The rule was initially proposed in October 2010 to restrict the exposure levels of coal dust from 2 milligrams of dust per cubic metre of air down to 1mg.

The limit aims to lower the incidents of black lung suffered by coal miners and will be phased in over the next two years.

The Mine Safety and Health Administration made the announcement regarding the completed rule in Morgantown, West Virginia overnight.

The MSHA has long argued that the standards, unchanged since 1972, were out-of-date.

The new rule wasn’t unanimously popular within the industry. The National Mining Association opposed the rule and claimed it was unwarranted and misdirected. It also questioned the validity of the scientific research put forward by the MSHA in support of the regulations.

A report by the Government Accountability Office confirmed the research the MSHA used to justify lowering the limit of coal dust exposure.

The regulation is due to come into effect August 1 2014.

The rule will tighten the limit from the current 2mg per cubic meter to 1.7 mg per cubic meter within six months, then to 1.5mg next year, and to 1mg in two years.