Silica (Crystalline) Occupational Exposures

Silica (Crystalline) Occupational Exposures

Overview

Inhalation is the most important route of occupational exposure to silica.[1]

CAREX Canada estimates that approximately 380,000 Canadians are exposed to silica in their workplace. The largest industrial group exposed is construction. In terms of occupation, the largest occupational groups exposed to silica were construction trades labourers, heavy equipment operators, and plasterers and drywallers.

Exposure to crystalline silica, especially quartz, may also occur in a number of other industries and occupations due to its wide and variable use.[2] Workers can be exposed in industries such as mining, agriculture, and various manufacturing industries.[1,2,3] Job tasks that are typically associated with exposure to silica include grinding, sandblasting, crushing, chipping, mixing, and plowing.

Prevalence Estimate

Results indicate that approximately 380,000 Canadians are occupationally exposed to silica; 93% of these workers are male. The largest industry exposed is construction, where the building construction industry and trade contractors together account for approximately 54% of exposed workers.

When exposure is examined by occupation, the largest exposed groups for both men and women are construction trades labourers (105,000 people exposed), heavy equipment operators (41,000 people exposed), and plasterers and drywallers (34,000 people exposed).

Workers exposed to silica by industry

Workers exposed to silica by region

Click the second tab to view total number of workers exposed.

* = < 50 workers

Level of exposure

In total, approximately 380,000 Canadians are exposed to silica in their workplaces. The majority of workers exposed to silica are in the high exposure category. A significant number of silica-exposed workers are also at risk of low and moderate exposure.

Level of Exposure by Industry

Identifying industries with either 1) workers exposed to high levels of silica or 2) a larger number of workers exposed to silica is important in guiding cancer prevention efforts to prioritize exposed groups and target resources most effectively.

The table below shows the number of workers exposed by industry group and level of exposure to silica. These results highlight industries with the most number of workers, as well as industries with the highest levels of exposure. Data for those industries with at least 5,000 workers exposed is shown. For example, in the construction industries (236, 237, 238), the largest industrial groups, the majority of exposed workers are in the high exposure category (as of 2018 updates to our silica estimates). In other industries, such as Truck Transportation, where exposure sources are more dispersed and ventilation may be better, workers fall into the low exposure category. Depending on the goals of a prevention campaign, exposure reduction in the large industrial group might be a useful strategy, or reducing exposure to those at highest risk of exposure could be seen as a priority. For silica, these two strategies both highlight the construction industry.

*Numbers may not add up due to rounding

 

Methods and Data

Our Occupational Approach page outlines the general approach used to calculate prevalence and exposure level estimates for workplace exposures.

Data Sources

Data used in developing the occupational estimates for crystalline silica were collected from several sources:

  1. The Canadian Workplace Exposure Database (CWED) contains approximately 7,600 measurements for crystalline silica exposure. These measurements were collected during the years 1981 to 2004 in Ontario and British Columbia workplaces.
  2. Noticing a lack of exposure measurements from the construction industry, data from this particular industry was added from Manitoba and Saskatchewan exposure databases, as well as some current monitoring data from Alberta, in 2018. An additional 2,400 samples in construction were used for this update.
  3. Canadian and US scientific peer reviewed publications that addressed crystalline silica exposure in Canada and the United States.
  4. Grey literature including technical reports from governments and international bodies.

Prevalence Estimate Method

CAREX defines exposure to crystalline silica as inhalation at work to levels significantly exceeding non-occupational background levels.

To determine the number of workers potentially exposed to crystalline silica at work, CAREX occupational exposure experts used methods previously established in other peer-reviewed CAREX projects in Europe. A series of steps were taken to assign exposure proportions to occupations and industries at risk of exposure to crystalline silica. Please note that the 2018 data update described in the Data Sources section above did not affect the prevalence of exposure, only the exposure levels.

  1. Occupations and industries at risk of possible exposure to crystalline silica were identified using any combination of data sources described above.
  2. The total number of workers in each identified occupation and industry intersection was obtained from Statistics Canada 2006 census data.
  3. A percentage of workers exposed was assigned to that occupation and industry intersection. Percentages were determined by consultation with existing evidence in the data sources, previously established methods from the Europe CAREX estimates and the expert judgement of CAREX occupational hygienists.
  4. The number of workers in the identified group is multiplied by the assigned percentage to calculate the prevalence estimate of workers exposed to crystalline silica.

Exposure Level Method

CAREX uses available workplace exposure measurements in the CWED to create exposure level categories by industry and occupation. For the 2018 update, we sought out and included 2,400 additional silica measurements in the construction industry, and applied the same categorization process as was used in initial estimates. For silica, these categories are:

Category 1: Low Exposure

A group of workers (people in the same job category and industry) is put in this exposure category for one of two reasons:

  1. The are no valid measurements, but a hygienist identified this group as typically exposed during literature and other reviews;
  2. There are valid exposure measurements in the CWED and a hygienist review determined that exposure is plausible; AND EITHER:
    1. There are less than 10 samples available in the CWED, OR
    2. There are ≥10 measurements available but they do not meet the criteria for Moderate Exposure.

Category 2: Moderate Exposure

A group of workers is put in this exposure category if:

  1. There are at least 25 individual samples in the CWED, AND
  2. 20% or more samples have a value higher than 0.0125 mg/m3 (which is ½ of the current occupational exposure limit for crystalline silica).

OR

  1. There are at least 10, but less than 25, individual samples in the CWED, AND
  2. 20% or more samples have a value higher than 0.025 mg/m3 (which is the current occupational exposure limit for silica).

Category 3: High Exposure

A group of workers is put in this exposure category if both these criteria are met:

  1. There are at least 25 individual samples in the CWED, AND
  2. 20% or more samples have a value higher than 0.025 mg/m3 (which is the current occupational exposure limit for silica).
Sources

1. National Toxicology Program (NTP). 14th Report on Carcinogens for Silica, Crystalline (2016) (PDF)
2. International Agency for Research on Cancer (IARC). Monograph summary, Volume 68 (1997) (PDF)
3. Occupational Safety and Health Administration (OSHA). Fact Sheet: Crystalline Silica Exposure Health Hazard Information (2002) (PDF)

Subscribe to our newsletters

The CAREX Canada team offers two regular newsletters: the biannual e-Bulletin summarizing information on upcoming webinars, new publications, and updates to estimates and tools; and the monthly Carcinogens in the News, a digest of media articles, government reports, and academic literature related to the carcinogens we’ve classified as important for surveillance in Canada. Sign up for one or both of these newsletters below.

CAREX Canada

Faculty of Health Sciences

Simon Fraser University
Harbour Centre Campus
2602 - 515 West Hastings St
Vancouver, BC  V6B 5K3
CANADA

© 2019 CAREX Canada