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.

What’s in the air in St John the Baptist parish?

There are nearly 50 toxic chemicals in the air here, including known carcinogens like benzene, 1,3 butadiene and formaldehyde. But according America’s environmental regulatory agency, the EPA, the two most pressing concerns for residents in St John the Baptist are chloroprene and ethylene oxide [EtO].

The chloroprene comes from the Pontchartrain Works facility, which straddles the towns of LaPlace and Reserve in St John the Baptist. The plant was built by chemicals company DuPont and sold to the Japanese firm Denka in 2015. According to EPA data, the facility presents the greatest risk of cancer from air pollution of any factory in the US.

In the area closest to the factory, residents face a cancer risk of 50 times the national average – primarily due to chloroprene, but also EtO, which is emitted by several nearby facilities.

The agency has been monitoring chloroprene levels in St John since 2016. According to the EPA, the safest limit for chloroprene is 0.2 micrograms per cubic meter (μg/m3). But readings throughout Reserve and LaPlace far exceed this on a regular basis.

What is chloroprene and where does it come from?

Chloroprene is the primary ingredient in the production of neoprene, a synthetic rubber developed by the DuPont company in 1931. Neoprene is used in a wide variety of products from electrical insulation, belts, and gaskets, to consumer athletic and aquatic products, like wetsuits.

From 1948 t0 2008, DuPont produced the majority of its neoprene at a facility in Louisville, Kentucky. But DuPont shuttered the facility in 2008 after concerns over toxic pollution pushed the city to adopt a “toxic air reduction” Program.

In the area closest to the factory, residents face a cancer risk of 50 times the national average

When that plant closed, DuPont’s facility in St John the Baptist became the sole producer of chloroprene and neoprene in the US. In 2015, the plant was sold to the Japanese company Denka Performance Elastomers. DuPont still owns the land beneath the plant, and other facilities on the site.

What is ethylene oxide (EtO) and where does it come from?

Ethylene oxide emissions are much more common than chloroprene in the US. The chemical is produced by nine different companies at 15 different facilities in the US, mostly in Texas and Louisiana.

EtO is used in the production of many plastics, household cleaners, adhesives, and detergents. It is also sometimes used directly for the sterilization of medical equipment.

In 2016, the Union Carbide facility in Hahnville, Louisiana was the No 2 emitter of EtO in the US. It sits just across the river from Reserve.

How serious a cancer risk does chloroprene present, and how do we know?

It’s substantial. The National Toxicology Program has described chloroprene as “reasonably anticipated to be a human carcinogen” based on the observation of tumor formation at multiple sites in animal studies.

Quantifying those risks in humans, however, is complex, because it involves calculating the impact of exposure to relatively small amounts of the chemical over long periods of time. Currently, the most comprehensive research comes from an EPA program called the Integrated Risk Information System, or Iris.

Iris uses a peer-reviewed method that pulls together the best studies that exist on an individual chemical, and converts them into a numerical estimation of the risks of exposure over a lifespan. According to the “weight of the evidence” reviewed by Iris chloroprene is “likely to be carcinogenic to humans”.

What about EtO and cancer?

Ethylene oxide is characterized as “carcinogenic to humans” by the EPA, based on evidence of lymphomas and breast cancer in workers exposed to EtO, along with tumors discovered in laboratory tests on animals. EtO was subject to the same peer-reviewed Iris process as chloroprene in 2016.

Are there risks from these chemicals besides cancer?

Aside from cancer, studies have found that chloroprene exposure can cause irritation to the skin, lungs and eyes; chest pains, rapid and irregular heartbeat, dizziness, insomnia, headache, and fatigue; and changes in the nervous and cardiovascular systems.

Ethylene oxide exposure has been linked to eye and upper respiratory tract irritation, nausea, vomiting, diarrhea, headache, dizziness, malaise, fatigue, muscle weakness, and signs and symptoms of peripheral neuropathy. Other effects noted in some studies include dry mouth, sore throat, runny nose, shortness of breath, apnea, memory loss, and seizures.

What do the manufacturers say?

In a statement to the Guardian, Denka said that the health and safety of residents and employees were “the company’s top priorities” and said it operates the facility in Saint John the Baptist within all state permits.

But the company has repeatedly criticised the EPA’s 0.2 μg/m3 guidance. Denka instead touts the conclusions of a toxicological assessment it commissioned itself, which found that the safe amount of chloroprene in the air should be 156 times greater than what the EPA has determined.

But Denka doesn’t have many options besides to dispute the science. Company officials acknowledged that the EPA’s safe air quality levels are “technologically impossible to achieve,” and that the current Iris calculations “threaten the very survival of [Denka’s] neoprene production facility,” in a 2017 letter to the EPA.

Denka also highlights the fact that it entered a voluntary agreement with the state of Louisiana to reduce emissions in 2016 after EPA data was first revealed demonstrating the risks from chloroprene.

“Despite disagreement with the report’s suggestions … DPE voluntarily designed and committed to a plan to significantly reduce its emissions beyond the emissions control technology already in place at the plant … at a total cost of over $35m,” Denka spokeperson Jim Harris said.

The industry response to the EPA’s evaluation of ethylene oxide has been similar. The American Chemical Council, a powerful lobbying group for the chemical industry, calls the EPA’s risk value for EtO “significantly flawed” due to “selective science”.