When silica is part of industrial processes, airborne silica dust is a significant health concern. Silica does not naturally fracture smaller than 10 micrometers (μm), but in road construction, non-metallic mining, sand-blasting, and other high-pressure activities, it may break down to less than 5 μm, a size at which it becomes “respirable.” Respirable dust can travel deep into the lungs’ smallest structures, alveoli, where oxygen is exchanged. All respirable particles interfere with breathing and are difficult for the body to remove, but one type of respirable silica — respirable crystalline silica — also cuts and scars the lungs, creating a condition known as silicosis, and is carcinogenic.1
In occupational settings, respirable crystalline silica dust is regulated by the Occupational Safety and Health Administration (OSHA). Non-occupational exposure to respirable crystalline silica is regulated in just six states, whose ambient exposure limits are based on OSHA’s 8-hour workplace exposure limit. OSHA has set a new limit of 50 micrograms of silica per cubic meter of air (50 μg/m3), going into effect June 23, 2016. This new limit may affect state non-occupational exposure rules. Converting between occupational and non-occupational exposure requires accounting for both exposure time and exposure risk. Occupational exposure time is assumed to be 40 hours per week, while ambient, chronic exposure time is a full week of 168 hours. Workers, who are protected by the OSHA laws, are also assumed to be healthier than vulnerable segments of the general population, such as children and the elderly. A margin of safety (usually a factor of 30-100) is therefore built into chronic exposure limits to account for risks to these vulnerable populations. An occupational exposure limit of 50 μg/m3 therefore may have a corresponding chronic exposure limit near 0.4 μg/m3. Currently no states routinely measure respirable crystalline silica to assess chronic exposure, and the methods to do so are still debated.2
_Image credit: Center for Disease Control _
The federal Environmental Protection Agency (EPA) does not regulate respirable silica or other respirable particles, but it does set ambient airborne concentration limits on two size categories of particles, without regard to their chemical composition: particles up to 10 μm diameter (PM10) and particles up to 2.5 μm diameter (PM2.5). PM10 are considered “inhalable” because they travel only into the upper reaches of the respiratory system, while PM2.5 is a subset of respirable particle sizes that travel all the way to the alveoli. Respirable silica is one of the components of particulate matter.
_Image Credit: EPA 454-R-04-002 _
Agencies use a rough estimate for the composition of particles they expect to be in any given sample. They estimate that 10% of particulate matter is silica, which includes respirable crystalline silica and other forms of particulate silica. However, it is acknowledged that the percentage of total silica, and the percentage of respirable crystalline silica, varies by location and nearby activities. At sand mining operations, where silica can constitute 95-99% of the mined sand, the percentage of PM10 that is total silica is likely to be higher than the assumed 10%. If the respirable crystalline silica percentage of PM2.5 near industrial sand mines is more than 3%, the area could be in compliance with the ambient air limit of 12 μg/m3 PM2.5, but still exceed a chronic exposure risk level for respirable crystalline silica.
References: 1. International Agency for Research on Cancer (IARC) report: http://monographs.iarc.fr/ENG/Monographs/vol100C/mono100C-14.pdf 2. Read more: https://publiclab.org/wiki/silica-monitoring