Black-damp is also known as choke-damp or stythe, and is a mixture of nitrogen and oxygen in proportion varying from 5 to 20 % of carbon dioxide.

With 5 % carbon dioxide it is about as heavy as air but increases in density according to the percentage of carbon dioxide.

It is not a poisonous gas except in proportions which would cause death by suffocation in a short time.

The properties of the gas vary with its composition, but they are approximately indicated where the combined effects of carbon dioxide and a shortage of oxygen are tabulated; the presence of black-damp would produce a somewhat similar atmosphere and therefore similar effect.

The gas occurs in old workings, badly ventilated dip workings, and in the coal and adjacent strata. As a rule it tends to accumulate on the floor, but this depends upon the percentage of the heavier carbon dioxide.<

Black-damp may be regarded as the residual nitrogen of the air mixed with carbon dioxide which has taken the place of oxygen due to oxidation, decay, etc. Though the whole of the oxygen may have been absorbed, only part of it is, as a rule, replaced by carbon dioxide, since some of the oxygen remains occluded in the coal. Therefore black-damp will normally contain more nitrogen than ordinary air.

The following is a representative sample of return air