 In caves carbon monoxide can build up in enclosed chambers due to the presence of decomposing organic matter . In coal mines incomplete combustion may occur during explosions resulting in the production of afterdamp . The gas is up to 3 % CO and may be fatal after just a single breath . Following an explosion in a colliery adjacent , interconnected , mines may become dangerous due to the afterdamp leaking from mine to mine . Such an incident followed the <unk> Grange explosion which killed men in the <unk> mine . 
