We all know that air is a mixture of too many gases.  Gases are having some good, neutral, or bad impact to all living lives of the earth. We are going to study about two types of air compositions, one is general atmospheric air and other is mine air composition. 

This percentage get affected by thepresence of vapour in theair. The composition of air is different at different places onearth. It depends on surrounding environment, thesource of pollution and source of fresh air. Similarly in mines composition of air is quite different than thecomposition of air on thesurface. It is mainly due to theemission of gases from theseam, theexhaust of machinery, theexhaust of human working belowground. This type of mine air composition may consist following types of gases. Non-toxic gases like methane, hydrogen, and other hydrocarbons. This gases are not directly harmful to human life. Mild toxic gases like carbon dioxide and toxic gases like carbon monoxide nitrogen oxides, sulphur dioxide.  It may also have dust particles, humidity, explosive particles, lubricated oil and acid vapour.

Damps in mines

Impurity in mine air mixture is called damp. Dam does not mean impurities of particularly single gas. There are mainly seven types of damps are found in mine.

1. Firedamp: This damp is a mixture of methane and other gases. Methane may be explosive if it is present in certain quantity and suitable condition.  But its presence may decrease thepercentage of oxygen.
2. Blackdamp: This damp is a mixture of carbon dioxide and nitrogen. It is also known as a chokedamp.
3. Afterdamp: This is a mixture of gases originated due to firedamp or coal dust explosion. Carbon monoxide plays important role in afterdamp.
4. Whitedamp:This damp contains carbon monoxide sourced other than anexplosion. The main source of carbon monoxide is strata emission and engine exhaust.
5. Stinkdamp:This gas contains hydrogen sulphide as the main gas. 
6. Nitrous fumes: All nitrous gases like NO, NO₂, N₂O₃, N₂O₄, N₂O originated due to blasting are covered in this damp.
7. Sulphur dioxide: This gas damp contains sulphur dioxide gas as the main gas.

Threshold limits of gases

As we know that in mine, due to some activity or tasks percentages of gases get changes frequently. This changes may cause danger if they cross its threshold limits.  Some gases have anadverse impact on human being either directly or indirectly if they got decrease from thecertain limit. On the other hand, some are having anadverse impact on ahuman being if they increase from thecertain limit. The worker may get faint or die if proper actions are not taken on time by management for controlling the situations. To identify danger it is necessary that we can easily check current percentages and status of various gas and its minimum and maximum permitted values.

The permissible limits of the various gases are also expressed as the MAC (Maximum allowable concentration) or TLV (Threshold Limit Value) for a duration of 8 hours shift of worker. If somehow duty hours are more than 8 hours than worker must be shifted to afresh environment.

The MAC or TLV are measured in percentage or in ppm (parts per million).

1 ppm is equal to 0.0001%

Similarly, 1 % is equivalent to 10000 ppm

Sometimes in anexam or in viva of thegas testing exam, percentages are converted to ppm for making question difficult or tricky, so you must be aware of theconversion rate of ppm to apercentage.  Generally, gases having threshold limit less than 0.01% are written in ppm unit to make it simple.

Let’s see some permissible limits of various gases.

Oxygen O₂: As par coal mines regulation, mine air must have minimum 19% oxygen. Physiological effects get started from 17%. The victim may die if he is not moved to fresh air or oxygen supply is not provided to him on time.

Carbon dioxide CO₂: TLV for CO₂is 0.5% or 5000 ppm.

Carbon monoxide: TLV or MAC value for carbon monoxide is 0.005% or 50ppm

Methane CH_4: percentage of methane must be less than 0.75% in general body of return air

And must be less than 1.25% in any part of the mine.

Nitrous fumesNO, NO₂, N₂O₃, N₂O₄, and N2O:TLV or MAC value fornitrous fumes is 5 ppm

Sulphur dioxide SO₂:TLV or MAC value forSulphur dioxide is 5 ppm

Hydrogen sulphide H₂S: TLV or MAC value forHydrogen sulphide is 10 ppm

Aldehydes & acrogens: TLV or MAC value is 0.1 ppm

Degree of gassiness

During coalification process, some amount of gas gets trapped between strata of coal, coal and adjunct mineral. As mining operation cut that strata or seam, that trapped gas start emission from the cracks and cuts. This gas mixture mainly contains methane in its composition. We all know that methane is anexplosive gas under certain condition. This gas has atendency to accumulate over roof and caves in mines. On getting enough temperature and oxygen this gas gets self-exploded.  As apercentage of emission of methane is not common in all types of gassy mines so Coal mines are categorised into three degrees depending on their emission rate and production rate as par coal mines regulation 1957.

1. First Degree: In this type of mines methane percentage does not exceed 0.1% and emission rate of methane does not exceed 1 m³per tonne of coal produced.
2. Second degree: In this types of mine methane percentage exceed 0.1% and/or the emission rate of methane exceed 1 m³ but does not exceed 10 m³ per tonne of coal produced.
3. Third degree: when emission rate of methane exceeds 10m³per tonne of coal produced.