Retraction to: Journal of Exposure Science and Environmental Epidemiology (2011) 21, 601–608; doi:10.1038/jes.2011.24; published online 13 July 2011

The publisher unintentionally published this earlier version of the manuscript. The correct, final version was subsequently published as Journal of Exposure Science and Environmental Epidemiology advance online publication, 14 September 2011; doi:10.1038/jes.2011.29. We regret the error.