30 CFR § 57.11050
Escapeways and refuges.
(a) Every mine shall have two or more separate, properly maintained
escapeways to the surface from the lowest levels which are so positioned that
damage to one shall not lessen the effectiveness of the others. A method of
refuge shall be provided while a second opening to the surface is being
developed. A second escapeway is recommended, but not required, during the
exploration or development of an ore body.
(b) In addition to separate escapeways, a method of refuge shall be
provided for every employee who cannot reach the surface from his working
place through at least two separate escapeways within a time limit of one
hour when using the normal exit method. These refuges must be positioned so
that the employee can reach one of them within 30 minutes from the time he
leaves his workplace.
For more information: See MSHA'S Program Policy Manual