Small silica dust is carcinogenic, and exposure to silica has been recognized an occupational health concern for decades. This size range of particle can travel long distances suspended in the atmosphere and particles smaller than 5 μm are easily lodged in the lungs.
In natural settings sand does not usually fracture this small, but under pressure in industrial operations and on roadways, silica can be ground to this respirable size. Road construction, non-metallic mining, glass grinding and sand-blasting are common sources of silica pollution.
Regulations on silica emissions and non-occupational exposures are fairly new, highly varied, and applicable only in a few states. Monitoring requirements and techniques are not yet standardized. No affordable, low-cost means of demonstrating regulatory exceedances of airborne silica concentrations currently exists, although Public Lab is pursuing the development of passive PM monitors for this purpose.
While there are few direct means of proving or registering non-occupational silica exposures, concerned communities can take a variety of actions to address sources of silica particles and advocate for stricter silica regulations. For more information about advocacy around industrial sand mining, see the Wisconsin frac sand advocacy leverage points page and for monitoring visible emissions, including fugitive emissions, please see the visible emissions and silica.
The following sections discuss regulations around silica and how they relate to existing occupational exposure and overall particulate matter regulations that do exist. It also highlights where current regulations on non-occupational exposure to silica do exist, and their approaches differ.
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Exposure to silica in occupational and non-occupational settings
Efforts to measure and regulate non-occupational exposure to silica are fairly recent. Occupational regulations around silica exposure, which started in the 1920s for the U.S. (OSHA 2008), are based on scientific findings that there are correlations between total airborne particles and lung damage. However, the concentrations of airborne particles that are likely to cause health effects in occupational settings are higher than concentrations of particles that are relevant to protect health in non-occupational exposure settings.
Only very small size-fractions of silica are transported and settle outside of occupational zones. Fine sand (~20-100 μm) can become airborne, but it settles nearby. Silica dust less than 10 μm is light enough and has enough surface area to stay airborne long enough to travel beyond occupational zones. A fraction of these smaller dust particles are also the most damaging to the lungs.
Silica dust less than 5 μm in diameter is respirable, meaning it can travel into the bronchial region and deposit in the gas-exchange zone of the lungs. There, they can cause scarring, swelling, and the growth of fibroids in alveoli, the deepest parts of the lungs. Silica dust less than 5 μm is of greatest concern in both occupational and non-occupational exposure. In occupational exposure, respirable silica is often correlated with larger particles, whereas in non-occupational settings respirable silica is not necessarily correlated with total coarse particulate matter. Occupational and non-occupational guidelines for silica exposure vary in whether they derive from estimates based on larger particle (PM10) monitoring data or respiratory-size specific data, but all non-occupational exposure limits are based on modifications of occupational exposure rules.
The concentration of particulate matter that is cause for concern in non-occupational exposure is much lower than in occupational exposure. A person is at work typically only one-third of the day, and usually spends more hours at home than work, including sleep. Also, the exposed population in a non-occupational setting includes more vulnerable people, such as children and the elderly, than the workforce (which is often estimated as healthy young-adult and middle-aged men in exposure risk studies). Children breathe more deeply than adults, and their smaller body mass means that their relative exposure to pollutants is much higher.
For all of these reasons, non-occupational exposure limits are set lower than occupational exposure limits to protect human health. For respirable crystalline silica, the difference between the two types of exposure limits can be orders of magnitude, as OSHA’s occupational exposure guidelines are to avoid exposures above 10 milligrams per cubic meter, while Vermont’s non-occupational exposure guideline is 0.12 micrograms per cubic meter.
Inhalation studies and studies of human cadavers have shown that crystalline silica particles less than 5 μm in diameter can travel deep into the lungs causing irritation and cancer. Respirable crystalline silica (silica particles that are less than 5 μm) has been identified as a human carcinogen by the International Agency for Research on Cancer (IARC). Non-occupational respirable silica emissions are not federally regulated, however six states have adopted ambient respirable silica exposure standards.
The Occupational Safety and Health Administration’s (OSHA) PM4 monitoring standard is a federal standard that state-level OSHA agencies implement and enforce.
OSHA’s current rules necessitate air sampling using a size-shearing pump to draw air onto a filter, and then analyze the filter for the concentration of crystalline silica. The methods that OSHA promotes are more performance-based than based on specific technology (whereas EPA methods are specific to certain technologies/instruments), but require using a devise (usually a cyclone) that can collect and retain 0% of particles that are 10 μm or larger, 25% of particles that are 5 μm, 50% of particles that are 3.5 μm, 75% of particles that are 2.5 μm, and 90% of particles that are 2.0 μm (OSHA Technical Manual, Section II, Chapter 1, Part III). The method is generally referred to as a method for “PM4” or particulate matter that is 4 μm, because size-shearing methods are often distinguished by their “50% cut point,” or the diameter of particle for which 50% are entrained into the cyclone and 50% impact the walls. The OSHA method is labeled for PM4 as short-hand rounding for a PM3.5 method. After the appropriately sized particles are collected on the filter, they are analyzed using X-ray diffraction (XRD) techniques, described below.
We predict that the sampling techniques outlined by OSHA for occupational silica exposure would systematically underestimate silica exposure in non-occupational ambient settings. The stipulated performance (described above) methodically under-samples particles on the larger end of the range (1-5 μm), with only 25% of PM5 being entrained into the sample stream, so a relatively larger proportion of much smaller particles (e.g. 90% of 2 μm particles) constitute the final sample. In occupational settings such as sandblasting where more than 90% of particles will be silica, under-sampling particles on the large-end of the respirable fraction, will not appreciably change the percentage of silica in the air.
However, in ambient situations where a significant portion of fine particulates (2.5 μm and smaller) derive from other sources such as diesel combustion products or atmospheric reactions of sulfur dioxide, the disproportionately large representation of these smallest particles on the sample filter might not be truly representative of what is respirable in the air. This is extremely important in measuring for respirable silica because the percentage of total particles that is crystalline silica will be assessed based on the percent of particles on the sample filter that are silica. Non-silica particles would constitute a disproportionate (erroneously high) fraction of the total particulate matter, and thus the calculated silica percentage would be erroneously low.
State non-occupational exposure rules
Six states, California, Minnesota, New Jersey, New York, Texas, and Vermont, have adopted ambient air quality standards or guidance for ambient respirable crystalline silica (less than 5 μm in diameter) based on concerns about its health effects. Any inhaled particles of this size are dangerous, but silica can be especially detrimental to people’s health.
In 2005, the California Office of Environmental Health Hazard Assessment (OEHHA) set forth a rule that chronic exposure (e.g. everyday exposure, at home or outside) to respirable crystalline silica should be less than 3 μg/m3. Minnesota – also a state facing potential frac sand mining like Wisconsin – and New Jersey have adopted California’s health-based standard of respirable crystalline silica at 3 μg/m3, Texas and New York have set their guidance at 2 μg/m3 (though prior to 2014, New York had set theirs at 0.06 μg/m3), and Vermont has set their guidelines much lower at 0.12 μg/m3.
To determine ambient air guidelines for respirable crystalline silica, states used occupational health guidelines and adapted them to be suitable for chronic exposure. The typical population in occupational exposure studies are healthy adult males. This population’s ability to deal with problematic exposures before experiencing negative health impacts is greater than any other population’s. Thus, adequate concentration limits for non-occupational exposure need to be lower than occupational exposure limits.
California, and subsequently Minnesota and New Jersey, adjusted occupational exposure for the increased number of hours exposure would occur (i.e. hours not included in the 40-hour work week), and an “intraspecies uncertainty factor of 3” (MN DOH Respirable Silica Toxicological Summary), which is an estimated factor to account for the differences in susceptibility between healthy adult males and more vulnerable populations. Texas and New York used slightly higher adjustment factors, and Vermont followed adjustment guidelines for most known carcinogens, adjusting by an overall factor of 100.
The nonprofit organization Environmental Working Group wrote an expository piece on ambient airborne silica, in which they urged more states to adopt respirable silica regulations and make the standards no higher than 0.3 μm/m3 in order to protect vulnerable populations.
State measurement programs
California, Minnesota, New Jersey, New York, Texas, and Vermont have added respirable crystalline silica as a Hazardous Air Pollutants and thus adopted ambient guidelines, but respirable crystalline silica is not routinely measured. Rather, industries known to emit silica must use computer simulations to estimate their respirable crystalline silica emissions before they can obtain a permit to build or operate a facility.
These estimate emissions are based on empirical conversion factors from PM10 emissions estimates, followed by air dispersion models. If a proposed facility’s emissions estimates indicate that they might emit an unacceptable amount of respirable silica, then the state would work with the proposed facility owner to discuss Best Available Control Technologies (BACT) to reduce their potential emissions. However, states may never actually monitor respirable crystalline silica. For example, in New York state, there has yet to be a case in which the state determined it must monitor respirable silica emissions based on emissions estimates and air dispersion models (personal communication).
Silica & PM10
The U.S. does have National Ambient Air Quality Standards for particulate matter (find more information here), including standards for “coarse” and “fine” particulate matter. Coarse particulate matter (PM10) is composed of airborne particles that are less than 10 μm in diameter. Analyses from different regions of the U.S. determined that silica composed anywhere from 0-25% of the total particles (by mass) in daily PM10 samples, and proposed estimating 10% silica by mass in PM10 samples (US EPA 1996).
Since silica is not federally regulated separately from general particulate matter, and analyses to identify silica (such as XRD, discussed below) can be very expensive, agencies use this very rough estimate that 10% of PM10 is silica, though it is acknowledged that the percentage silica in a sample varies by location and nearby activities. At sand mining operations, the percentage of particulate matter that is silica can be upwards of 90% (based on EPA’s emissions factor for sand and gravel processing), so the typical estimation of 10% may significantly underestimate the amount of airborne silica in areas near industrial sand mining.
"Inhalable" vs. "Respirable"
Coarse particulate matter is all “inhalable,” meaning that it can enter the upper respiratory system, but it is not all “respirable,” meaning it reaches the gas-exchange zone deep in the human lungs. Particulate matter that is less than 5 μm in diameter is considered respirable. Unfortunately, there have been few studies that have investigated what portion of PM10 is respirable, and it is likely to vary based on the composition of particulate matter in the sample.
This EPA study found an average of 20% PM4 (respirable fraction) in PM10 samples, but it ranged from 7 to 50%. Directly from PM10 measurements, it is difficult to ascertain the risk of respirable dust exposure. With the combined uncertainties of the portion of PM10 that is respirable and the percentage of PM10 that is silica, it is nearly impossible to adequately assess the risk of respirable silica exposure from PM10 measurements.
Silica & PM2.5
The U.S. has National Ambient Air Quality Standards for fine particulate matter (read more here), which is less than 2.5 μm in diameter (PM2.5). Much respirable silica is larger than PM2.5 (though smaller than PM10), and is excluded from sampling for PM2.5. Up to 90% of PM2.5 may be comprised of combustion byproducts and secondary particles. These make identification of respirable silica more challenging.
Particles this small are composed of “primary” and “secondary” particles, meaning particles that are directly emitted from a source and particles that are formed through reactions in the atmosphere, respectively. Chemicals that can react to create PM2.5 include nitrogen dioxide (NO2) and sulfur dioxide (SO2), which are hydroscopic and react with water droplets (read more on droplet formation).
Visible emissions are also regulated throughout the United States. Visible emissions are quantified by a measure of opacity, which the degree of light-scattering by particles, and akin to the lack of transparency in the sky. The EPA has two primary methods that citizens can conduct to measure the opacity of emissions, EPA methods 9 and 22. Read more about these methods here.
While visible emissions are not chemical-specific, monitoring and reporting visible emissions can be effective to bring enforcement for emissions violation. Emissions that are subject to opacity rules include primary emissions (e.g. through a smoke stack), and also fugitive emissions, such as leaky pipes, unpaved transport roads, or storage piles on industrial property. Often fugitive emissions are difficult to quantify or are neglected in permitting applications, so monitoring for visible fugitive emissions can be useful.
Silica can be speciated as crystalline minerals and amorphous silica in commercial lab services for filter-based pm monitoring tools using X-ray diffraction. Lab analysis costs over $100 per sample. See the EPA list of Federal Reference and Equivalent Methods for particulate matter, and OSHA's respirable silica method
It may be possible to use polarized light microscopy to identify silica on passive PM monitors. Manual identification of silica by scanning electron microscopy (SEM) and x-ray diffraction (XRD) is possible at many universities and some additional accredited laboratories. To read more about our plans to try this method, please see our PM monitoring development goals.
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