Dust is a problem with which we are all familiar in our homes, but it can be a much more serious problem in certain workplaces, especially when it comes to a factory setting. This article explores how the dust being produced on a daily basis can damage your health and could even prove fatal.
Employers have a legal responsibility to protect their employees and the surrounding environment from airborne contaminants, including dust, but they also have a social responsibility to look after the health and wellbeing of their employees. There are specific regulations which fall under the Control of Substances Hazardous to Health which outline how dust should be managed.
What is Dust?
This simple question is more complicated than it might sound. There are many definitions out there, but one of the most useful is to consider its physical characteristics. Dust can be defined as a fine, dry powder that is made up of tiny particles of earth or waste matter. It can be found on surfaces or it can be airborne.
Dust can be made up of many different kinds of material. For example, the majority of dust in the home is made up of particles of human skin or hair. It can also come from fibers of clothing, sawdust, metal dust, stone dust, or chemical powders that might be highly toxic.
The Health and Safety Executive classify dust in two different ways. The first is supplied dust, which is created when a powder is being handled and some gets disturbed and rises into the air or spreads across surfaces. The second is process dust, which is formed through a process such as drilling, cutting or crushing a material.
How Can Dust Be Hazardous?
The airborne factor is particularly important when considering dust as a health hazard. If dust is in the air then it means you can breathe it in, exposing your lungs and airways to it. This can cause physical blockage or irritation within the respiratory system and is extremely dangerous if the material is toxic or even carcinogenic as the lungs may be able to absorb it quickly into the bloodstream.
Dust may be made up of materials that are combustible and could, therefore, cause an explosion. In this case, dust doesn't need to enter a person's body to be dangerous. Many materials are not overly toxic but can be extremely hazardous because they create dust which can combust. For example, flour, wood or aluminum can create dusts that are potentially explosive and particular care needs to be taken when handling and transporting them in bulk. The ST value of a dust measures how combustible it is compared with other types of dust. If a dust has a ST value higher than zero then this indicates that there is a risk of explosion and the higher the ST number, the greater the risk of explosion.
Presence of a combustible dust in the air is not sufficient on its own to cause an explosion. There are upper and lower limits to the concentration of dust in the air which can lead to explosion and concentrations above or below these limits are less hazardous. In addition to the correct ratio of dust to air, for an explosion to happen, the dust particles need to be within a specific size range.
The most hazardous type of combustible dusts occur where the dust-to-air mixture is fuel-rich. This means that there is not enough oxygen in the air, so it cannot burn all of the dust in one go and partly burned particles remain. In turn, these glowing embers can ignite further dust clouds, leading to multiple explosions which are particularly damaging and can spread further.
Combustible dust is more hazardous when it is airborne because explosions can be fast, violent, and can cover a wide area. When the dust is in a heap or a pile, it will burn more slowly as there is a limited surface area that is exposed to the oxygen in the air. Although the results of a pile of burning dust may initially be less dramatic, they are still hazardous, especially if there is a large quantity of dust and other combustible materials, such as that to be found in a timber mill.
How Can You Protect Yourself from Hazardous Dust?
The first step to controlling dust is to examine how the dust is created in the first place. There may be ways to reduce the volume of dust that is made or to ensure that less of it becomes airborne. Alternatively, if a process could be automated then it may be possible to remove people from the immediate area and therefore reduce the risk of human exposure.
In addition, you can reduce the hazard level if you are able to switch materials to an alternative that is less toxic, or to one that does not create as much dust. If an alternative material produces larger particles that would be less invasive to the human body, then this could also reduce the risk of harm.
One of the most common ways in which you can control dust is to extract it from the environment using dust extraction technology. In trying to extract dust from an environment, it is important to consider whether the particles can be classed as inhalable dust or respirable dust. Inhalable dust is made up of particles which range from 0.01 to 100 microns. To give you an idea of scale, a human hair is typically between 30 and 100 microns thick. Inhalable dust is generally visible to the naked eye under normal light conditions, but it is small enough to get into a person's nose, throat, and mouth.
Respirable dust particles are smaller, typically below 10 microns and are too small for most people to be able to see with the naked eye so they may be unaware that there is any dust in the environment. These particles are small enough to get right into the smaller tubules of the lungs so respirable dust is generally considered to be more hazardous as it gets further into your system.
Removing Dust from the Environment
Systems that extract dust and fumes are also known as local exhaust ventilation systems. There are a variety of systems available in many shapes and sizes, but they all fall into two main types.
The vast majority of systems are ‘recirc’ or recirculation extractors. Recirculation extractors are devices that pass air through a filter to trap the dust and pass the clean air back into the local environment near the source. This allows you to easily manage the dust produced and dispose of it without making significant changes to your working environment. These extractors can be portable, with the advantage that they can be moved to the area where there is the greatest need for dust removal.
The other main type of extraction system is known as a 'vented to atmosphere' system, but these are mainly targeted at fume extraction rather than dusts. These are more traditional and involve a combination of ductwork, extraction hoods, and duct filters connected to a fan in the roof and an exhaust that releases air outside the building or to an area away from the initial source. These systems are predominantly utilized for hazardous fumes but can also extract small volumes of fine dusts when necessary.
Dust can be filtered in a variety of ways and there are many grades of dust filter. The most effective type and probably the most widely known is a High Efficiency Particulate Arrestor (HEPA). These filters are 99.99% effective at removing particles of 0.3 microns. Other types of filter include F8 filters, dust bag filters, course dust filters, M Class filters, and inline dust filters.
Extracting Combustible Dust
As discussed, dust that is combustible is particularly hazardous when it is airborne, so it is highly desirable to remove it from the atmosphere. However, there are special considerations when trying to remove these types of dust as the extraction system itself could create the necessary conditions for the dust to explode.
Pulling the dust into an extraction system may cause the concentration of the particles to fall within the limits for creating an explosive atmosphere. This cloud of explosive dust is then captured in a pressurized container and more air is forced through it creating prime conditions for an explosion.
One way to prevent the dust exploding within an extraction system is to eliminate potential ignition sources. The dust cannot combust without something to start the initial fire. For example, the dust could ignite if the extraction fan overheats, or if there are external factors nearby creating sparks. If there are grinding, drilling, or cutting works close to the dust extraction system then these could cause the dust to ignite. Hot works such as welding, common in metal working facilities, could also be a source of ignition.
There are several things you can do to reduce the risk of an explosion:
* Make sure you use non-sparking fans in your extraction system
* Maintain equipment properly and check it regularly so any faults are picked up quickly
* Train all employees to work within designated work areas and make sure they appreciate the risks of working in different areas (i.e. taking drilling work into an area that does not have an appropriate dust extraction system)
* Carry out comprehensive risk assessments and update them regularly
* Use inline spark arrestors and pre-separators in your extraction system that can help to prevent the dry dust contacting with hot materials
There are also steps you can take to reduce the danger if an explosion does occur. Firstly, you can install explosion relief panels that are slightly weaker than the rest of the system. These will then blow out first and direct the main force of the explosion away from areas where people may be working.
Secondly, all areas with potentially hazardous dust should have adequate fire suppression systems. The extraction unit used should include inbuilt extinguishers and sprinklers to suppress any fires that may start and prevent them from spreading to other areas or reaching the critical point where an explosion could occur.
Some types of dust, especially some metal dusts, can be extremely combustible and pose a high risk. It is not sufficient to use dry dust extraction for these dusts and you need to use wet extraction instead. Most wet extraction systems are either wet extraction benches or wet collector systems.
Titanium dust can be particularly hazardous with the size of the dust particles once again determining the degree of hazard. Fine dust ignites more easily than larger particles, but since larger chips are often mixed with fine particles, both should be considered dangerous. To stop titanium dust from exploding, it should be extracted using wet dust collectors that have a titanium specification. Systems must be especially designed with vents to allow the hydrogen to escape during the process. The extractors also control water level as titanium fires react violently with water at high temperatures. It is vital that water fire extinguishers are never used on titanium fires.
Magnesium in a block has a melting point of 650°C, but fine magnesium dust can be ignited at temperatures below 400°C so this needs to be considered when protecting yourself and others from this kind of dust. Like titanium dust management, the dust collector used should be designed to allow hydrogen gas to escape and it should have some kind of water level control. Magnesium dust collectors should also be grounded, and it is important to avoid mixing magnesium dust with dust from other metals.
Dust can be hazardous when inhaled and may represent a risk of explosion. There are ways in which you can reduce the production of dust or change the type of dust produced to reduce the hazard level. It is also possible to protect yourself and others through careful management of the environment and by using the appropriate dust extraction technology.
Paul Riddick is co-founder and technical director at fume and dust extraction specialists Vodex. For more information, visit www.vodex.co.uk.
For other news, articles, and equipment reviews, visit our Explosion Protection & Safety Equipment Zone
Click here for a List of Explosion Protection & Safety Equipment Manufacturers