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September 04, 2013
Runoff potential in OSHA's silica proposal

There is concern that U.S. OSHA’s proposed revision of its 40-year-old permissible exposure limit (PEL) for respirable silica may impact the environmental compliance requirements of small construction businesses. 

The issue, which was raised by small entity representatives when OSHA issued its draft of the PEL standard in 2003, is that the much more stringent PEL and the wet methods OSHA recommends to reduce exposure may trigger new stormwater requirements for outdoor construction projects.  In its proposal, OSHA states that it reviewed the environmental aspects of the new PEL as required by the National Environmental Policy Act (NEPA) and concluded that it “does not expect that the proposed rule, in and of itself, would create additional environmental issues.”

10 times more stringent

OSHA-proposed PELs for all industry sectors is calculated as an 8-hour time-weighted average of 50 micrograms of respirable crystalline silica per cubic meter of air (50 µg/m3).  For the construction and shipyard industries, OSHA’s existing PEL is a formula that is based on the concentration of respirable particles in the air.  On a mass concentration basis, OSHA says it believes the existing PEL for construction and shipyards is within a range of between about 0.25 mg/m3 (250 µg/m3) to 0.5 mg/m3 (500 µg/m3) expressed as respirable quartz.  In other words, the proposed PEL is up to 10 times more stringent than the existing standard.

EPA stormwater rules

In its 2003 report, the small entity representatives who were part of the U.S. Small Business Administration’s advocacy panel that reviewed OSHA’s draft indicated that use of the wet methods contemplated in the draft may lead to violations of EPA rules prohibiting suspended solids in runoff unless provision is made for recycling or settling the suspended solids out of the water. In the preamble to the current proposal, OSHA notes that EPA stormwater requirements apply to construction projects of 1 or more acres and generally require the use of such best management practices (BMPs) as stormwater collection to minimize pollution. 

While the use of wet methods to control silica dust is not directly addressed in the EPA requirements, OSHA recognizes that local governments might require compliance with EPA’s BMPs when granting construction permits to prevent water used for dust control from running into storm drains, drainage ditches, or surface waters.  OSHA notes that the California Department of Transportation is one regulatory body that requires use of such BMPs.

Pavement breakers

In its environmental assessment, OSHA says that of the six wet methods that were costed for silica control, only the use of pavement breakers (jackhammers) has the potential to directly result in runoff discharges to storm drains or surface waters.  The applicable dust-control method consists of a dedicated worker directing a constant mist with a garden-type hose at the jackhammer being operated by a second worker.  But OSHA has determined that this wet dust-control method would rarely create runoff potential. 

“Studies suggest that a water flow rate of 1/8 to 1/4 gallons per minute is best for silica dust control,” says OSHA.  “At this rate, about 7.5 to 15 gallons of water per hour would be applied to (i.e., sprayed on) the work area.  It is unclear whether this quantity of water applied to a moveable work area at a constant rate would produce a runoff.  If the work were in sufficient proximity to a storm drain or surface water, the contractor might need to use a simple barrier to prevent the water from entering the drain, or filter it. 

Because the volume of water is relatively small, the costs for such barriers are likely insubstantial.  However, because this type of runoff could happen occasionally, OSHA has added costs for barriers in costing silica controls for this task.”

Even with the potential for runoff posed by wet control of pavement breakers, OSHA concluded that the silica PEL as proposed would “have little potential impact on air, water, or soil quality; plant or animal life; the use of land; or aspects of the external environment.”

OSHA’s proposed silica PEL

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