Every couple of years, the Environmental Protection Agency puts out this report.

It's a series of spreadsheets mostly, with thousands of lines of data.

It's called the National Air Toxic Assessment.

And one of the things they do is really amazing the data is that they go through every

census tract in the United States.

And they estimate its cancer risk based on exposure to a number of air pollutants.

Environmental reporter Sharon Lerner waits for these reports.

Read some like a novel.

And a few years ago, she was flipping through one of those spreadsheets, looking for outliers.

Places where cancer risk seemed to cluster.

The EPA categorizes each census tract by how risky it is.

In one place, 30 cancers out of a million might have been caused by air pollution.

And another 35.

And then I saw a number that was over 800.

It was just way off the map.

It was so far above any of the rest of the numbers that I knew that something else was

going on.

I said, wait, what's that?

What is this?

This cluster is in St. John, Louisiana, just outside of New Orleans.

On one side of town, the Mississippi makes its way to the Gulf of Mexico.

On the other, a plant belches chloropene, the main ingredient in the synthetic rubber

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