Two recently banned pesticides have all but vanished from the atmosphere around the Great Lakes, but others phased out decades earlier don’t seem to be going anywhere.

Marta Venier, an environmental chemist at Indiana University, and her colleagues used a unique long-term dataset collected by the US Environmental Protection Agency, which has been sampling the air around the Great Lakes every 12 days since 1990. It measures the atmospheric concentration of hundreds of compounds, including pesticides, PCBs, and other persistent chemicals. They found that levels of two banned pesticides – lindane and endosulfan – can no longer be detected.

“The concentration of some chemicals has decreased so much, they have reached a level where we can say they have been virtually eliminated,” says Venier. The work was published in the journal Environmental Science and Technology in February.

Lindane, which was used to treat seeds prior to planting and has been linked to anemia, lung cancer, and lymphoma, was banned in 2006. Endosulfan, used on fruit and other agricultural products, and which can cause liver, kidney and brain damage, was banned in 2010.

But Venier also found that other persistent and potentially dangerous pesticides are not going anywhere fast, even though they have been banned for much longer than lindane and endosulfan. Levels of DDT, chlordane, and hexachlorobenzene have dropped only slowly since they were banned decades ago. DDT, for example, was phased out in the 1970s because of its detrimental effects on both the environment and human health. Exposure to DDT can cause developmental abnormalities, reproductive disease, neurological disease, and cancer.

“Those others haven’t changed much,” says Venier. “They’re going down, but have not been eliminated yet.”

And she doesn’t expect them to disappear anytime soon. Venier estimates that it will take another 17 years for the current concentration of DDT in the atmosphere to fall by half.

Venier thinks the difference is more a result of where and how the pesticides were used, rather than any differences in their chemical structures or reactions. Lindane and endosulfan were largely agricultural pesticides, while DDT, for example was used extensively in cities. Agricultural soil is churned up regularly, so any pesticides stored in it are frequently released into the atmosphere where they are broken down by chemical reactions, exposure to sunlight, and mechanical activity. Soil in cities, by contrast, is rarely disturbed so the pesticides are released more slowly.

“DDT therefore is still present in large reservoirs in cities,” she says.

Frank Wania, an environmental chemist at the University of Toronto, says the work shows the vital importance of long-term monitoring programs, but he is not convinced by the soil-mixing hypothesis. He thinks that the lack of mixing in urban soils would actually lead to a faster drop in the amount of DDT in the atmosphere, because the portion locked in the soil would stay there and not replenish the atmospheric portion.

Instead, he says it is more likely that DDT and chlordane decline in air more slowly than lindane because of their lower volatility and slower degradation rates in soil. So their stock in the soil reservoirs declines more slowly than the stock of lindane, regardless of whether the soil is mixed.

While Venier says the study shows how successful environmental regulations can be in reducing harmful chemicals in the atmosphere, Wania is not so sure. One reason for the lack of a decline in hexachlorobenzene might be due to its continued use in the production of aluminum and magnesium, which, he says, is a sign that regulations may not be working as intended. “Considering that HCB was included among the ‘dirty dozen’ of the Stockholm Convention, the lack of progress indicates a clear failure of the industries involved and of the governments that have made commitments in a globally binding agreement,” he says.