An estimated 2 million construction workers in the United States have been exposed to respirable silica dust, according to the Occupational Safety and Health Administration. That’s why OSHA’s Respirable Crystalline Silica standard requires that employers limit workers’ exposure to silica and take other steps to protect workers.

“Silica dust appears in asphalt, concrete and rocks,” said David Rigsbee, CEO at Chemtek Inc., Morrisville, North Carolina. “Any activity that involves breaking, cutting, demolishing, grinding, etc. of those elements will create silica dust.” That means many activities in our industry have the potential to create respirable crystalline silica dust that can be inhaled by nearby workers.

The standard (29 CFR 1926.1153) requires employers to limit its workers’ exposure to and provide protection from silica dust. Employers have the option of following a control method laid out in Table 1 of the construction standard or they can measure workers’ exposure and decide which dust control methods will reduce workers’ exposure to the permissible exposure limit (PEL).

Check out AsphaltPro’s article, New NIOSH Software Measures Silica Exposure FAST, for more information about measuring respirable crystalline silica levels.

How Silica Dust is Collected

According to OSHA, the main types of engineering controls for silica are wet methods and local exhaust ventilation.

Wet methods involve applying water or foam where the dust is generated to prevent it from getting into the air, whereas local exhaust ventilation removes dust by capturing it where it is created.

For example, a grinder, walk-behind milling machine or jackhammer must be operated with either a water delivery system or a shroud and vacuum dust collection system.

If using water to suppress dust, you must ensure an adequate supply of water, hoses are intact, and that the spray nozzle is working and pointed at the point of dust generation. OSHA’s Small Entity Compliance Guide for the Standard for Respirable Crystalline Silica for Construction doesn’t dictate a minimum flow rate, but states that the equipment must apply water at the flow rate specified by the manufacturer.

If using a dust collection system, it must have a filter with 99 percent or greater efficiency and a filter cleaning mechanism, and it must be able to maintain the air flow recommended by the manufacturer. On the job, be sure to check that the shroud is intact and properly installed, the hose connecting the tool to the vacuum is intact without any kinks, the filter is cleaned or changed to prevent clogging, and bags are regularly emptied to avoid overfilling.

Silica Collection for Milling

Within the world of asphalt, Rigsbee said, the lion’s share of silica dust comes from sweeping and milling operations. The most common containment method for sweeping and milling, Rigsbee said, is spraying water onto the dust. However, after OSHA passed 29 CFR 1926.1153 in 2017, reducing the amount of dust allowed from 250 micrograms per cubic foot to 50, water alone may no longer be sufficient to suppress silica dust to the standard.

“To give you an idea, if you have more dust onsite than the size of Abraham Lincoln’s forehead on a penny, you are now over the limit and could be fined by OSHA for tens of thousands of dollars,” Rigsbee said.

That dust is also hydrophobic, meaning it fails to mix with water. Pair that with the stricter requirements, and even contractors using wet method can struggle to contain silica dust. To combat this, some companies choose to add a silica dust suppressant, such as Chemek’s NeSilex, to the water to break down the barrier between the dust and water.

According to Steve Howard, safety and facilities manager at Wirtgen America, Antioch, Tennessee, its milling machines use water and a vacuum system to control dust. The vacuum system captures dust particles from the cutter house, transports them through a hose, and ejects them out the end of the conveyor.

Silica dust suppressants, such as Chemtek’s NeSilex, break down the barrier between dust and water to aid in silica control. These photos show the milling process with (above) and without (below) NeSilex. Photos courtesy Chemtek Inc.

Wirtgen was a part of the Silica/Asphalt Milling Machine Partnership, which was comprised of the National Asphalt Pavement Association, manufacturers, labor, academia and the National Institute for Occupational Safety and Health (NIOSH). The partnership “worked to design, test and implement engineering controls for milling machines that effectively reduce potential silica exposure below OSHA’s new proposed permissible exposure limit,” according to NAPA’s website.

“During those tests, we realized that water was a good method and the vacuum system was also good, but baghouses were not proven to be effective,” Howard said. “Baghouses work at an asphalt plant, but they are more equipped for that at a stationary plant. Not only would there be a whole big attachment on the milling machine, but also someone would have to get rid of that material at the end of the day and risk exposure then.”

“All milling machine manufacturers currently offer/use water and dust extraction on their milling machines,” said Matt White, North American Sales Manager for Milling and RS at BOMAG, Ridgeway, S.C. However, the vacuum system transfers the dust, versus reducing it, he added. “Transferring the silica dust is enough to meet today’s standard and ensure the operator isn’t exposed to it.”

White said that recent tests performed by BOMAG have indicated that dust particles could remain in the air on the job site for up to 40 hours after the milling machine has passed. That’s why BOMAG has developed its Ion Dust Shield for its 4-foot class machine.

According to White, the system reduces the PM 2.5 and PM 10 sized particles creating during the milling process by at least 80 percent.

“The system removes this dust by clumping or sticking it together by positively charging the particles,” White said. “This particle size is respirable dust that goes directly into the lungs and bloodstream, and is not seen by the naked eye.”

BOMAG plans to showcase its Ion Dust Shield in the American market in the fall of this year. The system is already available on milling machines sold in Europe.

“We’re preparing for the direction we’re headed,” White said. “Regulatory agencies aren’t going to make the standards any less stringent.”

Regardless of the system you employ, Howard reiterates that it’s imperative to use the vacuum system while milling.

“The employer has to make sure they use and maintain the system correctly,” Howard said. “If you have the system, you should use the system. If it breaks, you need to get it fixed.”

Silica Collection for Sweeping

“Most operations don’t create that much uncontrolled really fine dust,” said Brian Giles, product manager at Schwarze Industries, Huntsville, Alabama. What does create a good amount of the troublesome fugitive fines is equipment driving on top of larger particles, such as sand and powders and crushing them into dust. “The best role for the sweeper to play is to remove those larger particles to prevent from getting crushed into fine particles.”

Giles said the key to control fines generation is keeping the area clean. Once the particles become dust, he added, there’s only so much a sweeper can do.

“Catching the fine stuff is almost like catching smoke,” he said. Giles estimates a sweeper can’t efficiently control anything smaller than a PM 10, as it becomes airborne when anything comes close to it.

That’s where the wet dust control method is integral, to cluster those fine particles into larger particles that the sweeper can pick up and reduce any fugitive emissions from the sweeper.

For open air activities that generate dust, such as pavement milling operations, Giles said wetting down those particles is a must. If the sweeper follows closely enough behind the mill that is using adequate water, Giles said it may not need to use its own water to suppress the dust, though that is always an option if needed.

Micro milling, Giles said, is another game entirely. “Regular milling makes chunks, not powder,” he said. However, Giles compares the wetted down dust behind a micro mill to soupy black snow. For that, he recommends a vacuuming process along with traditional sweeping.

According to page 43 of OSHA’s compliance guide, slurry generated by wet methods should be cleaned up before it dries using a wet vacuum.

Vacuum manufacturers’ operator manuals typically provide instructions for changing dust bags and filters. For example, Rigsbee said, this could involve disposing of dust bags and filters in sealed, impermeable containers, such as heavy-gauge plastic bags, to prevent the release of dust particles into the air.

OSHA’s compliance guide states never to sweep or use compressed air on dried slurry. If slurry dries, they instruct immediately wetting it down and cleaning it up with a wet vacuum.

Some states may have more specific requirements for disposing of silica dust, so it’s best to contact your state Department of Environmental Protection or your regional office of the Federal Environmental Protection Agency for more information.

Dispose of Dust Collection Bags

If using dust collection as a method to protect workers from respirable crystalline silica dust, changing out the dust collection bags is inevitable.

According to Eric Massinon, business development manager at Chicago Pneumatic Power Technique, one bag can last for an entire day on the job before requiring replacement, depending on the media being worked.

In January of this year, Chicago Pneumatic released its new CDP 10 dust collection system kit to help protect workers from exposure to crystalline silica during common construction tasks. The kit includes a vacuum suction unit, 21-foot suction hose, universal shroud connector, three dust collection bags and three bellows.

For the CDP 10 dust collection system, the bags should be changed when filled to 25 percent (2 to 3 inches full). The bags allow visibility to see the level of dust inside of them.

The bags should be sealed and disposed of properly following local applicable environmental legislations for disposing of silica dust, CP said. As seen in the photo within this sidebar, a belt will tighten around the top of the bag to seal it in place.

The CDP 10 comes with three bags, and additional bags and filters can be purchased at any dealer for CP.

The filters used on the CDP 10, which has the capacity of 99.58 percent filtration of exhaust air, can and should be washed out and reused, CP said, adding that they should be replaced after a couple of cleanings. A vacuum indicator notifies the operator of a clogged filter. CP reminds us that operators should wear dust masks any time they are exposed to respirable crystalline silica dust.

CP released its new CDP 10 dust collection system kit to help protect workers from exposure to crystalline silica during common construction tasks.