Respirable Crystalline Silica

Terms you need to know

Industry wide recommendations by the manufacturers of joint compounds for the safe use of drywall finishing products have been virtually the same since Freeman Products, Inc. began in 1979. Wet sponge sanding has been the recommended method and when not wet sponge sanding: wear a NIOSH approved respirator, adequately ventilate the jobsite and, once they became available, use of a vacuum sander.  Again, these are industry wide recommendations, not just Freeman’s.

What is now changing are the OSHA rules relating to “Respirable Crystalline Silica”.  Respirable Crystalline Silica.  TWA.  PEL.  TLV.  Engineering Controls, PPE.   These are all terms that OSHA uses in the new rules taking effect on September 23, 2017.   The following provides definitions and reference sources that explains these terms.

  1. Respirable Crystalline Silica: Crystalline Silica is found in mined mineral products, anything coming out of the ground contains some amount.   One of the most common forms of crystalline silica is quartz, the second most common mineral in the Earth’s crust.  Crystalline silica is therefore all around us in our everyday life.  Crystalline silica becomes a potentially serious health problem when it becomes respirable.  For a particle to be “respirable” it must be very tiny, only a few microns (thousandths of a millimeter) in order to penetrate to the smallest parts of the lung (alveoli).  So when we talk about respirable crystalline silica we are talking about the amount, or fraction/percent, of particles present in a sample that are small enough to penetrate the deep lung.  OSHA defines it as ‘The respirable dust fraction, which is comprised of particles sufficiently small to reach the deep lung.  The respirable fraction is broadly defined as particles having a diameter of 10µm or less (IARC, 1997).’(1)   Crystalline Silica is present all around us but it is potentially a serious health problem when it becomes a dust particle of such a small size as to get deep into a person’s lung.
  1. Time Weighted Average (TWA): OSHA defines TWA as ‘When an air sample is taken over a period of time, the resulting measure of exposure represents an average concentration to which a worker was exposed over the sampling period, this is referred to as a “time-weighted average” (TWA) concentration.’(1)    This means monitoring and measuring just how much “dust” a worker is exposed to over a day’s work would be a TWA.
  1. Permissible Exposure Limit (PEL): The PEL is not a simple value but a formula based on the percent quartz content of respirable dust for an 8-hour time-weighted average (TWA).  Exposures to dusts are expressed either as a particle concentration (millions of particles per cubic foot of air or mppcf) or a gravimetric concentration (unit mass of particles per unit volume of air, such as mg/m3).  This is the amount of exposure that is considered to be allowed for a worker during the 8 hour work day.
  1. Threshold Limit Value (TLV): The Threshold Limit Value (TLV) of a chemical substance is a level to which it is believed a worker can be exposed day after day for a working lifetime without adverse effects.  TLV is a reserved term of the American Conference of Governmental Industrial Hygienists (ACGIH).  TLV’s issued by the ACGIH are the most widely accepted occupational exposure limits both in the United States and most other countries.(2)(3) Its units are in parts per million (ppm) for gases and in milligrams per cubic meter (mg/m3) for particulates such as dust, smoke or mist.   TLV is what studies have estimated to be just how much a worker can be exposed to over their lifetime without damage to their health.
  1. Engineering Controls: As described by the National Institute of Occupational Safety and Health (NIOSH)   ‘Engineering controls protect workers by removing hazardous conditions or by placing a barrier between the worker and the hazard.’(4)  Wet Sanding, vacuum sanding and mechanical ventilation (exhaust fan) would be examples of engineering controls. These are actions you can take on the jobsite to reduce or limit the amount of dust created.
  1. Personal Protection Equipment (PPE): PPE is not an engineering control.  PPE is used when the engineering controls are not feasible or cannot control exposure enough to keep below the PEL.  When PPE is used the employer is required to implement a respiratory protection program that is specific to the current work task and jobsite. See OSHA 1910.134 and ANSI Z88.2 for details on respiratory protection requirements (OSHA, 2006; ANSI/ASSE, 2015). Respirators or dust masks are examples of PPE’s used by a worker while on the jobsite to reduce or limit his personal exposure to the levels of dust created on the jobsite.

If you have any questions about these definitions or need additional clarification we would recommend that you review the OSHA and NIOSH referenced documents. Understanding these terms will help you in following OSHA’s new rules.



← Back to Blog