Cancer and chemicals in Reserve, Louisiana: the science explained

In the area closest to the local chemicals factory, residents face a cancer risk of 50 times the national average. So what's in the air?

St John the Baptist parish sits about 20 miles from New Orleans in southern Louisiana. Home to about 45,000 people, this predominantly black parish contains the US neighborhoods most at risk of cancer due to air toxicity, according to government science. The parish sits at the heart of a region sometimes described as "Cancer Alley" because of the pollution from a high concentration of petrochemical plants.

Throughout the year, the Guardian will be reporting from St John and other places nearby. Here's a rundown of the common questions on the science behind the health hazards in this corner of Louisiana.