ExxonMobil Corp.'s (NYSE: XOM) XTO Energy unit said on March 7 it plugged a blown-out natural gas well in rural southeast Ohio that had been leaking for nearly three weeks.

The Feb. 15 blowout in Belmont County, Ohio, had spewed millions of cubic feet of gas into the air, triggering evacuations of nearby residences and raising concerns among environmental groups about health and environmental impacts.

XTO spokeswoman Karen Matusic said the company could not immediately say how much gas leaked from the well, which was about to be put into production after being drilled and fracked.

An initial report from the Environmental Protection Agency (EPA) on Feb. 17 estimated the natural gas flow rate from the well at 100 million cubic feet per day.

Exposure to low levels of natural gas is not harmful to human health, according to the National Institutes of Health, but extremely high levels can cause loss of consciousness or death by displacing oxygen.

Matusic said the company has been taking air samples since the blowout and "never picked up anything that would harm humans or animals."

Following the well blowout, emergency responders evacuated about 30 homes within one mile of the well. Residents of all but four homes located within a half mile of the well were able to return home within a few days, Matusic said.

Ohio Department of Natural Resources (DNR) is the lead government agency at the XTO well pad. Officials at the DNR were not immediately available for comment

The U.S. EPA said it responded to a fire at the well on Feb. 15 to provide technical assistance and air monitoring at the site. Because there were no apparent release of oil or hazardous substances, the EPA said it demobilized on Feb. 21.

An unknown quantity of brine and produced water, estimated to be more than 5,000 gallons, was initially discharged to streams that flow into the Ohio River, according to the EPA.