Silica Exposure in Construction – Final Revision

Abstract

Many construction related activities in the construction industry create dust; often this dust contains respirable crystalline silica. When dust from these activities are not controlled, they can cause exposures that exceed OSHA permissible exposure level (PEL) of 0.1 mg/m3. Exposure to silica has been found to lead occupational disease such as silicosis, asthma, and cancer. NIOSH as well as OSHA recommend the use of engineering controls to mitigate silica exposure. On-tool local exhaust ventilation (LEV) and water suppression are common methods of dust control. Do these engineering controls provide sufficient worker protection from silica exposure per OSHA’s PEL? Should airborne sampling be conducted to ensure dust controls are adequate protection? Should construction workers be required to utilize respiratory personnel protective equipment (PPE) in addition to implementing the engineering controls? This paper will seek out to validate the hypothesis that engineering controls for construction tools provide sufficient protection from silica exposure per OSHA’s current exposure level (PEL) of 0.1 mg/m3.

A literature review of existing airborne silica exposure will be collected for a limited number of construction tools. The tools will include concrete surface grinding and finishing, tuck point grinding, rock & surface drilling with a rotary hammer, drywall sanding, tile cutting, brick and block cutting with a stationary saw, brick and block cutting with a handheld saw, and jack hammering. Data will be collected and categorized per tool; with sub categories to include tool used with no controls, tool used while utilizing on-tool LEV, and tool used while utilizing water suppression methods. Categories lacking sufficient data will be supplemented by airborne sampling conducted at construction sites. The data will be analyzed the hypothesis tested.

Topic

Crystalline silica is a basic component of 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. Crystalline silica exposure remains a serious threat to construction cement finishers who are involved in concrete work like cutting, mixing, grinding, drilling, and chipping.

This study will examine silica exposure while completing various tasks without engineering controls as well as exposure levels found while utilizing on-tool local exhaust ventilation (LEV) and on-tool water dust suppression. The review will seek to validate the hypothesis that engineering controls for construction tools provide sufficient protection from silica exposure per OSHA’s current permissible exposure level (PEL) of 0.1 mg/m3.

Introduction

Description of the Hazard

Silica (SiO2) is a compound resulting from the combination of one atom of silicon with two atoms of oxygen. It is the second most common material in the earth’s crust and is a major component of sand, rock, and mineral ores. “Based on epidemiological studies, crystalline silica dust has been classified as a known human carcinogen.” (1) Breathing too much dust containing the crystalline silica particles small enough to enter the deep parts of the lung can cause silicosis (scarring of the lung tissues), cancer, and other forms of lung disease including an increased risk of getting tuberculosis.
The Occupational Safety and Health Administration (OSHA) estimates that out of the 2.2 million individuals exposed to silica in the workplace, 1.85 million of them are employed in the construction industry. Approximately 59,000 of the exposed workers will develop silicosis during their lifetime. (1)

This paper reviews the silica exposure levels of several construction activities while utilizing two types of engineering controls; local exhaust ventilation (LEV) and on-tool water suppression methods. Both types of engineering controls will be applied to construction related processes, such as tuck-point grinding, surface grinding, polishing, brick, block and tile cutting, and floor and drywall sanding. Are these engineering control methods capable of significant reductions in silica dust exposure? Are they adequate measures to eliminate the use of respiratory protection below OSHA’s current permissible exposure level (PEL) of 0.1 milligrams per cubic meter of air; or if approved, OSHA’s proposed PEL of 0.05 mg/m3? (NIOSH currently recommends 0.05 mg/m3 (2002) while ACGIH recommends exposures be limited to 0.025 mg/m3 (2008))

Description of the Environment and Where the Hazards Exist in that Environment
Exposure to crystalline silica is common in several construction trades due not only to its presence in many materials such as concrete, mortar, and brick but also to the processes involving operations such as breaking, grinding or sawing.
This paper reviews a number of commonplace construction activities that have been found to exposure workers to significant silica levels. Activities include surface grinding and finishing (2x’s the PEL), tuck-point grinding (2x’s the PEL), rock and surface drilling/rotary hammers (3x’s the PEL), dry wall sanding (15x’s the PEL), tile cutting, brick and concrete block cutting with a stationary saw (20x’s the PEL), brick and block cutting handheld (14x’s the PEL), jack hammering (8x’s the PEL). (2)

The prolonged inhalation of respirable dust containing crystalline silica may result in silicosis, a disease characterized by progressive fibrosis of the lungs. Silicosis symptoms are shortness of breath and impaired lung function which may result in complications that can result in death. The development rate and the severity of silicosis depends on the airborne concentration and duration of silica dust exposure. Crystalline silica may be harmful following several weeks of high levels of exposures as well as long term, low level exposures. There are three major types of silicosis, chronic, accelerated, and acute.

The International Agency of Research on Cancer (IARC) has concluded that crystalline silica inhaled in the form of quartz or cristobalite is carcinogenic to humans and has classified these forms of silica as Group 1 carcinogens. In addition, the American Conference of Governmental Industrial Hygienists (ACGIH) has classifies quartz as a suspected human carcinogen with an A2 classification.

Route of Exposure

Occupational exposure to silica occurs through inhalation of small airborne particles of silica dust that is in the range of 5.0 um to 0.5 um. These small particles become lodged and are not expelled from the lungs; they are deposited in lymph nodes where over time calcium can deposit and settle along the rim of the lymph node. This condition is known as egg-shell calcification. In some cases, silica particles are carried into the lungs where a scar may form around the particles. Over time the hardened scars gradually start to show up on the chest x-ray as fibrosis of the lung.

Data Representing the Degree of Exposure

The following tasks were researched to determine the level of silica exposure without engineering controls compared to those same tasks when engineering controls were applied. Engineering controls included the use of on tool local exhaust vacuums (LEV) and on tool water suppression systems. OSHA’s current silica PEL is 0.1mg/m3; OSHA’s proposed PEL is 0.05mg/m3.

Tuck Point Grinding:

In a study for NIOSH of silica exposures in the construction industry in the U.S., (3) “found exposures of 1 – 3 mg/m-3 caused by tuck-point grinding. Tuck point grinding is a task associated with the restoration of damaged or old brick. The existing mortar is ground out using a grinding to completed restoration. In a study of the effectiveness of on-tool LEV” (4) measured “exposures of 3.04 mg/m-3 caused by uncontrolled tuck-point grinding. On-tool LEV applied to right angle grinders for mortar removal has been tested both in the field and in a laboratory setting. The effectiveness of the LEV controls range from completely ineffective to being able to reduce exposure to respirable dusts by 99 %”.
(4) “fitted a water suppression device to one of the right-angle grinders tested after the on-tool LEV device had failed and retested it with a wet and dry vacuum cleaner. This reduced exposure to 0.38 mg/m-3 (uncontrolled exposure 1-3 mg/m-3) providing a maximum reduction in exposure of 87 %.”

Surface Grinding:

On-tool LEV for use with surface grinders has been tested in the field and in the laboratory in a variety of configurations. Manufacturer supplied LEV’s were found to reduce exposure from 37 % to 99 %. (5) conducted testing on manufacturer supplied LEV for a Hitachi grinder and found the dust concentration were reduced from 7.73 mg/m3 without the control to 4.87 mg/m3 with the control; a 37% reduction. Factors that affected the performance included maintaining an adequate volume flow rate, and user training and operation.
In 2007 two studies conducted by the Akbar-Khanzaden team compared water suppression control to on-tool LEV control (6). They found a 98% reduction from 25.4mg/m3 to 0.148 mg/m3 when using the on-tool LEV and an exposure level of 0.521 mg/m3, a 99% reduction when the controls were water suppression.

Cut-off Saws:

A study (7) conducted a study on three cut-off saws in the construction industry. “The controls tested were, water suppression from a pressurized tank, water suppression from water mains, and an on-tool LEV system each using a resin bonded blade and diamond tipped blade. Exposures to respirable dust were reduced by 94 %, 96 % and 91 % for the pressurized water, water mains water, and LEV controls while using a diamond tipped saw. For the resin bonded saw, exposures to respirable dust were reduced by 47 %, 97 % and 98 % for pressurized water, mains water and LEV respectively”. Exposure levels using cut-offs saw have been measured as high as 50mg/m3.

Masonry Saw:

(8) Meeker measured exposure during block and brick cutting using a hand-held saw fitted with on-tool LEV and a stationary wet saw in the field. “When cutting blocks the saw with LEV and wet saw produced reductions in exposure of 96 % and 93 %. (0.11 mg/m-3, 0.21 mg/m-3 controlled and 2.83 mg/m-3 uncontrolled)”.
(9) Carlo performed a laboratory evaluation of an on-tool LEV and a water suppression system for a hand-held masonry saw. “The water suppression system reduced exposure to respirable dust by 99 % and the LEV system by 88 %”.

Hammer Drill:

(10) Shepherd evaluated an on-tool system for a hammer drill, two hood types and two vacuum cleaners were assessed. “Exposure was reduced by 91-98% by the four combinations (0.308 mg/m-3 uncontrolled, 0.006 – 0.028 mg/m-3 controlled)”.

Dry Wall Sanding:

A (11) NIOSH article states that “drywall sanders can be exposed to dust levels of 10 mg/m-3 and 4 mg/m-3”. The NIOSH article advises that the use of “vacuum sanding tools can reduce exposure by 80-97 %”. (12) Young-Corbett compared “hand sanding to pole sanding (hand sanding with the use of a pole to separate the workers breathing zone from the dust source), wet sponge sanding and vacuum sanding. They found that exposures were reduced by 58 %, 60 % and 88 % respectively”.

Workers at Risk

The following chart describes workers silica exposure taken from OSHA sampling from January 1, 1999 through December 31, 2002. A value (severity) above 1.0 indicates that the exposure limit has been exceeded. The method of the PEL calculation is listed below the chart. Of the 738 construction samples taken by OSHA, 214 or 29% of them were found to exceed OSHA’s silica exposure limits.

TABLE I – Results of time weighted average (TWA) exposure respirable crystalline silica samples for construction [January 1, 1997–December 31, 2002]

Exposure (severity relative to the PEL)

< 1 PEL: Number of samples ……… 424 Percent …………….. 58 1 × PEL to < 2 x PEL: Number of samples ………. 86 Percent …………….. 12 2 × PEL to < 3 × PEL: Number of samples ………. 48 Percent ………………. 6 ≥ 3 × PEL and higher (3+): Number of samples ………. 180 Percent ……………… 24 Total # of samples ………… 738 Source: OSHA Integrated Management Information System. Table explanation: Calculation example of exposure severity relative to the PEL PEL: This number is the calculated permissible exposure limit for dust to which the worker may be exposed. It uses the data you supplied and is based on the percent crystalline silica in the sample. For example: 1.6 mg/m3 Exposure: This number is the actual amount of dust that is in your work environment, for example: 4.8 mg/m3 Severity: This number is derived by dividing the exposure by the PEL, for example: 4.8/1.6 = 3.0. If the severity number is higher than 1.0 you are above the limit. If the severity number is less than 1.0 you are below the limit. Surveillance data is unavailable that accurately estimates the total number of silicosis cases in the United States. Due to a lack of national medical monitoring surveillance programs as well as the misdiagnosis of silicosis and subsequent failure to list it as the cause of death on a death certificate, NIOSH indicates that “the true extent of the problem is probably greater than indicated by the available data”. (12) Silicosis often occurs after a worker has retired or left an occupation or worksite were exposure may have occurred making it difficult for the Bureau of Labor to accurately report the disease as work related. Even so, over the period of 2000-2005, an average of 162 individuals die annually due to silicosis as reported U.S. mortality surveillance. According to formulas published by Rosenman, 162 silicosis deaths annual deaths would point to approximately 1,975 new cases per year. (12) Summary of Literature Review Both engineering controls were found to be to capable of reducing exposures created by tuck-point grinding, surface grinding, finishing and polishing, block, slab, brick and tile cutting, and drywall sanding. In most cases exposure reductions of greater than 90 % were achieved and no significant differences were noted between the effectiveness of on-tool local exhaust ventilation and water suppression systems. Even with exposure reductions of over 90 %; on-tool engineering controls never completely eliminated the silica exposure and did not always reduce it to below the OSHA’s current exposure limit. To ensure worker safety, respiratory protection may be required. Study Design Hypothesis Null: Engineering controls for construction tools provide sufficient protection from silica exposure per OSHA’s current exposure level (PEL) of 0.1 mg/m3. Alternate: Engineering controls for construction tools do not provide sufficient protection from silica expossure per OSHA’s current exposure level (PEL) of 0.1 mg/m3. Research Plan to test Hypothesis The objective of the research will be to test the hypothesis that engineering controls for construction tools provide sufficient protection from silica exposure of 0.1 mg/m3. The scope of the study will be limited number of commonplace construction tools. Specific tools that will be reviewed in the study will include concrete surface grinding and finishing, tuck point grinding, rock & surface drilling with a rotary hammer, drywall sanding, tile cutting, brick and block cutting with a stationary saw, brick and block cutting with a handheld saw, and jack hammering. Historical air sampling silica exposure data for these tools will be obtained from literature review. A list of the silica air sampling data base literature sources are stated in “Data Gathering Methods” section of this paper. Insufficient data gaps will be filled by airborne silica sampling conducted on construction sites. A cyclone with a filter cassette will be utilized per NIOSH Method 7500. The air sampling data will be complied into three categories; silica exposure without engineering controls, silica exposures with LEV controls, and silica exposure with water suppression controls. OSHA’s current silica PEL of 0.1 mg/m3 will be used as the benchmark to determine if the engineering method provides sufficient protection. Data Gathering Methods Literature research will be conducted. Data base and silica exposure study sources will include: IRSST Publication, “Construction Workers’ Exposure to Crystalline Silica Literature Review and Analysis” – literature contains exposure database for 4251 air borne with 76 parameters that include the occupation, task, tool, and control method used, Journal of Occupational and Environmental Hygiene Publication, “Controlling Dust from Concrete Cutting” – 178 air borne samples, Journal of Occupational and Environmental Hygiene Publication, “Occupational Exposure to Silica in Construction Workers: A Literature Based Exposure Database” – 2858 airborne samples including information on trade, task, tools, and control measures, Journal of Environmental Monitoring, “Statistical modeling of crystalline silica exposure by trade in the construction industry using a database compiled from the literature” – 1346 airborne samples including information on trade, task, tool, and control measures, HSE Executive Publication, “Statistical modeling of crystalline silica exposure by trade in the construction industry using a database compiled from the literature” – airborne samples for construction tools including information on trade, task, tool, and control measures, U.S. National Library of Medicine National Institute of Health, “Engineering control technologies to reduce occupational silica exposures in masonry cutting and tuck pointing” – study specific to tuck pointing, includes airborne sampling results as well as information on task, tool, and control measures, Journal or Occupational and Environmental Hygiene, “Silica Exposure on Construction Sites: Results of Exposure Monitoring Data Compilation Project” – 1374 airborne sample data base including information on task, tool, and control measures. These publication as well as additional silica data bases and studies obtained from the Occupational Health and Safety Administration (OHSA), the National Institute for Occupational Safety and Health (NIOSH), American Conference of Governmental Industrial Hygienists (ACGIH), the Health and Safety Executive (HSE), the Health and Safety Laboratory (HSL), the Centers for Disease Control and Prevention (CDC) will provide the airborne silica data base for the study. Insufficient data gaps will be filled through the means of airborne silica sampling. Sampling will be conducted when data from the literature review does not provide adequate airborne silica sampling data for each tool that is being studied. Airborne testing will be conducted under the guidance of a Certified Industrial Hygienist to ensure proper sampling methods are implemented. Bias/Confounding Factors Proper use of attachments: Manufacturer on-tool LEV or water suppression systems must be properly secured to the tool. They must also remain on when the tool is in use and not removed by workers who are unfamiliar or uncomfortable using them. Workers will be asked not remove the attachments while the airborne tests are being conducted. Proper maintenance of LEV filters: Filters need to be properly cleaned and disposed per the manufacturer recommendations. Improper filtering can adversely affect the ability of an LEV system to capture airborne contaminants. Prior to air sampling, filters will be replaced or cleaned to ensure the LEV is functioning properly. Wind velocity and direction: A strong wind blowing away from the worker can result in a lowered air borne silica concentration. This will result in faulty data. Air sampling data will not be collected during days of strong velocity winds. Water flow for water suppression devices: Water flow must be maintained at the manufacturer’s specified level. Water flowing too slowly will impair the ability of a water suppression system to mitigate airborne dust. Prior to air sampling water flow will be checked to determine if suppression system is properly functioning. Water suppression is appropriate for many types of construction tasks and may result in additional hazards including electrocution, slipping and falling hazards: Some tasks and tools will be unable to safely utilize water suppression as a means of engineering controls due to increased safety hazards. Workers commented that on-tool engineering controls can be burdensome and less productive making them less apt to utilize them if required to do so: Workers need to be trained and assured that loss of production to ensure a reduction in silica airborne levels will not negatively impact their employment. Type of Study Design Experimental Study. Airborne silica sampling will be conducted to fill insufficient data gaps from the literature review. Sampling will be conducted while engineering controls are not in use, while on-tool LEV controls are in use, and while water suppression controls are implemented. Explanatory Variable Implemented on-tool LEV engineering control or lack of engineering control and implemented on-tool water suppression engineering control or lack of engineering control Outcome Variable Airborne silica concentrations greater than OHSA’s current PEL silica exposure level of 0.10 mg/m3. Analysis Technique/Method Airborne concentration data will be compiled per the tool utilizing manufacturer engineering controls. The tasks and tools are concrete surface grinding and finishing, tuck point grinding, rock & surface drilling with a rotary hammer, drywall sanding, tile cutting, brick and block cutting with a stationary saw, brick and block cutting with a handheld saw, and jack hammering. This data will then be graphed using OSHA’s PEL of 0.1 mg/m3 as the comparison level. Data above this level will indicate overexposure while levels below this level will indicate acceptable exposure per OSHA PEL of 0.1 mg/m3. Predicted Outcome The null hypothesis will be rejected. Engineering controls, specific to manufacturer supplied on-tool LEV and water suppression methods do not provide sufficient silica exposure mitigation to control the hazard below OSHA’s construction silica standard PEL of 0.1 mg/m3. “Even with exposure reductions of 90 %, on-tool controls never completely eliminated exposure. This may mean that the use of supplementary respiratory protective equipment (RPE) is required, especially where materials contained silica.” (14) Workers maybe unwittingly exposed to crystalline silica despite their efforts to control the airborne hazard with engineering controls. Conclusion in Respect to Hypothesis Manufacturer supplied, engineering controls are the current gold standard in the effort to mitigate construction employees’ overexposure to silica dust inhalation. Unfortunately, these efforts may not be enough to ensure the short and long term health of construction workers. Construction managers, HSE managers, and trade personnel will need to understand that airborne sampling should be conducted to ensure silica exposure levels do not exceed OSHA limitations even while engineering controls are properly implemented. References 1.) Harding, Anne, “OSHA Plans to Slash Silica Workplace Exposure Limits”, Reutgers, Jan. 1, 2014 2.) OSHA, “Controlling Silica Exposures in Construction”, OSHA 3362-05, 2009 3.) Heitbrink, WA, “In-depth Survey Report: Control Technology for Crystalline Silica Exposures in Construction: Exposures and Preliminary Control for Evaluation” NIOSH, 2000 4.) Croteau, GA, “The Effect of Local Exhaust Ventilation Controls on Dust Exposures During Concrete Cutting and Grinding Activities”, AIHA Journal, 2002 5.) Ojima, J, “Efficiency of a Tool Mounted LEV System for Controlling Dust Exposures During Metal Grinding Operations”, Industrial Health 2007 6.) Akbar-Khanzadeh, F, “Crystalline Silica Dust and Respirable Particulate Matter During Indoor Concrete Grinding Compared with Conventional Grinding”, Journal Of Occupational and Environmental Hygiene, 2007 7.) Thorpe, A, “Measurement of the Effectiveness of Dust Control on Cut-off Saws Used in the Construction Industry”, Occupational Hygiene, 1993 8.) Meeker, JD, “Engineering Control Technologies to Reduce Occupational Silica Exposures in Masonry Cutting and Tuckpointing”, Public Health Records, 2009 9.) Carlo, RV, “Laboratory Evaluation to Reduce RCS Dust When Cutting Concrete Roofing Tiles Using a Masonry Saw”, Journal of Occupational and Environmental Hygiene, 2010 10.) Shepard, S, “Reducing Silica and Dust Exposures in Construction During the Use of Powered Concrete Cutting Hand Tools: Efficacy of LEV Hammer Drills”, Journal of Occupational Environmental Hygiene, 2001 11.) NIOSH, “Hazard Controls of Drywall Sanding Exposures”, Applied Occupational and Environmental Hygiene, 2000 12.) Young-Colbert D.E., “Dust Control Efficiency of Drywall Sanding Tools”, Journal of Occupational and Environmental Hygiene, 2009 13.) Weissman, David MD and Schutte, Paul MD, “The Continuing Persistence of Silicosis” NIOSH Science Blog, October 18, 20111 14.) Pocock, Dom, “On-tool Controls to Reduce Exposure to Respirable Dusts in the Construction Industry”, Health Safety Executive, 2012