Danger in the Air

Silica Particles from Frac Sand Mining Put Tens of Thousands at Risk

The boom in natural gas and oil exploration using hydraulic fracturing and horizontal drilling, commonly called fracking, has created a huge demand for the sand that drilling companies mix with water and toxic chemicals and inject underground to free gas and oil trapped in deep rock formations.

A 33-county area that spans southeastern and south-central Minnesota, southwestern Wisconsin and northeastern Iowa has become a major source for this now-valuable sand. As of March 2014, according to data compiled and mapped by the Environmental Working Group (EWG), the tri-state area was home to 71 operating silica sand mines and 27 sites for solely processing, transporting or loading sand onto trucks or rail cars. Another 82 mines or associated sites have been proposed or granted permits.

Across the region, EWG’s mapping shows that more than 58,000 people live within 750 meters (less than half a mile) of the existing permitted or proposed sand mines and related sites, which can release fine particles of silica into the air. Research has shown that these particles can degrade air quality as far as 750 meters away, leading to a variety of serious health problems, particularly in children and other vulnerable populations. More than 162,000 people make their homes within 1,500 meters of the frac sand-related sites in the region. In the absence of definitive evidence on the potential risk from airborne silica at 1,500 meters, EWG recommends that air monitoring be conducted to determine whether there is a danger to public health at that distance as well. (See Silica in Outdoor Air – The Danger of Frac Sand.)


The expansion of the frac sand industry in the region has been explosive. In 2004, there were only 20 mines and 20 mine-related facilities in operation, meaning that in the last decade the number of mines has increased by 145 percent. Some news reports have cited higher figures, and the rapid growth and patchy regulation of the industry make it likely that some sites have been overlooked. EWG used figures supplied by state, county and local agencies and jurisdictions.1

EWG’s mapping (Table 1) shows:

  • If there is a ground zero of the explosion in frac sand mining, it is Trempealeau County, Wis., with 29 active, permitted or proposed mines or associated sites. Ten frac sand mines and one loading site are currently in operation in the county; 14 others have received permits and four more have been proposed.
  • Four other Wisconsin counties also have high concentrations of frac sand sites. Barron County has six operating mines and four processing or loading sites, with nine more permitted or planned; Chippewa County has six mines, two related sites and six more on the way; Wood County has seven mines, three related sites and another proposed; and Monroe County has eight mines, one related site and two more proposed. Two Wisconsin counties, Jackson and Buffalo, have fewer operating sites, but between them there are 19 on the way. When all of the permitted and proposed sites are in operation, these six counties plus Trempealeau will have 99 sites in all, well over half of the facilities in the tri-state area.
  • In Minnesota, the most affected county is Winona, with three mines and five related sites in operation and five proposed. No other Minnesota county has more than five working, permitted or proposed sites.
  • There is only one operating mine in northeast Iowa, in Clayton County. Mines were proposed in Allamakee and Winneshiek counties in 2012, but public opposition led both to enact 18-month moratoriums. They are due to expire in the fall of 2014, and at least some local officials do not expect them to be extended.2

With the exception of Ramsey County, which includes St. Paul, Minn., most of the counties where frac sand facilities are located are rural or small-town areas. But the spread of the industry throughout the region means that tens of thousands of people now live or work near sand mines or their associated sites.

Using U.S. Census data and computer mapping, EWG calculated the number of people in each county who live within 750 meters and 1,500 meters of a frac sand site. (Table 2) The first zone, at 750 meters, is the distance downwind from a sand and gravel pit at which air monitoring studies have found that the level of airborne silica particles can be twice as high as it is upwind. The second zone, at 1,500 meters, is the distance from the sites at which EWG recommends that regulators monitor air quality. There is not enough evidence to know whether people living in the 750-to-1,500 meter zones are at risk, but the lack of certainty underscores the need for monitoring, health studies and adoption of enforceable standards.

EWG’s mapping also found:

  • By a wide margin, the place where the most people live in a zone of concern is Winona County, Minn., where more than 22,000 people live within 750 meters of frac sand sites and more than 62,000 live within 1,500 meters.
  • Other counties where more than 10,000 people live in the zones of concern are Monroe County, Wis., with more than 5,800 people within 750 meters and more than 13,700 within 1,500 meters; Ramsey County, Minn., with more than 16,500 people within 1,500 meters; and Wood County, Wis., with more than 11,000 people within 1,500 meters.
  • There are also frac sand sites in proximity to schools, hospitals and clinics, where children, patients and others with greater sensitivity to airborne silica may be exposed. In the 33 counties, 20 schools are within 750 meters of frac sand sites and 83 are within 1,500 meters; two hospitals or clinics are within 750 meters and 32 are within 1,500 meters.

Table 1. Existing, permitted and proposed frac sand mining sites

County Operational Permitted Proposed Total
Barron, Wis. 10 8 1 19
Mine 3 6   9
Mine + Processing 3   1 4
Processing   2   2
Processing + Rail Load-out 4     4
Blue Earth, Minn. 1     1
Mine + Processing + Rail Load-out 1     1
Buffalo, Wis. 2 4 4 10
Mine 1 2 1 4
Mine + Processing 1 2 2 5
Rail Load-out     1 1
Burnett. Wis. 1     1
Mine + Processing 1     1
Chippewa, Wis. 8 5 1 14
Mine 3 2   5
Mine + Processing 3 2 1 6
Processing + Rail Load-out 1 1   2
Rail Load-out 1     1
Chisago, Minn. 1     1
Processing + Rail Load-out 1     1
Clark, Wis. 4     4
Mine 2     2
Mine + Processing 2     2
Clayton, Iowa 1     1
Mine + Processing + Rail Load-out 1     1
Columbia, Wis. 1     1
Mine + Processing + Rail Load-out 1     1
Crawford, Wis. 2     2
Mine 1     1
Rail Load-out 1     1
Dunn, Wis. 2   1 3
Mine 1     1
Rail Load-out 1   1 2
Eau Claire, Wis. 1 2   3
Mine   2   2
Mine + Processing + Rail Load-out 1     1
Fillmore, Minn. 1   4 5
Mine 1   4 5
Goodhue, Minn. 1   1 2
Mine + Processing     1 1
Rail Load-out 1     1
Grant, Wis. 1     1
Mine 1     1
Green Lake, Wis. 4     4
Load-out 2     2
Mine + Processing 1     1
Mine  + Processing + Rail Load-out 1     1
Houston, Minn.     5 5
Mine     5 5
Jackson, Wis. 5 5 6 16
Mine   3 4 7
Mine + Processing 2 2 2 6
Mine + Processing + Rail Load-out 2     2
Processing _ Rail Load-out 1     1
Juneau, Wis.     1 1
Mine     1 1
Le Sueur, Minn. 2   2 4
Mine     1 1
Mine & Processing     1 1
Mine & Processing + Load-out 1     1
Mine & Processing + Rail Load-out 1     1
Monroe, Wis. 9   2 11
Mine 2     2
Mine + Processing 2   1 3
Mine + Processing + Rail Load-out 4   1 5
Rail Load-out 1     1
Olmsted, Minn. 1     1
Mine 1     1
Pepin, Wis. 1     1
Mine 1     1
Pierce, Wis. 4 1 1 6
Mine 1 1   2
Mine + Processing + Rail Load-out 1     1
Processing 1   1 2
Processing + Rail Load-out 1     1
Ramsey, Minn. 1     1
Rail Load-out 1     1
Rusk, Wis.     1 1
Rail Load-out     1 1
Scott, Minn. 1   2 3
Mine + Processing     1 1
Mine + Processing + Rail Load-out 1   1 2
St. Croix, Wis. 2   1 3
Mine 2     2
Mine + Processing     1 1
Trempealeau, Wis. 11 14 4 29
Mine 4 7 2 13
Mine + Processing 5 6 1 12
Mine  + Processing + Rail Load-out 1 1   2
Rail Load-out 1   1 2
Wabasha, Minn. 1     1
Rail Load-out 1     1
Washington, Minn. 1     1
Mine + Processing 1     1
Winona, Minn. 8   5 13
Barge Load-out 1     1
Mine 2   3 5
Mine + Processing 1     1
Processing 1   1 2
Processing + Rail Load-out     1 1
Rail Load-out 2     2
Rail Load-out + Barge Load-out 1     1
Wood, Wis. 10   1 11
Mine 7   1 8
Processing +  Rail Load-out 3     3
Total 98 39 43 180

Environmental Working Group, from state, county and local government data.

Table 2. Population living in proximity to frac sand sites

County, State Population 750m Population 1500m
Barron, Wis. 2,685 5,950
Blue Earth, Minn. 1,268 6,284
Buffalo, Wis. 742 1,197
Burnett, Wis. 166 237
Chippewa, Wis. 1,561 3,547
Chisago, Minn. 295 1,325
Clark, Wis. 540 868
Clayton, IA 12 29
Columbia, Wis. 674 883
Crawford, Wis. 1,517 3,976
Dunn, Wis. 773 1,921
Eau Claire, Wis. 459 752
Fillmore, Minn. 212 440
Goodhue, Minn. 1,702 4,254
Grant, Wis. 146 169
Green Lake, Wis. 303 710
Houston, Minn. 342 741
Jackson, Wis. 823 3,721
Juneau, Wis. 43 202
Le Sueur, Minn. 1,058 2,910
Monroe, Wis. 5,838 13,740
Olmsted, Minn. 220 873
Pepin, Wis. 87 166
Pierce, Wis. 1,002 1,587
Ramsey, Minn. 3,171 16,526
Rusk, Wis. 233 355
Scott, Minn. 599 1,590
St. Croix, Wis. 289 842
Trempealeau, Wis. 4,263 6,981
Wabasha, Minn. 799 1,301
Washington, Minn. 2,027 5,230
Winona, Minn. 22,061 62,292
Wood, Wis. 2,881 11,189
Total 58,791 162,788

Environmental Working Group, from U.S. Census data

To read an extensive report on frac sand mining published by EWG research partner Civil Society Institute, click here.

1 Iowa Department of Natural Resources; Minnesota Departments of Natural Resources and Transportation and ’Environmental Quality Board; Wisconsin Department of Natural Resources; Fillmore, LeSeur and Scott counties, Minn.; Monroe and Trempealeau counties, Wis.; City of Winona, Minn.

2 Frac sand mining: Its fate in NE Iowa should rest in local hands. Cedar Rapids Gazette, Feb. 16, 2014.

Health Concerns for Silica in Outdoor Air


Mining, processing and transporting sand generate large quantities of silica dust, which is notorious for the damage it does to the lungs and respiratory system when inhaled. In recent years, the dramatic expansion of hydraulic fracturing and horizontal drilling technology to extract gas and oil, commonly called “fracking,” has led to boom in sand mining across the upper Midwest, creating a significant public health threat in the region.

None of the states at the center of this “frac sand” mining boom have adopted air quality standards for silica that are adequate to protect people living or working near the scores of recently opened or proposed mining sites. The growing danger of airborne silica is especially acute for children and other vulnerable populations.

Silica can impede breathing and cause respiratory irritation, cough, airway obstruction and poor lung function (Rego 2008). Chronic or long-term exposure1 can lead to lung inflammation, bronchitis and emphysema and produce a severe lung disease known as silicosis, a form of pulmonary fibrosis (Hnizdo 2003).

Silica-related lung disease is incurable and can be fatal, killing hundreds of workers in the U.S. each year. The National Institute for Occupational Safety and Health (NIOSH) has estimated that at least 2.2 million workers in the mining and construction industries are exposed to inhalable silica each year. However, the Institute noted, “There are no surveillance data in the U.S. that permit us to estimate accurately the number of individuals with silicosis. The true extent of the problem is probably greater than indicated by available data” (Weissman and Schulte 2011).

The fracking boom has meant that many more workers are being exposed to silica dust at “frac sand” mines or at drilling sites, where the sand is mixed with water and toxic chemicals before being injected underground to extract gas or oil trapped in deep rock formations. In a 2011-12 study, NIOSH found that exposure to airborne silica exceeded its occupational health criteria at every one of 11 fracking sites it tested, in some cases by a factor of ten or more (Esswein 2013). In response, the Institute and the Occupational Safety and Health Administration (OSHA) issued a Hazard Alert to warn workers of the risk (OSHA 2012).

In 2013 OSHA proposed new rules for occupational exposure to silica dust that it estimates could save 700 lives and prevent 1,600 cases of silicosis a year. The new rules, the first revision in 40 years of the agency’s permissible exposure limits for silica, would limit workplace exposure to 50 micrograms per cubic meter of air (µg/m3), averaged over an eight-hour day (OSHA 2014). Last year, U.S. Assistant Labor Secretary David Michaels told an oil and gas industry task force that more than 60 percent of workers at fracking sites are being exposed to amounts of silica above the proposed limits (Grossman 2013). The new rules have not yet been finalized.

How Silica Damages Health

By itself, silica is not toxic. The health risk arises when silica particles are small enough to get into the deepest parts of the lungs, especially the alveoli where inhaled air passes into the bloodstream (US EPA 1996).

In addition to the severe damage silica dust does to the lungs and respiratory system, studies of miners have linked it to diseases such as rheumatoid arthritis, systemic lupus erythematosus, scleroderma and kidney damage (Makol 2011; Parks 1999). Exposure to high levels has also been linked to heart problems, since the heart must work harder when pulmonary tissues are injured. Workers exposed to silica in other industries have a higher risk of lung cancer, which has prompted government and international health agencies to declare silica a known human carcinogen (IARC 2012; NTP 2011; Steenland 2014).

Silica dust is one of the most harmful components of “particulate matter,” a mixture of small airborne particles of organic chemicals, metals, minerals and soil (Reff 2009). Smaller particles pose the greatest danger because they can get deeper into the respiratory system. Fine particles smaller than 2.5 micrometers in diameter – less than 1/30th the width of an average human hair – are more harmful than larger particles. (The shorthand designation for particulate matter of a given size is “PM” followed by the diameter, as in PM2.5, PM4, PM10 etc.) Epidemiological studies have shown that breathing air polluted with PM2.5 particles causes heart and lung problems and increases the death rate from heart disease and lung cancer (Lepeule 2012). Particles larger than 2.5 micrometers in diameter do not get as deep into the lungs, but PM10-size pollutants do exacerbate respiratory diseases, particularly asthma, and cause heart failure (Shah 2013; Weinmayr 2010).

The concentration of silica in the air is often estimated based on the percentage of crystalline silica in a given sample of PM10, PM4 or PM2.5 particles (ACGIH 2001; Davis 1984; EPA 1996). Depending on the source, the level of silica in inhalable particulates collected at quarries and sand pits can be as low as 1-2 percent or as high as 95 percent of total particulate matter (Environment Canada 2013).

When tiny silica particles lodge in the alveoli, they cause an ongoing inflammation that damages lung tissue and causes scarring and fibrosis, a precursor of silicosis and lung cancer (IARC 2012). Freshly crushed silica is more damaging to the respiratory system and produces a more severe inflammatory response than “aged” silica particles of the same size (Shoemaker 1995; Vallyathan 1995). Breathing sharp, freshly-cut sand dust, such as silica at sand mining and processing sites, carries a greater risk of pulmonary disease than breathing older, smoother particles weathered by heat, wind, and moisture – such as silica dust blown from cropland.

There is no federal standard for ambient air exposure to silica outside the workplace. Based on occupational data, the EPA came up with a health-protective benchmark for crystalline silica in PM10 particles of 3 micrograms per cubic meter (µg/m3) (Gift 1997; US EPA 1996). Crucially, however, EPA’s benchmark did not consider the risks of exposure to vulnerable populations such as children or people with respiratory disease. The federal air quality standard for long-term exposure to PM2.5 for the general population is 12 µg/m3 a year, and the 24-hour, or acute, PM10 standard is 150 µg/m3 (US EPA 2014).

State exposure limits are inadequate to protect children’s health

Silica exposure is a well-known danger for workers in mining and construction. With the spread of frac sand mining, however, silica air pollution has also become a danger for residents near sand mining and processing operations. Children, older adults and people with respiratory diseases are especially at risk. In the absence of a national air quality standard for silica outside the workplace, six states have developed their own standards or guidelines.

Table 3. State exposure limits for crystalline silica in air*

State Calif. Minn. New Jersey Texas Vermont** New York**
Limit (µg/m3) 3 3 3 2 0.12 0.06
Type of limit chronic reference exposure level chronic health-
based value
long-term reference concentration chronic reference value hazardous ambient
air standard (annual)
annual guideline concentration
Measured as PM4 PM4 PM10 PM4 PM10 PM10

* Long-term exposure limits for general population based on the risk of silicosis.
** General population exposure limits derived by state agencies from occupational exposure values established by the American Conference of Governmental Industrial Hygienists (New York State Department of Environmental Conservation 1997; Vermont Department of Environmental Conservation 1998).

EWG’s analysis concluded that the silica exposure limits adopted by California, Minnesota, New Jersey and Texas are insufficient to protect children and other vulnerable populations, for several reasons:

These exposure limits are based on epidemiologic studies of adult male miners, a population of typically healthy and robust workers. None of the studies included children or vulnerable populations, although they face unique risks. As the California Office of Environmental Health Hazard Assessment (OEHHA) noted, “exacerbation of asthma, which has a more severe impact on children than on adults, is a known response to some respiratory irritants” (OEHHA 2005). The agency added: “Since children have smaller airways than adults and breathe more air on a body weight basis, penetration and deposition of particles in the airways and alveoli in children is likely greater than that in adults exposed to the same concentration.” 

In setting their silica exposure values, California and Texas used epidemiological data from miner studies and applied a three-fold adjustment factor as a margin of safety to account for human variability. (Minnesota adopted the California standards.)

EWG strongly disagrees with this approach. A three-fold margin of safety is insufficient to account for the potentially elevated sensitivity to silica among children, the elderly and people with respiratory diseases. The California agency’s own guidelines for the Derivation of Non-cancer Reference Exposure Levels, finalized in 2008 – three years after it adopted its silica exposure limit – call for a higher adjustment factor to protect children’s health from air pollutants (OEHHA 2008). In fact, in the draft risk assessment for benzene the Office of Environmental Health Hazard Assessment published in January 2014, it called a 10-fold adjustment a “default” factor for air toxics to allow for the differences among infants, children and adults (OEHHA 2014). Similarly, the U.S. EPA also typically uses an additional safety factor of 10 in its risk assessments for certain exposures during vulnerable periods of development. In the case of pesticides, the Food Quality Protection Act of 1996 specifically requires consideration of children’s exposure (U.S. EPA 2002a; U.S. EPA 2002b).

Under the California office’s current rules for assessing the risks of air pollutants, the three-fold safety factor is first applied where toxicological data is based on studies of adults only, as is the case with silica. If there is reason to suspect additional susceptibility in children to a particular pollutant, such as potential exacerbation of asthma, the rules call for applying an additional factor of 10 (OEHHA 2008). In sum, an overall 30-fold adjustment factor would be appropriate for air pollutants that pose particular risk to children, and in some cases the factor could be even larger (OEHHA 2008).

In its ongoing analysis of chronic exposure to benzene, for example, the California OEHHA started by recommending an adjustment factor of 30 to account for general human variability and children’s particularly susceptibility, but it later published an update concluding that an adjustment factor of 60 would be more appropriate (OEHHA 2014). In the risk assessment document for 1,3-butadiene, an industrial chemical used in making synthetic rubber, the agency again applied an overall adjustment factor of 30 (OEHHA 2013). Similarly, in 2012, it adopted a 30-fold safety factor in setting exposure limits for nickel to account for children’s sensitivity (OEHHA 2012).

EWG believes that exposure limits for silica also require an additional adjustment factor of 10 to account for the absence of epidemiological studies on children and their greater susceptibility to respiratory toxicants and to chemicals that affect the immune system. The California, Minnesota and Texas silica exposure levels all disregarded potential vulnerability of children to air pollutants such as silica.

 Applying both the three-fold and 10-fold adjustment factors, EWG calculates that a truly health-based value for silica exposure in outdoor air should be no higher than 0.3 µg/m3, and it may need to be lower.

To date only Vermont and New York have met this threshold. Both states have, in fact, set even more stringent silica exposure guidelines of 0.12 µg/m3 and 0.06 µg/m3, respectively. In setting those limits for silica in ambient air, New York and Vermont used a different method than California or Texas. Both started from occupational exposure limits and applied an adjustment factor of 100 (10 x 10). This combined factor of 100 takes into account the inherent toxicity of silica and the variable vulnerabilities of the population.

How much silica are communities near frac sand mines breathing?

Data on air pollutants near the Midwest’s burgeoning sand mining and processing plants are limited. Detailed air monitoring studies are critically needed to track the levels of airborne silica and other air pollutants near sand mining and processing operations and along the routes driven by trucks transporting the sand. Such studies should measure both airborne silica levels and how far silica and other air sand-mining pollutants travel on the wind (WDNR 2012).

One study of a sand and gravel facility in California found that at 750 meters (almost half a mile) downwind, the furthest point monitored, the level of silica in the air was twice as high as at an upwind site (Shiraki 2002). The silica content in particulate matter samples decreased from 33 percent at the plant itself to 10 percent at 750 meters away (Shiraki 2002). EWG recommends that air quality should be monitored at up to 1,500 meters (almost a mile) from sand mining and processing facilities. Monitoring at even greater distances may be necessary if significant quantities of silica are found at 1,500 meters downwind. 

EWG’s accompanying interactive map of existing or proposed frac sand operations in a region that spans parts of Minnesota, Wisconsin and Iowa identifies zones of concern at distances of 750 meters and 1,500 meters from each site. The potential risk of airborne silica at any given location depends on both the size of the site and the type of activity. A 1,500-acre open-air sand mine would generate more silica dust and disperse it over a wider area than an indoor processing facility or a railway loading station. The mapped zones of concern should be considered only as indicators of potential risk. Further research may indicate that these zones should be larger.

Analyzing estimated silica levels near frac sand sites

In January 2013, a research group from the University of Wisconsin-Eau Claire did a study of PM2.5 particle pollution near sand mining and processing operations. The choice of PM2.5 particle size was based on several factors. First, these smaller particles are more toxic to the respiratory system. Second, PM2.5 particles are encompassed in the California, Minnesota and Texas silica limits based on PM4. In contrast, the larger PM10 size would include many particles that are excluded from the limits set by California, Minnesota and Texas. Finally, there is a federal PM2.5 annual standard for the general population that corresponds to chronic open-air exposure, but there is only a short-term exposure standard for PM10 particles (US EPA 2014).

The Wisconsin researchers collected 16 air samples at the fence line (within 10 to 30 meters) of the EOG Resources Inc. sand mine and processing plant in Chippewa Falls, Wis. They found that the levels of PM2.5 ranged from 33 to 57 micrograms per cubic meter (µg/m3), with an average level of 41 µg/m3. They further estimated the silica content of the samples at 15 percent, yielding an average level of silica of 6 µg/m3. This is twice California’s and Minnesota’s PM4 chronic exposure limit for silica and three times the level set by Texas. It is 20 times the long-term exposure level of 0.3 µg/m3 recommended by EWG to be fully protective of children’s health.

The 15 percent average silica content of the samples in the Wisconsin study is largely consistent with several other studies. Sampling of particulate matter collected at sand and gravel operations in California’s Central Valley found silica levels of 6-to-26 percent, while sampling from two coastal sand quarries found silica levels of 14-to-17 percent (US EPA 1996). An analysis of inhalable dust samples at sand mining sites in Wisconsin reported silica concentrations in the range of 1-to-40 percent, with an average of 14.5 percent (Pierce 2013). Finally, a recent study of workers at fracking sites in five states found that silica constituted 53 percent of inhalable dust (Esswein 2013).


Silica levels detected near frac sand facilities in Wisconsin and Minnesota are at least 10 times higher than the health-based exposure limit of 0.3 µg/m3 that EWG considers safe for children and other vulnerable populations. This represents a significant health risk for residents living in close proximity to these sand mining and processing facilities. Residents exposed to sand dust spreading along the sand truck driving routes may also be exposed to silica dust in amounts that should cause concern.


1 Chronic exposure is repeated, continuous exposure to a hazardous substance over an extended period, defined by the Environmental Protection Agency as more than 10 percent of a person’s lifetime and potentially the entire lifespan.
Long-term exposure is a less formal term that can be used synonymously with chronic but is often used to indicate exposure for longer than a year (annual exposure). Acute exposure is defined by the EPA as exposure for 24 hours or less.

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