How long does silica dust stay in the air?

How long silica dust stays in the air is a combination of factors such as particle size distribution, wind direction, wind speed and turbulence. It is not based on one factor, but multiple of factors which differ from one scenario to the other. Silica dust is an atmospheric contaminant that is emitted from different sources. It is amongst many types of airborne contaminants that may mix with and be diluted by ambient air and travel to a target person, object or area where it has the potential to cause health problems such as silicosis, chronic pulmonary obstructive disease (COPD) and lung cancer. Respirable crystalline silica dust is carcinogenic, hence knowing how long it stays in air will provide more clarity on effective dust control methods which deal with it before and after it becomes airborne. Understanding and dealing with silica dust behaviours in the air takes synergy between real-time air monitoring and silica dust control techniques. It is a classic case of ‘Where air monitoring meets dust control’ because the particle size ranges measured by real-time air monitoring can provide tangible data to enhance the efficiency of dust control methods that are specific to what the data is saying. The data-driven approach to silica dust control is seldom used in the industry and GRT is breaking that norm by showcasing the possibilities. After all, we are in the 4th industrial revolution where data is the currency used to make informed decisions. 

GRT explores how long silica dust stays in the air by answering the following:

• What factors affect silica dust staying in the air?
• Where is silica found?
• What are the global workplace exposure limits (WELs) for silica dust?
• What can we learn from silica real-time dust monitoring?
• What role is GRT playing in silica dust control?

What factors affect silica dust staying in the air?

1. Size distribution is key to the discussion of how long silica dust stays in the air. This is because the particle size governs the length of time for which the particle will remain suspended in the air, the way it will settle, and the air velocity required to remove the particles from the workings. Silica dust behaves as an aerosol when suspended in the air. An aerosol consists of particles and the gas in which they are suspended. All aerosols are temporarily unstable. They experience change with the passage of time. Some important aerosol characteristics which can change are:

• Total mass concentration of a contaminant
• Fraction of a contaminant in the particle or vapor phase
• Particle size distribution

2. Wind direction at the source of silica dust determines initial direction of transport processes such as convection and advection, which mediate silica dust buoyancy in the air. Silica dust concentrations are sensitive to wind direction. There is normally considerable change of direction with height, especially near the ground. Although surface friction causes the wind to shift clockwise (veer) with height near the ground, the horizontal thermal structure of the atmosphere may exert a dominating influence at higher altitudes, such that the wind will shift counterclockwise (back) with additional height. Cold air advection in air layer will cause the wind to back with height through the layer. Warm air advection will cause veering with height.
3. Wind speed increases with height. One of the effects of wind speed is to dilute continuously released silica dust at the point of emission. Whether the source is at the surface or elevated this dilution takes place in the direction of plume transport. Wind speed also affects the travel time from source to receptor; halving the wind speed will double the travel time. For buoyant sources, plume rise is affected by wind speed. The stronger the wind, the lower the plume.
4. Turbulence is highly irregular motion of the wind. The atmosphere does not flow smoothly but has seemingly random, rapidly varying erratic motions. The most important mixing process in the atmosphere which causes the dispersion of silica dust is eddy diffusion. The atmospheric eddies cause a breaking apart of atmospheric parcels which mixes polluted air with relatively unpolluted air, causing polluted air at lower and lower concentrations to occupy successively larger volumes of air.

Where is silica found?

The most common form of silica is quartz. There is growing concern that silica dust exposure levels and measures instituted to control exposure seem to forget the quartz component which also determines the level of risk. Knowledge of this information about silica also helps in the dose/response approaches to dust control at different sources of silica dust. The amounts of silica in various types of stone is as follows:

• Sandstone, gritstone, quartzite – more than 70%
• Concrete, mortar – 25-70%
• Shale – 40-60%
• China stone – up to 50%
• Slate – up to 40%
• Brick – up to 30%
• Granite – up to 30%
• Ironstone – up to 15%
• Basalt, dolerite – up to 5%
• Limestone, chalk, marble – up to 2%

What are the global workplace exposure limits (WELs) for silica dust?

There is a growing awareness which we hopefully aim to turn into knowledge that ALL dust and not just the toxic fraction can impact on a person’s health. Silica dust generating industries work on eight-hour Time Weighted Average (TWA) and 15-minute Short-Term Exposure Limits (STEL) which form part of what is considered in creating the legislated maximum allowed concentration in workplace air for workplace exposure limits (WELs).

Silica WELs around the world are as follows:

• Australia recently cut from 0.1 to 0.05 mg/m3
• Canada (British Columbia): 0.025 mg/m3
• Finland, Ireland, Italy, Portugal: 0.05 mg/m3
• Netherlands: 0.075 mg/m3
• Poland: 0.3 mg/m3
• United Kingdom: 0.1 mg/m3
• United States of America: 0.05 mg/m3

The 0.1 mg/m3 standard represents a 2.5% risk of developing cancer. Measurement of silica dust exposure in real time tackles the challenges arounds WELs as dust monitors can be set to standards best suited to reduce the risk of developing cancer and still notify using an alarm if those standards are exceeded. 

What can we learn from silica dust real-time monitoring?

We had a chat with Steve Holland, Managing Director of leading safety technology company, Trolex and these were his insights on the synergy between real time dust monitoring and dust suppression technologies. 

“In our experience to make good simple decisions you need clear simple data. We developed the AirXD and XD One real time monitoring with this in mind , in order to utilise your dust suppression at the right time in the right place, real time monitoring is simply the only solution, you can’t accurately do this with an TWA over 8 hours.

 The ultimate pairing of world class dust suppression products from commendable companies like GRT and real time data has the ability to save more lives and be more cost efficient. “If you’re going to do it, do it right” is the message we need to emphasise, utilising BreatheLITE software you can simply see where and when you need dust suppression, you can analyse your trends easily and store 10 years of data on device or locally. 

The beauty of all this is maximising safety, saving lives, minimising cost and having data records readily available for compliance or analysis whenever you like.”

What role is GRT playing in silica dust control?

The source of silica dust may vary from mining, civil construction, renewable energy, oil and gas and rural applications. We will evaluate each of the given sources looking at specific sources of silica dust in the different operations and mention which GRT products applied using superior dust suppression units can be used from the product portfolio. 

Mining: Underground and open-cut rock mining activities generate silica dust which becomes airborne. Activities such as drilling, blasting, stockpiling, haul road transport and conveyor belts and transfer points require dust suppression to prevent silica dust exposure. 

Products: GRT: Haul-Loc, GRT: ACTIVATE, GRT: 12X, GRT: DC Binder, GRT: Wet-Loc.

Civil construction: Different projects in civil construction generate airborne silica dust. Road construction, road maintenance, airport runway construction amongst many others require silica dust control. 

Products: GRT: Enviro-Binder, GRT: Haul-Loc, GRT: Soil-Loc, GRT7000, GRT: Wet-Loc.

Renewable energy: It is important to protect the solar generating capacity of solar farms by making sure that dust does not foul the solar panels. Airborne silica dust suppression of solar farmyards and roads enables solar farms to operate at their maximum capabilities. 

Products: GRT: Enviro-Binder, GRT: Haul-Loc, GRT: Soil-Loc, GRT7000.

Oil and gas: The inroads of oil and gas facilities require consistent dust suppression to cater for workplace safety and health. Given the operations in these areas are very labor-intensive hence preventing exposure to airborne silica dust using the different GRT products saves lives.  

Products: GRT7000, GRT: Wet-Loc, GRT: Enviro-Binder, GRT: Haul-Loc, GRT: Soil-Loc.

Rural: Wind erosion and soil erosion are prone in unsealed roads and yards. Binding surfaces and preventing the effects of wind erosion also helps in dealing with silica dust at its source. It is better to control erosion and silica dust at it source rather than catch sediment. 

Products: GRT: Wet-Loc, GRT: Enviro-Binder, GRT: Soil-Loc, GRT5000

Sports and recreation: Airborne silica dust is dislodged from on BMX race tracks, footpaths, mountain bike tracks, walking trails, and outdoor areas and is a nuisance. Dealing with silica dust in these sports and recreation facilities is key for improving air quality in these areas. 

Products: GRT: BMX 

Silica dust – takeaway points

The particle size of silica dust, wind direction, wind speed and turbulence all contribute to how long it stays in the air. Its physical dimensions may be nanometers to micrometers, and this governs its buoyancy and how long it stays in air based on distinction into different particulate matter (PM) classification of PM1.0, PM2.5, PM4.25, PM10, total deposited particles (TDP) and total suspended particles (TSP). The various wind-related aspects contribute massively to the motion of these different particle sizes in the atmosphere. GRT is taking it a step further to be a knowledge hub for dust control techniques to deal with airborne silica dust at its source. 

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REFERENCES

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• Jahn, D., Bullock, W.H., and Ignacio, J.S. 2015. A Strategy for Assessing and Managing Occupational Exposures. 4th Edition. American Industrial Hygiene Association Exposure Assessment Strategies Committee.
• Logachev, I.N., and Logachev, K.I. 2014. Air Quality and Ventilation: Controlling Dust Emissions. CRC Press. Taylor & Francis Group. London.
• Ramachandran, G. 2005. Occupational Exposure Assessment for Air Contaminants. Taylor & Francis Group. London.