Regulators have long sought to curb dioxin levels because the toxic pollutants persist in the environment and climb the food chain all the way to our dinner tables. Now a study of airborne dioxin levels in the U.K. points to significant reductions during the 1990s.
"Dioxins" is an imprecise term: The pollutant class actually includes polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), both products of combustion from sources such as cement kilns and furnaces. Because PCDDs and PCDFs each have eight possible chlorination sites, 210 structural permutations, or congeners, are possible. But the 17 congeners that have 2,3,7,8-tetrachlorinated structures are the most toxic.
As part of an ongoing air quality survey, Kevin Jones and colleagues at Lancaster University in the U.K. have monitored airborne levels of dioxins since 1991. The researchers measured the pollutants at six locations in the UK, including the city of Manchester and rural locales such as High Muffles in northern England.
The scientists report (Environ. Sci. Technol. DOI 10.1021/es1009828) that airborne levels of those 17 most toxic congeners decreased dramatically—by 80 to 90%—in urban areas in the 1990s. Urban dioxin levels dropped so much that they reached the low levels found in rural areas, which did not change during the survey. New regulations in the U.K. on waste incineration, a particularly concentrated source of dioxins, may have spurred this urban dioxin decline.
But since the late 1990s, the urban dioxin drop has stalled in the U.K., the researchers report. These direct measurements parallel estimates from The National Atmospheric Emission Inventory, which approximates yearly dioxin emissions in the U.K. based on data from industry. The Inventory estimates that emissions decreased significantly over the course of the 1990s, but by smaller amounts thereafter.
The U.K.'s urban dioxin trend also may be more general to Western Europe and the U.S., says atmospheric chemist Evris Stephanou of the University of Crete: "Their meticulously collected data are similar to short-term studies from different countries, such as Greece, Spain, Portugal, and the US." Data points from these shorter studies followed the general drop of the U.K. study.
And the 1990s reduction could constitute the tail end of a longer downward trend, Jones and colleagues report. They overlaid their graphs with data from a 100-year retrospective study of dioxin levels measured in plant samples collected by botanists. From those data, the scientists think that dioxin airborne concentrations peaked between the 1950s and 1970s and then started to drop.
The timing makes sense, the researchers say: The U.K. Clean Air Act, which called for pollution reductions across the board, went into effect in 1956 and around that time people started to move away from burning coal and wood in their homes. So the decline in the 1990s, Jones says, "is apparently something that had been going on for quite a long time—decades, in fact."