The Stockholm Convention on Persistent Organic Pollutants is an international treaty signed in 2001. The treaty became effective in May 2004. Its aim is to limit the use and production of Persistent Organic Pollutants. Examples of substances affected by the treaty are Polychlorinated biphenyl and Dichlorodiphenyltrichloroethane. An extension to the treaty in 2009 added other substances, like Lindane. Critics have said that the treaty limits the effectiveness of the fight against diseases such as malaria. This is not the case, as the use of certain substances for vector control is permitted; this includes the use against mosquitoes.