NIOSH found the position of coal dust along the perimeter of an entry was a more important factor affecting explosion propagation than was often recognised.

Researchers said the dust on the ribs, roof and other elevated surfaces (overhead dust) could be dispersed much more readily by an explosion than dust on the floor.

They said if the overhead dust was mainly coal dust, the explosion hazard was intensified, but their research showed that when overhead dust was primarily rock dust, the explosion hazard was reduced.

During the experimental mine explosion tests, trays of colour-coded dust layers were substituted for the floor layer in strategic locations throughout the test zone.

The results showed that, for a typical float coal dust explosion, only the top 2-4mm of the floor dust layer was stripped off or entrained in the air.

It was also found that a minimum layer, about the thickness of a sheet of paper, of pulverised float coal dust deposited on top of a uniform concentration of 80% rock dust and 20% float coal dust would propagate an explosion.

The thicker the float coal floor layer, the more violent the explosion.

NIOSH said depending on the quantity, the overhead rock dust could compensate for a deficiency of rock dust on the floor. However, rock dust on the floor could not compensate for float coal dust on above surfaces, so special attention towards increasing the rock dust content on these elevated surfaces was recommended.

They also found float coal dust deposits could be neutralised by new applications of rock dust (such as trickle rock dusting or bulk rock dusting), or by mixing the float coal dust with underlying rock dust.

NIOSH said generalised rock dust was currently the primary means of defence against coal dust explosions in US mines and regulations stated rock dust had to be used in bituminous coal mines and distributed on the top, floor and sides of all underground areas of a coal mine.