From her home in Donaldsonville, La., less than three miles from the world’s largest ammonia plant, Ashley Gaignard says the air itself carries a chemical edge.

The odor, she said, is sharp and lingering. Years ago, when her son attended an elementary school about a mile from the massive CF Industries ammonia production facility, he would begin wheezing during recess, she recalled. His breathing problems eased only after he transferred to a school several miles farther away.

Now, along Louisiana’s Mississippi River corridor, fertilizer giant CF Industries and other companies are placing multibillion-dollar bets on “blue ammonia” — a product made from fossil fuels but with extra technology to capture planet-warming gases and pipe them underground for storage.

An extensive review by Floodlight found no evidence that existing carbon capture projects anywhere in the world have achieved anything close to the emissions cuts companies like CF Industries are promising. Permit documents, meanwhile, show that the proposed plants combined could be allowed to discharge more than 2,800 tons each year of air pollutants (not greenhouse gases), including more than 400 tons of ammonia.