote polar regions. Finally, they modeled how dioxin deposited to land moves through waterways to reach marine coastal areas and other reservoirs. They used salinity data for coastal areas to determine inputs of freshwater from rivers to marine environments.
The model estimated that North America, Europe, South Asia, and East Asia are global dioxin production hot spots, with 30% of emissions coming from China, Japan, and the United States. The model showed that 57% of dioxin was deposited on land, and 40% was deposited into the oceans, whereas a previous study had estimated marine deposits as 5% of total emissions. The model suggests that oceans are a more significant sink for global dioxin emissions than previously thought.
To visualize relative deposition, researchers used “toxic equivalents” (TEQs). This method involves calculating the amount of toxicity associated with different dioxin compounds relative to the most toxic compound in the group, 2,3,7,8-tetrachloro-p-dioxin. In this way, quantities of different compounds can be expressed on a common scale.
The researchers observed that polar regions received little dioxin in this simulation. They state that this is because the model accounts for pollutant transport over only one annual cycle, and accumulation in polar regions takes more time. They postulate that models showing long-term transport would eventually depict the accumulation of dioxin in polar regions.
Despite all of the health concerns associated with dioxins, detailed budgets on global production and deposition have been scarce, partially because these compounds are released unintentionally. This study made an effort to identify the most heavily impacted regions of the globe, and in doing so, highlights how environmental issues cannot be isolated on a country-to-country basis, but are a global problem that requires global solutions.
The study stresses the complexity of global models for atmospheric transport. Emission and deposition of dioxins presented here do not match, meaning that some releases are still unaccounted for. This is an issue common to many budgets for ubiquitous global pollutants.
Additionally, the study’s suggestion that marine environments may be more heavily impacted by dioxin emissions than previous models had shown emphasizes of the ocean as a sink for global pollutants. Impacts to marine ecosystems are especially important, considering that fish are particularly vulnerable to bioaccumulation, and many populations rely on fish from some of these impacted regions as a primary food source.
I am the founder of oceanbites, and a postdoctoral fellow in the Higgins Lab at Colorado School of Mines, where I study poly- and perfluorinated chemicals. I got my Ph.D. in the Lohmann Lab at the University of Rhode Island Graduate School of Oceanography, where my research focused on how toxic chemicals like flame retardants end up in our lakes and oceans. Before graduate school, I earned a B.Sc. in chemistry from MIT and spent two years in environmental consulting. When I’m not doing chemistry in the lab, I’m doing chemistry at home (brewing beer).