Crystalline Silica
Full Profile
CAS No. 14464-46-1 (Cristabolite)
General Information
Silica is one of the most common minerals on earth and is a basic component of soil, sand, and rocks including granite and quartzite. Silica exists in both crystalline and amorphous (non-crystalline) forms.[2] Conversion from amorphous to crystalline form can occur at high heat.[3] Quartz is the most common form of crystalline silica and the most commonly used industrially.[4] Cristobalite also has important industrial uses. Tridymite is found in rocks and is not an important industrial product.[4] There are numerous other synonyms and product names for silica; see IARC for more information.[2]
Crystalline silica is used extensively in many industrial applications because of its unique physical and chemical properties.[4] Health concerns arise when silica containing products are disturbed by grinding, cutting, drilling or chipping, creating respirable particulate.[5]
IARC’s classification in 1997 of crystalline silica as a Group 1 carcinogen (carcinogenic to humans) is specifically for quartz and cristobalite silica inhaled from occupational sources.[2] A recent IARC review of Class 1 carcinogens reaffirmed this classification.[15] Epidemiological studies have shown a relationship between occupational exposure to crystalline silica and increased risk of lung cancer, with the strongest link in quarry and granite workers and workers involved in ceramic, pottery, refractory brick and diatomaceous earth industries.[4] Increased risk was not evident with exposure to amorphous silica.[2]
Silicosis, a non-reversible fibrotic lung disease, is caused by the inhalation of crystalline silica particles.[6] Silicosis is typically categorized as chronic (> 10 years exposure), accelerated (high concentrations over 5-10 years) or acute (short term exposure at high concentrations).[5] Besides silicosis and lung cancer, occupational silica exposure has also been linked to pulmonary tuberculosis, chronic obstructive pulmonary disease, and autoimmune disease (rheumatoid arthritis).[7,8]
Crystalline silica has been ranked by CAREX Canada as Group A (immediate high priority) for occupational settings. Silica is not considered an environmental carcinogen. Prioritization was based on the carcinogenicity and other toxic properties of the substance, the prevalence of occupational exposure in Canada, and the feasibility of assessing exposure.
Regulations and Guidelines
Occupational Exposure Limits (OEL)
| Canadian Jurisdictions | OEL1 (mg/m3) |
|---|---|
| Canada Labour Code | 0.025 [r] |
| BC, AB, MB, NB, NS, NL, PE | 0.025 [r] |
| SK | 0.05 [r] |
| ON | 0.05 [cristobalite] 0.1 [quartz, tripoli] |
| QC | 0.05 [r, cristobalite, tridymite] 0.1 [r, quartz, tripoli] |
| YT | 300 particles/mL2 [quartz] 150 [cristobalite] |
| Other Jurisdiction | OEL (mg/m3) |
| ACGIH 2011 TLV | 0.025 [r] |
Canadian Environmental Guidelines
| Jurisdiction | Limit | Year |
|---|---|---|
| Health Canada | DSL – high priority substances with greatest potential for exposure (quartz cristobalite) | 2006[9] |
| Challenge to Industry | Batch 12B (Health) for quartz and cristobalite | 2006[10] |
Crystalline silica was not included in other Canadian government environmental guidelines reviewed.[11] For further information on silica and the Hazardous Products Act and Controlled Products Regulations in Canada, please refer to the Health Canada pages for non-respirable silica[12] and diatomaceous earth.[13]
Main Uses
Applications for silica differ depending on the particle size, which is divided into three general size categories: lump silica, 0.3 – 15 cm; sand, 75 μm – 3 mm; and flour, < 75 μm. The approximate proportion of use per size category in Canada in 2005 were lump (29%), sand (68%) and flour (3%).[14]
Lump silica is used as flux for smelting operations, in silicon and ferrosilicon alloys and for silica brick.[14] This size fraction is not of concern for health effects as it is too large to inhale. Sand, the most common size fraction of natural crystalline silica, has many applications. For example, it may be used in foundry castings, Portland cement, abrasives and sandblasting materials, and hydraulic fracturing.[4,14] It may also be used as a raw material for the production of silicon and ferrosilicon metals, or as a filter for large volumes of water, i.e. in municipal water and sewage treatment plants.[4,14] When sand has more than 98% silica and low iron content it can be used for glass and ceramic production.[4] Flours are formed by the grinding or quartz, quartzite, sand and sandstone. Flours are very fine grades of crystalline silica and are used in the ceramic and pottery industry, in the manufacturing of chrysotile cement, as a filler in rubber and paints and as an abrasive in soaps and cleaners.[14]
Canadian Production and Trade
Quebec, Ontario and Alberta are the primary silica producers in Canada, followed by Saskatchewan, BC and Nova Scotia. There are silica deposits in all Canadian provinces, however not all are in commercial operation.[14]
Canadian production of silica fulfills most of its domestic requirements, however high quality sands for glass and foundry applications are imported from the US.[14]
Production and Trade
| Activity | Quantity | Year |
|---|---|---|
| Canadian Production* |
ON: 435,537 t
B: 515,409 t
QC: 523,313 t
TOTAL: 1,893,022 t
|
2006[14] |
| Industrial Consumption of Silica | 2,568,393 t | 2003-2005[14] |
| Export: Mainly to US | 154,903 t of ‘silica sands and quartz sands’ | 2010[17] |
| Import: Mainly from US | 1,271,444 t of ‘silica sands and quartz sands’ | 2010[17] |
t = tonne
Occupational Exposures
Inhalation is the most important route of occupational exposure.[4]
Workers may be exposed to crystalline silica, especially quartz, in a number of industries and occupations due to its wide and variable use.[2] Exposures can occur in, for example:[2,4,5]
- Construction: sandblasting, rock drilling masonry work, jack hammering, tunneling
- Mining: metal, non-metal and coal miners, granite quarrying and processing
- Stone cutting (including jewellery): sawing, abrasive blasting, crushing, chipping and grinding
- Ceramics, clay and pottery industries
- Manufacturing: glass industries, soaps, detergents and abrasives
- Agriculture: dust from plowing or harvesting
- Foundries: casting, abrasive blasting
- Mills & refractory brick work: maintenance, repair and replacement
- Automotive industry: sandblasting
- Diatomaceous earth: production and use (i.e. as a filtering agent in the food and beverage industries)
- Dental offices: polishing and sand blasting
Sources
- Wikimedia Commons Photo: A-quartz
- IARC monograph summary, Volume 68 (1997) (PDF)
- Key-Schwartz, R., et al (2003) and NOISH 2002; NIOSH Manual of Analytical Methods: Determination of Airborne Crystalline Silica
- NTP 12th Report on Carcinogens for Silica, Crystalline (2011) (PDF)
- OSHA Fact Sheet: Crystalline Silica Exposure Health Hazard Information (2002) (PDF)
- CICAD Document No. 24: Crystalline Silica, Quartz (2000) (PDF)
- G M Calvert, F L Rice, J M Boiano, J W Sheehy, W T Sanderson (2003). ‘Occupational silica exposure and risk of various diseases: an analysis using death certificates from 27 states of the United States.’ Occupational & Environmental Medicine; 60:122-129
- Parks, C. et al (1999). ‘Occupational Exposure to Crystalline Silica and Autoimmune Disease.’ Environmental Health Perspectives, Vol. 105, Supplement 7, pp. 793-802
- Health Canada’s Prioritization of the DSL
- Challenge to Industry List of Substances
- CEPA List of Toxic Substances
- Health Canada Environmental and Workplace Health. Substance Specific Issues: Silica
- Health Canada Environmental and Workplace Health. Substance Specific Issues: Diatomaceous earth (calcined) with respect to carcinogenicity
- Canadian Minerals Yearbooks: Silica/Quartz, 2006
- IARC Monograph Volume 100: A review of human carcinogens—Part C: metals, arsenic, dusts, and fibres (2009)
- US EPA: Ambient Levels and Non-cancer Health Effects of Inhaled Crystalline and Amorphous Silica Health Issue Assessment (1996)
- TradeMap (Free subscription required)
