U.S. soldiers applying DDT-based insecticide during WWII. Image credit: Wikimedia Commons
An estimated 1.5 million metric tons of DDT were used between the 1940s and the 1970s, when strict restrictions were placed on the chemicals worldwide. Though the use has been cut dramatically since then, new research has found that DDT is still being emitted by the world's oceans, and that concentrations in the northern oceans may actually be increasing.Used as an agricultural insecticide and to control mosquitoes and other disease carrying insects, DDT was banned after research showed its powerful impact on marine life and birds. Despite bans, DDT is still in use in some places today. Most common in the southern hemisphere, the chemical still enters the environment through use as a malaria control, leakage from old containers, and its inclusion in other pesticides.
Still, new research shows that the levels in the oceans are greater than that being emitted today. The explanation, Irene Stemmler and Gerhard Lammel of the Max Planck Institute for Chemistry believe, is that legacy DDT has entered a cycle in which it evaporates from the oceans into the atmosphere and then is returned with rain before it has a chance to be broken down by exposure to the sun.
This cycle, they report, is likely to continue for some time because DDT is broken down very slowly in the environment. Taken alone, however, the process is not something people need to worry about. Environmental levels are still low and as Lammel explained, "cruise passengers will never suffer from this."
Increasing Concentrations in Models
There is evidence, however, that DDT may be increasing in concentration in northern waters. In warmer southern oceans, the chemical evaporates very quickly, Stemmler and Lammel explain, but in the colder waters of the north, it tends to stay longer, allowing concentrations to build.
This could, according to the team's computer models, persist to the point where DDT levels are toxic to marine life. Additionally, there is the risk that concentrations will increase up the food chain, through a process known as bioaccumulation, creating toxic meals for top-level predators including humans.
Stemmler and Lammel point out that, until recently, there has been very little research done on DDT in marine systems and because of this, they were not able to conclusively prove their model. Still, it certainly provides a compelling reason to conduct further research on the chemical's behavior in the environment.