Midwest School’s air quality was given another all-clear Monday, 18 months after the building was evacuated and closed because of a gas leak and nearly three months after students returned.

The report, distributed by the Casper-Natrona County Health Department, found that the air quality was acceptable for all chemicals tested. After installing a vapor mitigation system, the district conducted tests inside and outside the school in May, June, July and October.

Samples were taken in the library, the pool, the preschool, the kitchen, and two classrooms. They were also taken outside at the swing set and the weather station.

Originally scheduled for Oct. 21, the most recent testing was moved up to Oct. 7 because of an unidentified smell in room 105 — which had not been one of the sample locations in previous tests — and because of fluctuations in carbon dioxide concentrations, according to the health department.

Students were moved out of room 105 on Sept. 20, according to the report. Dennis Bay, executive director of business services for the Natrona County School District, said there was a problem with the furnace in the room letting off a gas smell, but that it had been fixed and the room was again in use.

Testing showed the classroom did not contain a high concentration of the unsafe chemicals, and there will not be further sampling of the room, according to the health department report.

Carbon dioxide alarms have been installed and activated throughout the school. A review of the concentration of CO2 had been conducted weekly for five weeks, as part of a “separate task that arose since school resumed,” according to the report. Those reviews have stopped after the alarms were turned on.

Bay said the CO2 fluctuations were because students were again filling the school and breathing, exhaling carbon dioxide and causing an increase in the chemical’s concentration in the air.

“We figured out what was going on there,” he said. “Everything’s been cool ever since.”

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. There are 120 abandoned wells in the 640 acres surrounding Midwest, according to state records.

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

Despite initial hopes that Midwest could reopen by the midway point of the 2016-17 school year, the school remained closed until summer 2017. Students were bused to schools in Casper.

The district — with FDL Energy, which operates the Salt Creek field — installed a mitigation system designed to pump air out from beneath the building into the atmosphere above it.

A similar incident closed the school’s kitchens in November 2014. An odor was detected, and two kitchen workers became sick. Local, state and national health officials conducted what they said was an exhaustive investigation and found nothing conclusive.

Follow education reporter Seth Klamann on Twitter @SethKlamann