It's The Externalities, Stupid.

Last week, the Lancet Commission on pollution and public health (free registration required) released a study on the annual costs of pollution.  There's bound to be argument about the specifics, but it's difficult to argue with the conclusion that those costs are really, really, big.  The study estimates the annual global welfare loss due to pollution at $4 trillion – $6 trillion.  The Lancet says that this is more than 6% of global economic output.  It's important to note that The Lancet treats "pollution" differently from climate change, so this study does not include losses related to climate change.

It's also important to note the range of impacts – and some of the progress we've made in the developing world.  The study points out that the elimination of lead from gasoline has increased the IQ of American children born since 1980 by 2-5 points.  The economic gain just from this intelligence increase is estimated to be in the trillions of dollars