Understanding Dust in Tunneling and Underground Projects

The natural atmosphere that we breathe contains not only gaseous components but also large numbers of liquid and solid particles originating from a combination of natural and industrial sources known as aerosols. Dust is the term used for the solid particles. The first regulations for mine dust appeared around 1912 in South Africa

Sources

Dust can be produced by different activities in tunnel and underground projects, including excavation, drilling, bulk material transportation, loading and unloading, open-air storage, and movement of equipment.

The main source of dust in tunneling and mining operations is from rocks and minerals, which is produced when they are broken-down and reduced to a size convenient for handling. Part of these materials are coarse enough to settle out rapidly by gravity fall. A certain percentage is so fine that they tend to remain suspended in air due to the combined effect of their low settling velocity and aerodynamic properties.

Size and Behavior

Dusts vary widely in shape and size. The simplest method to quantify size is projected area diameter or equivalent diameter. Particles do not become visible to the naked eye until they are more than 10 microns in equivalent diameter.

Dust includes a wide range of particle sizes from over 1 mm to less than 1 μm. Particle size significantly influences their behavior: particles greater than 10 μm settle; particles between 0.1-10 μm suspend in the air but settle at a constant velocity; and particles between 0.01-0.1 μm diffuse in the air and remain in a suspended state. The most hazardous particles are those with particle sizes < 5 μm – at this size, they are small enough to be inhaled.

Hazard Potential

Dusts can be classified based on their potential hazard to the health and safety of industrial workers. Toxic Dusts cause chemical reactions in the respiratory system or allow toxins to be adsorbed into the blood stream. Carcinogenic dusts (e.g., wood, diesel, and arsenic) cause a variety of cancers.

Fibrogenic dusts scar lung tissue. Silica and some silicates are some of the most common and hazardous dust particles found in tunnel construction. A risk assessment should consider the presence of silica and the likely generation of dust containing silica. Exposure to respirable crystalline silica is known to cause silicosis, a respiratory lung disease that can be fatal.

Explosive dusts consist of materials (e.g., organics, chemicals, metals) that become explosive when finely divided at high concentrations in air. Nuisance dusts cause irritation to the eyes, nose, and throat. These include salt, potash, gypsum, and limestone.

Control

Possible sources of the generation of dust in tunnels should be identified and control measures implemented to eliminate or minimize the generation of the dust at the source.

There are five general methods to control dusts.

1.     Prevention: The best method for dust control is to prevent it from becoming airborne. This is not always possible, but in some cases, such as dust generated by drill and blasting, the amount of dust generated can be minimized by controlling the powder factor (amount of explosive).

2.     Removal: Removal is a common practice, with many tunneling sites having dust removal systems. Types of dust collectors include centrifugal (i.e., cyclones), filter of fabric, wet scrubbers, and electrostatic precipitators.

3.     Suppression: Suppression is probably the easiest and most common method used for dust control. By using water sprays, foam, or an air/water mist (atomizers), dust can be effectively controlled.

4.     Isolation: Isolation involves restricting blasting times. This is a common practice to help workers avoid being in contact with contaminated air.

5.     Ventilation: Ventilation, a combination of dilution and removal, is a very common method employed underground.

Measuring

Anyone who attempts to measure dust exposure needs to understand the relationship between exposure, dose, and Time Weighted Average (TWA) values. Exposure is the level or concentration of an aerosol that a person is exposed to at a given time. Dose is the total accumulated exposure over a given time.

The TWA exposure can be expressed as a function of the dose: ?? (TWA) = ???????? / ?????????? ???????????????? ????m??

Measuring dust concentrations is difficult due to their dynamic nature and variability across project areas. Methods include particle counting, gravimetric, and photometric (light scattering) methods.

Masks

Dust Masks are commonly used in tunneling where dust generating operations are underway.

The following are few guidelines for the better use of the dust masks:

Always review the data safety sheets of the material you work with to assess the type of mask appropriate for the operation.

Dust masks are to be used for airborne particulates only and are not suitable for hazardous vapors.

Change your dust masks frequently and whenever you note a discoloration from the accumulation of particulates.

Ensure that your dust mask fits snugly. If you feel air leaking around the edges, adjust or switch to a different mask.

Beards may interfere with the proper fit, so other options may be necessary when respiratory protection is necessary.

Tunneling projects around the world continue to increase productivity as equipment and practices are improved. Unfortunately, increased production also results in the potential for increased respirable dust generation and worker exposure. In response, we need to apply best controls available and at the same time looking for new innovations to better control dust levels.

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