One of the first giveaways is the smell, says Shamell Lavigne of how to recognize that the area she grew up in is steeped in "filth." She was raised in the heart of what has become known as 'Cancer Alley,' an 85-mile-stretch between Baton Rouge and New Orleans where around 200 fossil fuel  and petrochemical plants are clustered along the banks of the mighty Mississippi River.   The region handles around a quarter of US petrochemical production, which feeds public demand for things like