After collecting air samples for three months, federal inspectors had planned to wrap up their study of cancer-causing ethylene oxide in west suburban neighborhoods near Sterigenics.

But when Gov. J.B. Pritzker’s administration banned the Willowbrook facility from using the highly toxic gas on Feb. 15, a team from the U.S. Environmental Protection Agency saw an opportunity rarely afforded to regulators.

Instead of dismantling the monitoring equipment posted near homes, schools, parks and government buildings, the EPA team kept it in place for the rest of the month, providing it with six days of results when the company wasn’t operating its sterilization chambers or releasing ethylene oxide into the community.

No spikes of the chemical were detected during the period, unlike many of the testing days before state regulators effectively shut down the Willowbrook facility, according to EPA documents released Thursday.

Sterigenics is attempting to persuade a federal judge to throw out the state’s seal order. On March 11, another federal judge rejected the company’s bid to resolve a related dispute in U.S. District Court in Chicago, sending back to DuPage County Circuit Court a lawsuit filed by the Illinois attorney general’s office and Robert Berlin, the DuPage County state’s attorney.

During the 1930s, a corporate predecessor to Sterigenics pioneered the use of ethylene oxide to ensure that heat-sensitive surgical equipment and other goods are germ-free. Medical devices and equipment account for the vast majority of products sterilized at the Willowbrook facility in recent years, according to company documents shared with state lawmakers.

The chemical is an effective fumigant. But by the late 1940s scientists realized that it also mutates DNA and triggers neurological problems.

The volatile, easily absorbed chemical has been on the federal list of carcinogens since 1985. In December 2016, the EPA released a long-delayed reassessment linking it more conclusively to breast cancer, leukemia and lymphomas.

Independent scientific reviewers agreed with EPA researchers who concluded the chemical is far more dangerous than previously thought. Long-term exposure to concentrations as small as 0.003 micrograms per cubic meter increases the chance a person will develop cancer, the EPA concluded.

Shortly after the EPA officially adopted its updated evaluation of the chemical, the agency began updating its semiregular National Air Toxics Assessment, a screening tool used to identify areas where more investigation is needed.

Out of 73,057 census tracts in the U.S., the EPA found, seven tracts near Sterigenics are among 109 with cancer risks exceeding the agency’s guidelines.

More than 25,000 people live within the tracts surrounding Sterigenics, which is located in a cluster of nondescript commercial and government buildings behind a Target store off Illinois Route 83 and the Stevenson Expressway.

Four schools and a day care center are within a mile of the facility, including Hinsdale South High School in Darien and Gower Middle School in Burr Ridge.

In October, Willowbrook Mayor Frank Trilla called the reports from federal agencies an “information dirty bomb.” Trilla and other local officials have scrambled to respond, hiring consultants to conduct their own air monitoring and holding public forums dominated by angry and frightened residents.

The Willowbrook facility is one of nine ethylene oxide sterilizers operated nationally by Sterigenics. The company has grown rapidly since it was bought for $675 million in 2011 by GTCR, the private equity firm formed by former Gov. Bruce Rauner and a colleague during the late 1990s after they dissolved another firm with the same name.

Sterigenics is now owned by Sotera, a suburban Cleveland-based venture between GTCR and Warburg Pincus, another private equity firm that acquired a majority stake in 2015. Sotera has been in talks to sell the company for as much as $5 billion, Reuters reported in September.

mhawthorne@chicagotribune.com

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