HERE, THERE, EVERYWHERE
Crystalline silica or silicon dioxide (SiO2) is one of the most abundant compounds in the earth's crust, says Dr. Leon Genesove, chief physician in the Occupational Health and Safety Branch of Ontario's Ministry of Labour in Toronto.
How abundant? CAREX Canada, based at the University of British Columbia in Vancouver, reports that silica is one of the most plentiful minerals on earth and is a basic component of soil, sand and rocks, including granite and quartzite.
As for activity, Natural Resources Canada reports that Quebec, Ontario and Alberta are the primary silica producers in the country, followed by Saskatchewan, British Columbia and Nova Scotia.
Despite its natural and widespread abundance, occupational health experts are not particularly worried about silica, per se. Pure silica is inert, insoluble and indigestible. It does not burn and it yields no dangerous fumes. Nor is non-crystalline or amorphous silica a serious work-related health threat, unless a person is subject to overwhelming exposures. Then, like any other fine dust, it can severely irritate the lungs and breathing passages.
It is fine particles of crystalline silica -- those measuring five to 0.5 microns in diameter and those freshly cleaved (through sandblasting or crushing) -- that are the real worry. Sonia Lal, an occupational hygienist/health and safety specialist with Occupational Health Clinics for Ontario Workers, Inc. (OHCOW) in Toronto, explains that particles five microns and smaller have a higher likelihood of reaching the lower respiratory tract and instigating damage.
If inhaled into the lungs, Dr. Genesove points out that fine particles can "lead, cause or contribute to a number of medical conditions, including silicosis, tuberculosis, lung cancer, connective tissue and kidney disease, emphysema and chronic bronchitis." Respirable silica dust is released when rock or silica-containing materials such as concrete, brick, sand castings, tile, marble, mortar and grout are drilled, blasted, crushed, pulverized, ground, cut or polished. Estimates compiled by CAREX Canada note almost 350,000 Canadian workers, 94 per cent of them men, are occupationally exposed to crystalline silica over the course of a year.
By industry, total exposures in specialty trade contractors (construction) are 140,000; building construction, 68,000; heavy and civil engineering construction, 31,000; metal ore mining, 9,900; cement and concrete product manufacturing, 8,800; support activities for road transport, 7,300; plastic product manufacturing, 7,100; truck transportation, 6,900; highway, street and bridge construction, 6,400; iron and steel mills, 4,900; glass and glass product manufacturing, 4,800; non-metallic mineral mining, 4,200; and other, 50,000.
While the majority of exposed individuals work in construction, there are other environments and work tasks of concern, such as the following:
• coalandhardrockminingoperations, granite quarrying and processing;
• casting and abrasives blasting in foundries;
• agriculture (dust from both ploughing and harvesting); and,
• polishingandsandblastingindental offices.
While silica is used in every province and territory, Ontario leads the country in terms of occupationally exposed workers (see chart).
With almost 350,000 Canadian workers exposed, CAREX Canada notes that crystalline silica is among the most common workplace carcinogens, behind solar radiation (with 1.5 million workers exposed) and diesel engine exhaust (804,000), and just ahead of polycyclic aromatic hydrocarbons (307,000), benzene (297,000) and wood dust (293,000).
Cheryl Peters, the lead scientist compiling the occupational exposure data for CAREX Canada, cautions that not all of the aforementioned workers are necessarily at risk. The numbers of potentially exposed workers are based on statistical protocols developed in the European Union, says Peters. "The criteria for what constitutes workplace exposure is very low. They do not relate to the risk of developing silicosis."
A WIDER VIEW
Canada-wide statistics compiled by the Association of Workers' Compensation Boards of Canada show that there were 21 silicosis-related deaths in 2008, although the actual number may be higher. In many cases, one of the medical complications of silicosis -- which include tuberculosis, lung cancer or chronic obstructive pulmonary disease -- rather than silicosis itself, may be listed as the official cause of death.
Whether the fatality rate is conservative or not, Dr. Genesove suggests that the toll is still too high. "All cases of silicosis are preventable," he argues.
Exposures, at least among the most heavily exposed sectors of the work force, have dropped significantly since the 1950s. However, several recent Canadian studies seem to indicate that the extent of the danger may not yet be fully recognized or appreciated.
Much of the pivotal research around silicosis to date has focused on the high-exposure work forces, such as sandblasters and drillers, hard rock miners, potters, stonemasons and foundry workers.
But a team of Quebec researchers recently examined the effects of silica exposure on a much broader range of occupational experience. They reconstructed the work histories and silica exposures for two large population-based groups: 857 cases of lung cancer diagnosed between 1979 and 1986, and a second group of 738 cases diagnosed between 1996 and 2001. Each cancer case was matched by sex and age with Montreal residents from the electoral rolls.
The findings, published in the June, 2010 issue of Cancer, Epidemiology, Biomarkers & Prevention, indicate that occupational exposure to silica may be responsible for about three per cent of all lung cancers.
After adjusting for a number of confounding variables, including smoking and past exposures to other occupational carcinogens, the team concluded that any occupational exposure to silica (above background levels) increases the risk of developing lung cancer by an estimated 30 per cent. "Substantial exposures" increased the risk by 70 per cent.
"An association between inhaled crystalline silica and lung cancer has important public health implications, not only for the small number of high-exposure industries that have been targeted in the cohort studies, but for the much larger number of workers exposed in other industries," the study points out.
"I think that respirable silica should, indeed, be a regulatory and research priority," says Dr. Stephen Vida, a member of the Environmental Epidemiology and Population Health Research Group at McGill University in Montreal. In support of that view, Dr. Vida cites the strong association between lung cancer and occupational exposure to respirable silica, the ubiquity of silica in the environment, and the substantial estimated frequency of occupational exposure.
The International Agency for Research on Cancer (IARC) in Lyon, France has classified inhaled crystalline silica (in the form of quartz or cristobalite) from occupational sources as a Group 1 (carcinogenic to humans) lung carcinogen. When the IARC report on silica came out in 1997, it sparked considerable debate.
"Silicosis is definitely a risk factor, while the carcinogenicity of silica itself is still a matter of debate," says Dr. Genesove. "Some studies show a slight positive effect, while others do not support an association. It's best to say the link is still unclear," he adds.
Still, based on carcinogenicity, other toxic properties and the prevalence of occupational exposure, CAREX Canada ranks crystalline silica as an "immediate high-priority" substance. Beyond identifying sectors that need more attention, Peters says CAREX data can "also indicate jurisdictions that may need to revisit their occupational exposure standards."
In Ontario, OHCOW's Lal says she believes silica should be included in the Workplace Safety and Insurance Board's (WSIB) "Road to Zero" plan to eliminate workplace injuries and fatalities. Even though exposure limits continue to be ratcheted down, "we aren't moving as quickly as we should to get this terrible disease under control," Lal says.
Over the last 10 years, the American Conference of Governmental Industrial Hygienists (ACGIH) in Cincinnati, Ohio has twice reduced the recommended Threshold Limit Values (TLV) for quartz and cristobalite: first from 0.1 milligrams per cubic metre (mg/m3) to 0.05 mg/m3, and then by half again to 0.025 mg/m3 in 2006.
A number of Canadian provinces and territories reference these TLVs in provincial legislation, although not all have adopted the most recent edition.
New Brunswick, as an example, still relies on the ACGIH's 1997 booklet of TLVs for its occupational exposure standards (with the exception of lead sulphide and formaldehyde). Ontario, Quebec, Saskatchewan, Alberta and the Yukon, among other jurisdictions, have adopted their own requirements related to silica.
"The mining industry has certainly been on the [regulators'] radar for some time," says Sudbury, Ontario-based Nancy Keller, manager of occupational health and medicine for mining giant, Vale. "The industry has been very conscientious about putting silica-control programs in place where risk is identified, as well as undertaking ongoing monitoring and ensuring workplace controls are used," Keller says.
That approach, which she suggests is readily transferrable to other sectors where silica is a potential hazard, is based on five fundamentals:
• Incorporating a risk management process into all planned work to identify hazards, evaluate exposures and implement appropriate controls.
• Monitoring the effectiveness of the controls by measuring air quality and conducting worker medical surveillance.
• Training employees on the importance of maintaining and using workplace controls, the health effects of silica, and good hygiene practices.
• Continuing to search for new technology to reduce risk to as low as reasonably achievable.
• Ensuring management commitment to, and employee participation in, the silica risk management program. "Through the ongoing review of identified hazards, we can ensure that they are managed and prioritized as required," Keller suggests.
Medical surveillance is an important tool because it aids in the early detection of adverse health effects, she says. "The data can also be used to guide efforts to improve worker safe- ty and health, and to monitor trends and progress over time."
The approach appears to be working. A review of the company's WSIB claims database "indicates that we have had no recorded silicosis claims for workers that retired from Vale after 1985," Keller reports.
In the past, big mining companies -- the smokestack industries -- and other traditional sources of excessive silica exposures have been targeted by enforcement agencies. But additional attention may be needed elsewhere. (CAREX Canada reports that construction trades and contracting accounts for about 60 per cent of exposed workers.)
The Quebec research, which looked at silica exposures associated with many occupations and industries, found that close to half of all exposed jobs were in construction trades.
Although the data is still scanty, there are indications that construction workers are routinely exposed to levels of silica dust between two and 20 times higher than the occupational exposure limits (OELs). In a paper published in the Annals of Occupational Hygiene in March, 2003, Stephen Rappaport, Ph. D., and his team analyzed breathing zone exposure data for labourers, bricklayers, heavy equipment operators and painters at a number of construction sites across the United States. While there was a great deal of variation, both within and between trades, more than three-quarters of the measurements exceeded the OEL for crystalline silica.
Researchers report that few of the heavy equipment operators or labourers bothered with any kind of respiratory protection at all. If the bricklayers wore respiratory protection, it was usually the disposable, paper type. As for painters preparing roads and bridges for painting, the study found that all donned supplied-air respirators while using abrasive blasting equipment.
The problem was that because silica levels were so high, even higher levels of protection (that is, pressure-demand or positive pressure respirators) were warranted.
The paper concluded that "such grossly unacceptable exposures" portend a serious health threat requiring determined action. "The numbers were so unequivocal," says Dr. Rappaport, director and primary investigator of the Berkeley Center for Exposure Biology at the University of California Berkeley.
"When I walk past construction sites today, it's clear that not much has changed. You still see lots of open cutting and tuckpointing with no precautions being taken. It's really pretty egregious," he contends.
Dr. Rappaport says inexpensive controls, especially the use of water, can suppress dust levels by three-fold, while isolating employees in ventilated cabs can cut exposures six-fold. As well, he suggests that respiratory protection programs be instituted at every construction site, and new methods for locally ventilating sources of dust generation be developed.
Research published in the April issue of the Journal of Occupational and Environmental Hygiene (JOEH) suggests that respirable dust concentrations can be reduced 99 per cent by water controls and 91 per cent by local exhaust ventilation (LEV) systems.
A team from the University of Washington (UW) in Seattle also identified exposures of concern. Researchers analyzed the data provided by 1,374 personal silica exposure samples collected on construction sites by a variety of private, research and regulatory groups. Findings in the March, 2006 edition of JOEH confirmed that a large portion of the samples were at or over the quartz OEL, including all eight trades studied, 13 of the 16 construction tasks, and 12 of the 16 tool categories.
Even the average exposures were more than double the OEL, the study results indicate. The highest exposures arose during the use of abrasive blasters, surface and tuckpoint grinders, jackhammers and rock drills.
One of the UW team members was Noah Seixas, Ph. D., a professor in the university's Department of Environmental and Occupational Health Sciences. "Mining and construction have a lot in common," says Dr. Seixas. Both entail tough physical labour, require similar kinds of site prepa- ration, and employ the same type of heavy equipment -- drills, jackhammers and saws -- that can raise dense clouds of silica dust.
The primary difference, from a health and safety perspective, may lie in the attention paid by regulatory authorities to the mining industry over the past 60 or more years. This attention has compelled the sector, both collectively and on individual sites, to make a concerted and ongoing effort to reduce exposures.
"The mining sector hasn't completely solved their silica problems, but they have certainly devoted a lot more attention and resources toward solving them," Dr. Seixas suggests. "With the same level of regulatory attention, we should expect to see similar gains in the construction sector."
But the inherent characteristics of the sector present challenges. It can be much more complicated to promote proper use of safety equipment at sites where exposures are intermittent, dangerously high for short periods and well within exposure thresholds for much of the day.
"A worker needs to know when protection is needed and when it isn't," Dr. Seixas says. "Nobody wants to wear a respirator all day if it isn't necessary."
He cites a training intervention study which found that about 80 per cent of construction workers either wore their protective gear (in this case, related to hearing) all the time or never at all. When researchers added an exposure indicator, a device that flashed a warning when threshold levels were exceeded, worker compliance increased by about 15 per cent.
Silica has been recognized as an occupational health problem in some form or another for centuries, says Dr. Genesove. "Over the last 400 years, we have also learned a lot about prevention. It's extremely important for employers, with the active support of their staff and employees, to follow the requirements of the oh&s legislation and regulations and, hopefully, implement best practices in an effort to prevent silica exposures," he adds.
Effective silica control is a multi-level problem: individual workers have a responsibility to wear safety gear and use the equipment that is provided to them. Contractors and employers must be diligent in training workers about the risks of silica exposure, providing the appropriate equipment, implementing precautions, and imposing disciplinary action when protections are breached. Regulatory authorities, for their part, must maintain overarching regulatory attention across the entire industry.
Lal sees too many cases of silicosis. In OHCOW's Hamilton, Sarnia and Toronto clinics, most of these are construction workers, while in the Thunder Bay and Sudbury offices, staff see more miners.
A patient will come in complaining that he cannot climb stairs like he used to, she says. "You take a work history, you see the nodules on the chest X-ray, you perform a pulmonary function test, and you diagnose silicosis. It's almost a no-brainer," Lal says. "It's also so tragically sad."
A patient might be provided with cough suppressants, a bronchodilator or even a mobile oxygen tank to ease breathing problems. Antibiotics will help prevent secondary infections, while a corticosteroid can reduce lung inflammation.
But it's really just a "temporary Band-Aid," says Lal. Even if exposures cease, the body's macrophages will not stop their assault against the silica already trapped in the lungs and will continue to produce hard, inelastic scar tissue.
Once the nodes start getting bigger, it makes it more difficult to take a deep breath. That's also when complications such as pneumonia and tuberculosis can take hold. The macrophages are so busy trying to kill silica particles that they cannot counter the invading bacteria.
After a worker is diagnosed with silicosis, "all you can do is send him or her home," Lal says. "The only things that would help are the things they should have done years ago."
William M. Glenn is associate editor of hazardous substances.
No Treatment, No Cure
Information compiled from numerous sources shows how silica can send an exposed worker on a long, hard journey.
Once deep in the lungs, silica particles quickly become trapped in the tiny air sacs, or alveoli, where oxygen is absorbed and carbon dioxide expelled. White blood cells called macrophages are activated and rush to break down the silica, releasing enzymes that inflame the surrounding lung tissue.
Eventually, clumps of dead cells and fibrous scar tissue build up around the trapped silica particles, sealing off the reactive area. With the damaged areas of the lung no longer able to exchange gases efficiently, the lungs become less flexible and breathing becomes more difficult.
The effect is not always the same. Many patients with simple, chronic silicosis are asymptomatic. Others may complain of a nagging cough, become tired more easily or feel short of breath (dyspnea) after exertion.
A chest X-ray is needed for an accurate diagnosis. The scarred areas in the lungs will resemble tiny round nodes-- approximately one to three millimetres in size; certainly no bigger than one centimetre-- on the X-ray. In complicated silicosis, nodes combine into larger fibrous masses.
Chronic silicosis is caused by exposure, typically 10 years or more, to excessive levels of silica dust, those at or above the current occupational exposure limit. The first health symptoms may not arise for 20 or even 30 years. More advanced cases may be marked by laboured, rapid breathing (tachypnea), loss of appetite and weight loss, chest pain, fever, a blue tinge to the skin (cyanosis) and heart disease.
But while slow to develop, the damage is irreversible. Other than providing some temporary relief, there is no effective treatment and no cure for silicosis.
There is also accelerated silicosis, which follows the same pathological mechanism as chronic silicosis, but develops much more quickly following exposure to high levels of respirable silica, and acute silicosis, also known as silicoproteinosis, which is very rare in North America. Following exposure to dense clouds of respirable silica, the activated macrophages proliferate and fill the lungs with a protein fluid. Debilitating symptoms can appear within just a few weeks; in severe cases, respiratory failure and death is likely to occur in just a year or two.
About 600 cases of acute silicosis have been diagnosed over the last year or so among workers in the Turkish textile business. Fine silica was used to sandblast denim jeans to produce the "stone wash" look. Even though Turkey banned the sandblasting process in March of 2009, it is estimated that as many as 10,000 workers may have been exposed to the unsafe conditions.
*Sources: Dr. Leon Genesove; Ministry of Labour (Ontario); Canadian Centre for Occupational Health and Safety; Occupational Health Clinics for Ontario Workers, Inc.; and various scientific studies.
Road to Prevention
There is no shortage of advice on how best to avoid silica exposure -- exposure that can set the path for harm down the road. Publications from the Department of Labor and the Mine Safety and Health Administration, both in the United States, as well as the Infrastructure Health and Safety Association and the Ministry of Labour, both in Ontario, recommend that the following precautions be taken:
• Never blow clean a dusty work site with compressed air. Instead, wet sweep floors, hose down surfaces or vacuum dust using a machine equipped with a high-efficiency particulate air (HEPA) filter.
• Where feasible, reduce exposure levels through the use of engineering controls such as local exhaust ventilation, dust collection, water sprays or wet drilling systems.
• Where practicable, enclose silica-generating operations. Use automatic-blast cleaning machines or cabinets that allow machine operation from outside using gloved armholes.
• Wear, correctly use and maintain approved particulate respirators when engineering controls alone are not adequate to reduce exposures below permissible levels.
• Substitute sand abrasives with alternatives containing less than one per cent silica.
• Post warning signs to identify work areas where respirable silica is present.
• When using heavy equipment, consider the wind direction and try to limit the amount of dust that blows through the work area.
• Undertake regular air monitoring of work sites, both as needed and when required by law, and take corrective action when silica levels are excessive.
• Provide medical examinations for employees who may be exposed to respirable crystalline silica, and have X-rays examined by a specialist in dust diseases. Plan to reduce exposures for any employees who show signs of silicosis.
• Avoid eating, drinking, using tobacco products or chewing gum in work areas where there is dust or other toxic materials.