CASPER, Wyo. (AP) — Officials say Midwest School's air quality has reached acceptable levels, 18 months after the building was evacuated and closed because of a gas leak and nearly three months after students returned.

The Casper Star-Tribune reports that the Casper-Natrona County Health Department cleared the school on Monday.

The school installed a mitigation system designed to pump air out from beneath the building into the atmosphere above it.

Midwest School was evacuated and subsequently closed in May 2016 after an odor was discovered and testing revealed a gas leak coming from an uncapped well in the nearby Salt Creek oil field.

Many students reported having headaches and other symptoms around the time that the gas leak was discovered.

The school remained closed until summer 2017. Students were bused to schools in Casper.

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Information from: Casper (Wyo.) Star-Tribune, http://www.trib.com