West Virginia ? Underground coal mines must provide up to four days of breathable air to keep miners alive in emergencies such as an explosion or a tunnel collapse, federal regulators announced Thursday.

A law enacted last year after a string of deadly accidents, including the deaths of 12 miners in January 2006 at the Sago Mine, required mine operators to provide enough air to keep miners alive in an emergency but did not specify how much.

On Thursday, the federal Mine Safety and Health Administration gave mine operators several options to comply with the requirement.

Among the options were providing a 96-hour supply of air in a shelter or drilling boreholes to provide a constant flow of fresh air to a designated area or shelter.