Residents sounded the alarm about the plant's emissions of ethylene oxide, a chemical used in the sterilization process that can cause cancer.
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Elevated levels of ethylene oxide were recorded at two locations near the plant, including Willowbrook Village Hall. Six other monitors placed at schools and residential locations did not register measurable amounts of the chemical.
The samples were taken in mid-November after Sterigenics installed new exhaust filters to reduce emissions of the toxin.
The EPA said it's premature to draw conclusions from the data, which will be included as part of a comprehensive assessment that will take into account three months' worth of air samples.
Representative Dan Lipinski said Sterigenics should cease operations while federal agents conduct further testing.
"This certainly shows to me that Sterigenics is a public health threat, and it should be shut down," he said.
Neringa Zymancius, a member of the community group Stop Sterigenics, agreed.
"If there's any amount of detectable level of EO, it should be shut down immediately," she said. "This is a cancer-causing chemical and it is in our air."
Sterigenics maintains that it is operating within its permit.
"EO levels in the air vary greatly based on a number of factors including proximity to sources of EO, of which there are many including exhaust from diesel trucks and automobiles, and the weather conditions on the days that samples were collected," a Sterigenics spokesperson said in a statement.
The EPA acknowledged last month that its air testing methods were flawed and that measurement procedures would be reworked. The new data reflects those updated methods.