Dust is an inevitable part of materials mining. It’s an unfortunate truth that something so small can cause far-reaching health, safety and environmental impacts.
In this article, we’ll take a look at the different types of dust generated by mining operations, specifically PM10 dust. We’ll also explore the long term consequences of breathing in PM10 dust, relevant legislation in regards to exposure limits, and how mines can implement highly effective control measures for reducing worker and community exposure.
What is particulate matter?
Commonly just called ‘dust’, the term particulate matter (PM) is used to describe the range of particles that exist in the air we breathe. Dust can be broken down into two different categories - organic and inorganic. Organic dust is made up of biological matter such as cane fibre, cotton, tobacco and grain dust. Inorganic dust comes from non-living matters such as coal, silica, asbestos and iron.
The type and size of PM in the air has impacts on the surrounding environment and on the health of those breathing it in. The World Health Organisation (WHO) identifies two size fractions when classifying dust:
Inhalable dust is the fraction of a dust cloud that can be breathed into the nose or mouth. Inhalable dust particles are less than 100 microns in size and can be seen by the naked eye.
Respirable dust is the fraction of inhaled airborne particles that can penetrate beyond the terminal bronchioles into the gas-exchange region of the lungs. Respirable dust is too small to be seen by the naked eye.
The size of dust is measured in microns. PM10 dust (also referred to as airborne dust) is dust with particles smaller than 10 microns in diameter (smaller than 1/7th of a hair width). It can be made up of both inhalable and respirable dust and can therefore become lodged in the lower respiratory tract.
Particles in the size range 2.5 microns to 10 microns in diameter are referred to as coarse particles (PM 2.5-10). Particles which are less than 2.5 microns are referred to as fine particles.
PM10 dust in mining operations
Around 40% of dust from mining activities consists of PM10 dust and is generated from activities such as mechanical disturbance of rock and soil materials by dragline or shovel, bulldozing, blasting and vehicles on haul roads.
PM2.5 dust (i.e. fine particles) accounts for about 5% of the particles emitted during mining processes. Fine particles produced at mine sites are mainly from vehicle and mobile equipment exhausts.
Health impacts of mine generated PM10 dust
Because such a high percentage of dust generated at mine sites is PM10 or smaller, it’s a common cause of concern for health, safety and environmental managers.
Prolonged exposure to PM10 has been proven to have many adverse effects on mine workers and people living in surrounding communities, such as:
• Ongoing coughing and shortness of breath
• Worsening asthma
• Increased need for medication
• Respiratory failure
Certain materials pose greater health risks than others. Below is an overview of the most severe types of health conditions as a result of mining different materials.
Environmental and community impacts of mine generated PM10 dust
The impact of mining dust on a nearby community depends on the distance from the mine site and climatic conditions such as wind.
Common community concerns about mine site dust often relate to visibility of dust plumes and dust sources. Visible dust is usually due to short-term episodes of high emissions, such as from blasting.
Other community impacts include dust depositing on fabrics (such as washing) or on house roofs, and the transport of dust from roofs to water tanks, during rain.
In terms of environmental impacts, PM10 dust particles can be carried over long distances by wind and then settle on ground or in water. The effects of this settling may include:
• Changing the nutrient balance in coastal waters and large river basins
• Depleting the nutrients in soil
• Damaging sensitive forests and farm crops
• Affecting the diversity of ecosystems
What legislation is in place to limit PM10 exposure?
Occupational Exposure Limits (OELs) are a key element in dust risk management for mines and are often stipulated in relevant regulations. Although obvious exposure to known harmful agents should be controlled regardless of any existing regulation, establishment of a control limit often draws attention to a substance.
OELs are usually expressed in one of the following forms:
• Time-weighted average concentration (TWA), which is the average concentration over a full shift, usually 8 hours.
• Ceiling concentration, which is an instantaneous concentration not to be exceeded at any time.
• Short-term exposure limit (STEL), which is the average concentration over a specified time, e.g. 15 minutes.
For types of dust which has effects from long-term exposure, such as coal, OELs are usually given as TWAs.
The WHO provides scientific guidance to countries on dust levels that adversely affect human health. However, this guidance doesn’t take into consideration certain factors prevalent within an individual country such as climate, socio-economic conditions and typical worker schedules. As a result, the WHO produces guidelines that a country can then use to inform the development of its own standards.
In South Africa, dust exposure limits are regulated in SANS (South African National Standards) limits (SANS 1929: 2005). This standard takes into account the specific climate and social-economic conditions of South Africa as well as typical worker schedules. Below is a comparison of air quality standards for PM10 dust in different countries.
How can PM10 dust levels be reduced in mine sites?
Dust control measures should include a mixture of engineering controls (such as enclosure of conveyors) and suppression measures (such as watering stockpiles and materials in tipping areas and at transfer points).
While using water bowsers is a primary dust control measure for many mines in South Africa, there are numerous limits to this approach, such as:
• Excessive water pooling and run-off, leading to health and safety risks.
• Chutes and conveyors becoming clogged.
• Increased corrosion of conveyors and other infrastructure as water is carried through the entire plant.
• Excessive moisture content building up in the materials that are being watered.
Perhaps most importantly, low pressure water is not able to sufficiently subdue PM10 sized dust particles. The more effective approach is the implementation of high pressure dust suppression systems.
Dust suppression systems are misting systems which use low volumes of water at high pressure (more than 50 bar) to add a fine atomised mist to the air to capture and subdue airborne dust.
Unlike standard water bowsers, which use large volumes of water at low pressure, high pressure suppression systems are specifically calibrated to capture PM10 dust.
The secret to successful dust suppression is ensuring that the atomised water droplets and the target dust particles are of equivalent size. Spray nozzles need to be exactly calibrated to the right pressure and angle, depending on the type of material being treated. This allows the atomised water droplets to collide and agglomerate to airborne dust particles increasing their mass thus allowing them to fall to the ground due to gravity. If the water droplets are too large, the fast-moving airborne dust will simply flow around them.
High pressure dust suppression systems can reduce levels of PM10 dust by 80%+. This makes dust suppression a highly effective strategy in:
• Reducing the likelihood of workers developing dust related health problems
• Improving on-site safety
• Improving community and environmental outcomes
Want to learn more about control measures for PM10 dust at your mine site?
As leading providers of dust suppression systems, Dust-A-Side understands that the risks of PM10 dust can simply not be ignored.
To learn more about our custom-built dust suppression systems, call us on +27 (0) 12 648 8900. Or you can get in touch by clicking here.