Dust Suppression and Respirable Crystalline Silica

Construction season is here. Road construction, commercial construction and infrastructure projects are switching into high gear, and so is the dust that accompanies excavation and building.

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Construction season is here. Road construction, commercial construction and infrastructure projects are switching into high gear, and so is the dust that accompanies excavation and building. Without proper equipment, workers may be exposed to dangerous airborne particulate matter, including crystalline silica dust. Inhaling this dust puts workers at risk of developing life threatening medical conditions. According to the Centers for Disease Control and Prevention (CDC), millions of U.S. workers are exposed to respirable crystalline silica (RCS) in their workplaces, including 2 million construction workers and 300,000 workers in general industry, maritime and hydraulic fracturing. Exposure to RCS is related to the development of silicosis, lung cancer, pulmonary tuberculosis and other airways diseases.

What is crystalline silica? Silica is a mineral that occurs naturally in crystalline or non-crystalline form. The most abundant crystalline form is quartz, which is the most common mineral on earth’s continents. Crystalline quartz is found in sand, sandstone, shale and granite. Drilling, crushing, cutting, chipping, breaking, sawing or polishing materials containing silica can create a large amount of reparable dust. The smallest, and arguably the most dangerous, dust particles, 10 micrometers and smaller, are too small to see, but can penetrate to the deepest part of the human lung when inhaled.
 
According to the National Institute of Environmental Health Services, the size of dust emissions from silica-containing substances varies from .01 micrometers to 100 micrometers in diameter. Particulate matter (PM) of less than or equal to 10 micrometers (PM10) is the approximate size for particles to be able to penetrate the body’s natural defenses and reach deep into the lungs, potentially causing serious health issues. National air-quality standards for PM were first established in 1971 and were not significantly revised until 1987 when the Environmental Protection Agency (EPA) changed the indicator to focus on “inhalable particles”, which are particles equal to or smaller than 10 micrometers. As of 2012, the EPA finalized the annual standard and set the federal health standard for PM10 to a maximum of 50 micrograms per cubic meter for an annual average. The shear particle sizes of RCS and the health impacts drove OSHA to set new standards.
 
In 1974, the National Institute for Occupational Safety and Health (NIOSH) researchers had proposed a recommended exposure limit (REL) of 50 micrograms per cubic meter for inhalation of dust containing RCS, as a time-weighted average for up to a 10-hour workday as part of a 40-hour workweek. The U.S. Department of Labor’s Occupational Safety and Health Administration (OSHA) issued the latest limits on June 23, 2016. They included standards for construction, 29 CFR 1926.1153 and for general industry and maritime, 29 CFR 1910.1053. There are two separate standards because exposure conditions vary so greatly between these industries. The two silica standards are similar in scope, but have slightly different requirements for employers in each industry. The OSHA final rule reduces the permissible exposure limit (PEL) for RCS to 50 micrograms per cubic meter of air averaged over an eight-hour day. This level is the same for all workplaces covered by the standard, general industry, maritime and construction.
 
How can you determine the amount of RCS? A personal dust monitor will check the amount of RCS at your jobsite to help ensure the exposure meets appropriate levels. OSHA established a PEL of 50 micrograms per cubic meter because they determined that occupational exposure to RCS at the previous PELs resulted in a significant risk of developing or dying from silicosis. Silicosis is a form of occupational lung disease marked by inflammation and scarring in the form of nodular lesions in the upper lobes of the lungs. Symptoms of silicosis include shortness of breath, cough, fever and cyanosis. The rule also instates an action level of 25 micrograms per cubic meter averaged over an 8-hour day. In short, employers must either demonstrate that employees will not be exposed to RCS at or above this level in any foreseeable circumstance. If RCS levels could reach this action level, employers must meet additional requirements. Those requirements might include medical surveillance, hazard communication, record keeping of silica exposure results and medical exams as well as additional compliance methods.
 
Employee exposure testing is required if RCS exposure is at or above the action level. OSHA has given general instructions for compliance, advising companies to:
 
  • Use engineering controls
  • Provide respirators to workers
  • Limit worker access to high exposure areas
  • Develop a written exposure control
  • Offer medical exams to highly exposed workers
  • Train workers on silica risks and how to limit exposures
Depending on the industry, implementation dates for the new rule range from 2017 to 2021. According to OSHA, the new occupational exposure limit is expected to save over 600 lives and prevent more than 900 new cases of silicosis annually. OSHA stated about 676,000 workplaces will be affected and the rule will result in annual costs of about $1,524 for the average workplace. The annual cost to a firm with fewer than 20 employees will be less, averaging about $560.
 
In order to control silica exposure at or below the PEL, OSHA has instructed employers to use new engineering controls and work practices. Engineering controls include wetting down work operations, before wet vacuuming it up for proper disposal, or using local exhaust ventilation, such as vacuums, to keep dust-containing silica out of the air. Respirators alone cannot achieve compliance. They are only allowed when engineering and work practice controls cannot maintain exposures at or below the PEL. Even when respirators are selected, fitted and maintained correctly, they must be worn consistently and correctly by workers to be effective. For construction, the standard includes Table 1, a list of common construction tasks along with exposure control methods and work practices that work well for those tasks and can be used to comply with the requirements of the standard.
 
Among the engineering controls, water is the most used method of controlling dust at a work site. Spraying water in processes such as grinding or drilling can reduce the amount of dust by as much as 75%. Wet spray systems can offer significant advantages over ventilation/exhaust systems or structural enclosures. Water is highly effective, more economical to operate, quickly installed, more durable and a reliable way to control dust. Using water as dust control involves a combination of sprays, mists and water guns. Light, frequent application is better than infrequent and heavy usage as excess water can cause erosion problems. To suppress the dust, it is important to match water drop size to the particle size of the dust. Water drops larger than dust particles avoid collision. Water drops smaller than dust may collide, but the water evaporates too quickly. To properly suppress the dust, the diameter of the water drop size and the dust particle most be comparable.
 
Purposed-designed dust mitigation equipment like Generac Mobile’s dust suppression systems, nebulize the water, creating a fine mist that evenly covers the area. Nebulized water particles combine themselves with dust particulate and drag them to the ground. Traditional systems, like a water hose, produce water droplets of about 1000 micrometers in size, which only capture larger dust particles. For more on Generac Mobile’s dust suppression systems, visit this page.
 
OSHA has several tools that can help with understanding the latest standards and what it takes to be compliant. The agency has a frequently asked questions section on their website, including a series of videos that help instruct users on methods for controlling exposure to silica dust when performing common construction tasks.