June 15, 2018
by Nayab Sultan
According to the International Labour Organization (ILO), an estimated 160 million people suffer from work-related diseases, with some two million dying annually as a result. About eight per cent of these occupational disease deaths can be directly attributed to respiratory complications, namely chronic obstructive pulmonary disorders (COPD), asbestos-related diseases (ARDs), pneumoconiosis, and silicosis. What exactly is silicosis, though, and what should design/construction professionals know to help reduce its impact on those in the industry?
Conservative estimates from the World Health Organization (WHO) estimate 46,000 annual deaths from exposure to harmful levels of respirable crystalline silica (RCS) dust before progression to silicosis and other diseases. However, with under-reporting and misdiagnosis being common, tens of millions remain vulnerable, including thousands of Canadian workers. Therefore, the actual number of deaths from silicosis is likely to be far higher, particularly in developing countries. This is then further increased by considering environmental exposures.
CAREX Canada, a national surveillance project for estimating the number of workers exposed to substances associated with cancer in the workplace, reports approximately 380,000 Canadians (93 per cent males) are exposed to silica, primarily in the construction sector. In contrast, the figures for asbestos stand at 152,000 for the same sector. According to 2011 cancer statistics from CAREX, 570 lung cancer cases (i.e. 2.4 per cent overall) were attributed to occupational exposure to crystalline silica across the country. (Exposure is considered moderate to high amongst specialist trade contractors and building construction professionals followed by the heavy and civil engineering construction industry in Ontario, Quebec, Alberta, and British Columbia, respectively.)
All dust is not the same
Respirable crystalline silica is a naturally occurring substance found in stone, rocks, sand, and soil, as well as in concrete, fly ash, asphalt, clay, brick, grout, clay and ceramic tiles, some composite materials, and some metallic ores. Shingles, mortar, plaster, drywall, and many products commonly found in construction projects across Canada. Figure 1 offers some examples, while Figure 2 gives a range of RCS commonly found in the Canadian building industry.
RCS is harmful to health when it is inhaled deep into the lungs. The dust that is particularly harmful is smaller than a fine grain of sand. To put this into perspective, the size of a period is about 200 to 300 μm (7.8 to 11.8 mils) in diameter, whereas the RCS dust is about 5 μm (0.2 mils)—so small the particles cannot be seen with the naked eye.
If a person is exposed to high levels of RCS, harmful effects and onset of acute respiratory conditions can start in as little as a few weeks, especially where exposed to particularly high grades of silica dust and with no or ineffective respiratory protective equipment.
There are less-harmful forms of silica dust (e.g. amorphous, diatomaceous earth, or silica gel) that are not recognized to lead to silicosis, lung cancer, or other known diseases commonly associated with RCS. That said, it is possible to chemically change amorphous silica at temperatures above 1300 C (2372 F), where it can transform into RCS.
There are set standards regulatory authorities determine to be maximum levels of exposure before there is likely to be any significant harm. In Canada, this depends on the province (Figure 3).
Exposure needs to be monitored and the health of workers kept under surveillance to ensure levels are not harmful. Attempting to use sight to determine levels does not work—by the time a dust cloud is observed, levels are generally likely to be in excess of what is considered to be safe and of the province’s occupational exposure limit (OEL).
What is silicosis?
The name silicosis is derived from the Latin word Silex or flint, and is associated with inflammation and scarring of the upper lobes of the lungs in the form of lesions. While symptoms related to asbestos may take between 10 and 50 years to manifest, the effects of silicosis can start within a short period as seen in the Hawks Nest Disaster (Figure 4). In this incident from the 1930s, hundreds of construction workers died as a result of acute silicosis and associated conditions due to exposure of high levels of RCS dust while building a tunnel through sandstone-based rock with little (or no) respiratory protection. Individual susceptibility is based on factors such as amount of dust, particle size, respiratory protective equipment worn, the individual’s overall health status, genes, and whether the person is a smoker.
In acute form (i.e. short-term, severe, or sudden), the symptoms are typically bluish skin, breath shortness, coughing, and fever. It is not uncommon for this to be misdiagnosed as pulmonary oedema (i.e. water in the lungs), pneumonia, or tuberculosis. Symptoms can continue to develop even after exposure has stopped. Other conditions can also occur, such as bronchitis, lung cancer, or arthritis.
So how can construction professionals work safely without being harmed? If the threat of exposure to RCS cannot be eliminated altogether, then there are a few control measures that may work in keeping dust to relatively safe levels. To ensure compliance with various legislative requirements, the local rules must be followed. Generally, they are based on the principles of occupational hygiene: assess, control, and review.
Assessing and controlling the risks
High dust levels are caused by one or more of the following:
For specific activities where RCS could become airborne, separating the workers and the airborne materials can be a successful solution. In some cases, the means can be as simple as dousing the dust source with a water stream and standing upwind. Where possible, it is also important to specify materials that have less silica content or do not produce large volumes of dust.
Stopping or reducing the dust before work starts is also crucial for controlling the risks. This might mean different materials (e.g. the right size of building materials reduces cutting or preparation), less-powerful tools, or other work methods.
Regardless, the most important action is to stop the dust getting into the air, and more critically into the breathing zone of the workers. There are two main ways of doing this effectively: water and on-tool extraction.
Water damps down dust clouds, but to be effective, this needs to be in a fine mist or deluge form at a steady volume rate. Simply dampening material beforehand may not be as effective.
On-tool extraction removes dust as it is being produced. This type of local exhaust ventilation (LEV) fits directly onto the plant, tools, and equipment. The “system” consists of several individual parts—the tool, capturing hood, extraction unit, and tubing. It is important to use an extraction unit to the correct specification (i.e. H [high], M [medium], or L [low] class of filter unit). An industrial vacuum cleaner with a high-efficiency particulate air (HEPA) filter should be employed.
One size does not fit all: Thoughts on RPE
Before selecting respiratory protective equipment (RPE), there are various considerations ranging from the type to the duration of activities. Generally, it is far easier and more efficient to use engineering controls to protect a wider range of workers than ill-fitting RPE that may give a false sense of security and endanger the lives of wearers.
Refer to Canadian Standards Association (CSA) Z.94.4-11, Selection, Care and Use of Respirators, before selection of RPE, which must be adequate for the amount and type of dust; it should have an assigned protection factor (APF) showing how much protection it gives the wearer. The general level for RCS dust is an APF of 40 based on the activity—this means the wearer only breathes 1/40th of the amount of dust in the air.
The RPE must also be suitable for the job. Disposable masks or half-masks can become uncomfortable to wear for long periods, whereas powered air-supply RPE helps minimize this. When people are working for more than an hour, it is also important to consider whether the RPE is compatible with other items of protective equipment. Face-fit testing can ensure it is being worn correctly (ideally, anyone using tight-fitting masks will be clean-shaven), but RPE is the last line of protection.
The correct HEPA-type cartridge should be fitted to protect workers against silica dust exposure. In Canada, this tends to be magenta or purple, but one should always refer to the manufacturer’s safety data sheet or follow recommendations based on findings of air-monitoring. It should be remembered that on-tool extraction coupled with water dousing and suitable RPE provides significantly greater protection than RPE alone.
Reviewing the controls
Even with all the right controls in place, they must be checked to ensure they are working properly. This means:
Personal or background air-monitoring should be carried out to accurately determine the levels of dust generation at various positions in the worksite. Monitoring will enable the correct selection of dust control methods and respiratory protection. Regular air-monitoring on existing and new work methods will help ensure in-place dust control methods remain effective and provide adequate protection for workers.
A health surveillance program will be important to verify controls are protecting staff. The surveillance program should be under the direction of a suitably qualified medical professional and can include pre-employment health screening, occupational history and respiratory questionnaires to set baselines, coupled with suitable lung function testing and biological effect monitoring to monitor actual exposure levels. The frequency of testing should be determined by risk factors rather than a generic pre-determined testing period.
It is vital dusty clothes do not contaminate cars, homes, or other areas outside of the worksite. Workers should have disposable overalls or washable clothes to change into at the site. Before leaving, workers should shower if possible and then change into clean clothes without shaking these out. Workers should not be taking dusty clothing home to wash—this may expose family to harmful levels of RCS dust. There are documented cases of family members getting sick from indirect exposures to silica dust and asbestos.
Workers should be provided with competency-based training on silica dust awareness. This includes information about the health effects of exposure to silica dust, work practices to follow when silica dust is created at a worksite, and the appropriate use and care of protective equipment. It is vital to ensure workers actually understand rather than just sit through the talk.
Prevention is the best form of protection against silicosis and other illnesses, by either complete elimination of the silica dust hazard or introducing suitable dust-control strategies using dry air-filtering and water spray where dust emanates. That said, each worksite needs to have a comprehensive review of the hazards covering which technique is likely to be appropriate and establishing suitable and effective risk-control measures such as development of an exposure control plan (ECP)—required in most workplaces where high levels of dust are identified. Some provinces such as British Columbia have developed online tools for the construction sector.
Based on current medical knowledge, silicosis is not considered to be curable. (Therapy can alleviate some of the symptoms and provide some relief.) It can develop within a few weeks of heavy exposure and continue even years after exposure ceases. (Smoking can also exacerbate the symptoms.) In many cases, silicosis is considered asymptomatic—a person may not show classic signs of silicosis, which is one of the reasons why misdiagnosis is widespread. Prevention of exposure is the key.
Nayab Sultan is a researcher at Silicosis Research, and is currently pursuing his PhD in silicosis prevalence in Canada. He holds M.Sc. and M.Phil. degrees in the science of occupational health, safety, and the environment. Sultan has more than 25 years of experience in construction and engineering, and has worked as a subject matter expert to several organizations, both in Canada and beyond. He can be reached at firstname.lastname@example.org.
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