Carbon dioxide in coal mines

Exposure Standard first adopted in 1990

No standard should be applied without reference to Guidance on the interpretation of Workplace exposure standards for airborne contaminants.

Documentation notice: National Occupational Health and Safety Commission documentation available for these values. For this standard considerations, such as, economic, social or technological implications, or sampling and analytical limitations, have also been taken into account.

* Note: These exposure standards apply ONLY to underground coal mining where carbon dioxide emission from the coal strata is a natural phenomenon. For other work situations, the Exposure Standards Working Group has recommended for carbon dioxide a time-weighted average (TWA) exposure standard of 5,000ppm and a STEL of 30,000ppm ⁽¹⁾ and readers should refer to the relevant ACGIH documentation ⁽²⁾ for substantiation for these values.

1. HEALTH EFFECTS OF CARBON DIOXIDE

The general health effects of carbon dioxide have been reviewed^(2,3) . The Exposure Standards Working Group has considered these reviews and is of the view that 1.5% (15,000ppm) should represent the upper limit of tolerance for carbon dioxide for prolonged exposure, below which measurable decrement in basic physiological functions or in psychomotor performance should not be observed. For short-term exposure, a small reduction in exercise tolerance can be observed for carbon dioxide concentrations ranging from 3-4% (30,000-40,000ppm). The depth of respiration is markedly increased as concentrations exceed 4%.

At a concentration of 10% or more, carbon dioxide can cause unconsciousness to workers. The gas does not give a warning of its presence in an asphyxiating concentration and a worker may enter a confined space, or a tank or vessel, and be overcome before he becomes aware of the danger and make his escape.

2. UNDERGROUND COAL MINING

The atmospheric environment in underground coal mines differs from other work situations in many respects. The mining procedures usually generate considerable amount of airborne dust, which requires special dust control techniques. (Note: the Exposure Standards Working Group has recommended an exposure standard for coal dust⁽¹⁾.)

In underground coal mines, oxygen deficiency (blackdamp) and the accompanying formation of carbon dioxide gas usually occur in abandoned workings. Variations in atmospheric pressure and mine ventilation would cause the spread of carbon dioxide to other areas of the underground mines. Other contributing sources of carbon dioxide would also include diesel engine emissions⁽⁴⁾. (Note: other mine gases, such as methane (firedamp), are also present in different concentrations⁽⁵⁾.) It has been reported that the background levels of carbon dioxide in New South Wales⁽⁶⁾ and Western Australia^(7,8) underground coal mines are of the order of 0.5% (5,000ppm) with short-term exposure ranging from 0.5-1.0% (5,000-10,000ppm). The oxygen content is about 20.2%.

Both the Western Australia and New South Wales legislation^(9,10) requires that the air in underground coal mines shall not contain more than 1.25% (12,500ppm) carbon dioxide. Requirements for environmental monitoring and ventilator control of airborne contaminants are also stipulated. There are also international initiatives to deal with the health and safety in underground coal mines^(11,12).

3. RECOMMENDATION FOR EXPOSURE STANDARDS

The Exposure Standards Working Group recommends a TWA exposure standard of 12,500ppm and a STEL of 30,000ppm for carbon dioxide in underground coal mines, based on both health and practicability considerations. The Working Group also recognises that these standards may only provide a relatively small margin of safety for protecting workers' health from prolonged exposure to carbon dioxide in coal mines. For this reason, it is advisable that exposure to mine gases should be reduced to the lowest workable levels through proper engineering control measures, and under no circumstances should these exposure standards be exceeded.

REFERENCES

1. National Occupational Health and Safety Commission ( NOHSC ), Exposure Standards for Atmospheric Contaminants in the Occupational Environment - Guidance Note and National Exposure Standards, AGPS , Canberra , May 1990

2. American Conference of Governmental Industrial Hygienists, Documentation of the Threshold Limit Values and Biological Exposure Indices, 5th edition, Ohio , 1986

3. Beard RR, Inorganic Compounds of Oxygen, Nitrogen, and Carbon, In: Patty's Industrial Hygiene and Toxicology, ed by Clayton GD & Clayton FE, 3rd revised edition, Vol 2C, pp.4125-4126, 1982

4. Reger R et al, "Coal miners exposed to diesel exhaust emissions", Ann Occup Hyg , 26, 799-815, 1982

5. International Labour Officer ( ILO ), Encyclopaedia of Occupational Health and Safety, ed by Parmeggiani L, Vol 2, third revised edition, Geneva , pp.1362-1399, 1983

6. New South Wales Coal Association, Communication to the Exposure Standards Working Group, 1988

7. Western Australia Department of Mines, Communication to the Exposure Standards Working Group, 1988

8. The Chamber of Mines of Western Australia (Incorporated), Communication to the Exposure Standards Working Group, 1988

9. The Coal Mines Regulation Act 1982 (NSW) and regulations

10. Coal Mines Regulation Act 1946 ( WA ) and regulations

11. International Labour Office ( ILO ), Safety and Health in Coal Mines, ILO Code of Practice, Geneva , 1986

12. International Labour Office ( ILO ), Prevention of Accidents due to Explosions Underground in Coal Mines, ILO Code of Practice, Geneva , 1974

Footnotes:

Documentation notice:

Entries carrying a notice for National Occupational Health and Safety Commission documentation indicate that these substances have been reviewed in detail by the Exposure Standards Expert Working Group and that documentation supporting the adopted national values is available in the National Commission's Documentation of the Exposure Standards [NOHSC:10003(1997)].