What is crystalline silica? [1]

Crystalline silica (SiO2) is an element - basic compound found in soil, sand, granite, and many other minerals. 
Quartz is the most common form of crystalline silica. Cristobalite and tridymite are two other forms of crystalline silica. All three forms may become respirable size particles when workers chip, cut, drill, or grind objects that contain crystalline silica.  This form of silicon dioxide is also referred to as Respirable Crystalline Silica (RCS), Respirable Crystlline Quartz (RCQ).
What is Respirable Crystalline Silica? [2]
Not all dust is the same! 
For any kind of dust, there are different particle sizes, often referred to as dust fractions. When dust is inhaled, its point of deposition within the human respiratory system is very much dependent upon the range of particle sizes present in the dust.  
Three dust fractions are of main concern: the inhalable, thoracic and respirable dust fractions.  
What are the hazards of crystalline silica?
Respirable dust can penetrate deep into the lungs. The body’s natural defence mechanisms may eliminate much of the respirable dust inhaled. However, in case of prolonged exposure to excessive levels of this dust, it becomes difficult to clear the respirable dust from the lungs and an accumulation of dust can, in the long term, lead to irreversible health effects
Crystalline silica is recognised to be a human lung carcinogen. Additionally, breathing crystalline silica dust can cause silicosis, which in severe cases can be disabling, or even fatal. The respirable silica dust enters the lungs and causes the formation of scar tissue, thus reducing the lungs’ ability to take in oxygen. There is no cure for silicosis. Since silicosis affects lung function, it makes one more susceptible to lung infections like tuberculosis. In addition, smoking causes lung damage and adds to the damage caused by breathing silica dust. [1]
What are the symptoms of silicosis? [1] 
Silicosis is divided into three types: chronic/classic, accelerated, and acute.
Chronic/classic silicosis, the most common, occurs after 15–20 years of moderate to low exposures to respirable crystalline silica. Symptoms associated with chronic silicosis may or may not be obvious; therefore, workers need to have a chest x-ray to determine if there is lung damage. As the disease progresses, the worker may experience shortness of breath upon exercising and have clinical signs of poor oxygen/carbon dioxide exchange. In the later stages, the worker may experience fatigue, extreme shortness of breath, chest pain, or respiratory failure.
Accelerated silicosis can occur after 5-10 years of high exposures to respirable crystalline silica. Symptoms include severe shortness of breath, weakness, and weight loss. The onset of symptoms takes longer than in acute silicosis.
Acute silicosis occurs after a few months or as long as 2 years following exposures to extremely high concentrations of respirable crystalline silica. Symptoms of acute silicosis include severe disabling shortness of breath, weakness, and weight loss, which often leads to death.
Examples of work activities where exposure review process may be required involving crystalline silica: [3]
Excavation, 
Earth moving and drilling plant operations
Clay and stone processing machine operations 
Paving and surfacing 
Mining and mineral ore treating processes 
Construction labouring activities „ 
Brick, concrete or stone cutting, especially using dry methods 
Abrasive blasting—blasting agent must not contain >1 per cent crystalline silica  
Foundry casting 
Australian Exposure Standard (ES) for Respirable Crystalline Silica?
The AIOH supports the workplace exposure standard of 0.1 mg/m3 for respirable crystalline silica. However, a “no observable adverse effects level” (NOAEL) cannot be demonstrated. Risks to health are occurring at levels previously thought to be acceptable. Limitations in technology make it difficult to determine a NOAEL if it occurs at very low levels of exposure [6]. 
Factors thought to influence the potential for respirable crystalline silica to cause silicosis include the following [6]: 
polymorphic type of crystalline silica with Cristobalite, tridymite and quartz appearing more reactive and cytotoxic than Coesite and Shishovite
presence of other minerals, for example aluminium containing materials reduces the toxic effect of quartz, however, this may only be a temporary effect 
total surface area which is related to particle number, size and surface area of individual particles. Smaller particle size fractions would be expected to cause more lung damage than larger size fractions 
freshly fractured versus ‘aged’ surfaces. Increased cytotoxicity occurs when crystalline silica particles are cleaved into smaller fragments with reactive free radical species forming on the surface of the particles. There is an ‘aging’ process where free radical activity decays with time. This occurs slowly in air but rapidly in water.
Health monitoring for Respirable Crystalline Silica Exposure
Health monitoring using spirometry testing is important for people at high risk as it can detect loss in lung function before significant changes are detected by radiography. The results of spirometry testing can then assist to eliminate the worker from the source of respirable crystalline silica exposure, before significant lung function is lost.
Overseas Guides:
What can employers/employees do to protect against exposures to crystalline silica? [1,4]
HSE UK- Control cabins with forced filtration 
Consult a qualified ventilation engineer to assure that the design will cope with the anticipated dust levels. The design should cover the following points:
pre-filters, to protect the main filter if coarse silica dusts are present; 
HEPA filters (BSEN 1822): – type H11 for external RCS concentrations below 1 mg/m3; – type H12 or H13 for external RCS concentrations above 1 mg/m3; 
pressure gauges to show the system is working properly; 
alarms to sound when filters clog; 
overpressure around 10 Pa inside the cabin to prevent dusty air ingress; 
flaps to release excess pressure;   door seals - heavy-duty neoprene or other suitable material; and   self-closing doors.
door seals - heavy-duty neoprene or other suitable material; and
self-closing doors.
Using control cabins 
Abrasive dusts can wear out equipment quickly. Plan regular checks and maintenance of the critical parts. 
Check that the clean air supply is turned on and working at the start of work
Check pre-filters regularly - keep spares 
Check integrity of filter seals daily if they are accessible. If they are not, check monthly and carry out a smoke test at the mid-point of the month.
Change inlet air HEPA filters as advised by the manufacturer, but at least after every 250 hours’ use.
Keep doors and windows closed. 
Reduce dust being trailed in - use sticky mats or overshoes.
Clean the control cabin at least once a week. Use a Type H vacuum cleaner fitted with a HEPA filter, or wet clean. 
Define and provide personal protective equipment (PPE) for work outside the cabin. Don’t clean up with a brush or with compressed air.
Maintenance, examination and testing
Get a competent ventilation engineer to examine the system thoroughly and test its performance at least once every 14 months. See the HSE publication HSG54 - see ‘Further information’. 
Keep records of all examinations and tests for at least five years. 
Review records - failure patterns show where preventive maintenance is needed. 
Carry out air sampling to check that the controls are working well. 
Vehicle cabs with forced filtration 
High dust levels result from construction. 
Wash down metalled roadways regularly and limit vehicle speed.
The cab should have the following features:
pre-filter to protect the main HEPA filter; 
pressure gauge to show the system is working properly; 
overpressure around 10 Pa inside the cab to prevent dusty air ingress; and 
door and window seals - heavy-duty neoprene or other suitable material.
Using cabs with filtered air 
Abrasive dusts can wear out equipment quickly. Plan regular checks and maintenance of the critical parts.  
Check that the control cab clean air supply is turned on and working at the start of work. 
Check pre-filters regularly - keep spares. 
Change inlet air HEPA filters as advised by the manufacturer, but at least after every 250 hours’ use.
Keep doors and windows closed.
Check any air conditioning self-test every time the machine is started. 
Vacuum clean the vehicle cab at least once a week. Use a Type H vacuum cleaner fitted with a HEPA filter. Caution: 
Caution: Don’t clean up with a brush or with compressed air.
Recommendations from OSHA:  
Replace crystalline silica materials with safer substitutes, whenever possible.
Provide engineering or administrative controls, where feasible, such as local exhaust ventilation, and blasting cabinets. Where necessary to reduce exposures below the PEL, use protective equipment or other protective measures.
Use all available work practices to control dust exposures, such as water sprays.
Wear only a N95 NIOSH certified respirator, if respirator protection is required. Do not alter the respirator. Do not wear a tight-fitting respirator with a beard or moustache that prevents a good seal between the respirator and the face.
Wear only a Type CE abrasive-blast supplied-air respirator for abrasive blasting.
Wear disposable or washable work clothes and shower if facilities are available. Vacuum the dust from your clothes or change into clean clothing before leaving the work site.
Participate in training, exposure monitoring, and health screening and surveillance programs to monitor any adverse health effects caused by crystalline silica exposures.
Be aware of the operations and job tasks creating crystalline silica exposures in your workplace environment and know how to protect yourself.
Be aware of the health hazards related to exposures to crystalline silica.
Smoking adds to the lung damage caused by silica exposures.
Do not eat, drink, smoke, or apply cosmetics in areas where crystalline silica dust is present. Wash your hands and face outside of dusty areas before performing any of these activities.
Remember: If it’s silica, it’s not just dust.
 
 
 
Notes: This document contains notes on good practice which are not compulsory but which you may find helpful in considering what you need to do. 
© Crown copyright If you wish to reuse this information visit www.hse.gov.uk/copyright.htm for details. First published 10/06
Notes: This is one in a series of informational fact sheets highlighting OSHA programs, policies, or standards. It does not impose any new compliance requirements. 
For a comprehensive list of compliance requirements of OSHA standards or regulations, refer to Title 29 of the Code of Federal Regulations . 
This information will be made available to sensory-impaired individuals upon request. The voice phone is (202) 693–1999. See also OSHA’s website at www.osha.gov.
1    www.dol.gov.
2    www.nepsi.eu
3    Australian Institute of Occupational Hygienists Inc., AIOH Position Paper: Respirable Crystalline Silica and Occupational Health Issues, AIOH, Feb 2009.
4    www.hse.gov.uk/
5    www.safeworkaustralia.gov.au/