Reining in fugitive dust
- By Paul Simon
- Mar 01, 1999
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Anthropodust - fugitive dust generated from human activities - is more than a visual nuisance.
Soil-derived anthropodust not only worsens outdoor air quality, which impairs lung function and exacerbates lung and heart diseases, but also transmits infectious illnesses by carrying pathogenic bacteria and viruses into the respiratory system. A preliminary study by Brooklyn Veterans Affairs Medical Center in New York City of troops returned from the Persian Gulf War with respiratory complaints suggests the fine sand dust of the Saudi Arabian peninsula may have played a significant role in the respiratory ailments suffered back home.
Wind erosion of disturbed earth surfaces, including agricultural lands, uncovered storage piles, construction activities, mining operations and resuspension of dust from roads are the principal sources of anthropodust, which composes nearly 90 percent of inhalable particulate emissions.
The initial National Ambient Air Quality Standard (NAAQS) for particulate matter was set in 1971 by the U.S. Environmental Protection Agency (EPA) to encompass all forms of particulate matter, termed total suspended particulates (TSP). Later health studies revealed that airborne particles larger than 10 micrometers (µm) in diameter are trapped in the nose and in the nasal cavity, and are expelled by coughing and sneezing.
In 1987, EPA replaced the TSP standard with a new standard for inhalable particulate matter having an aerodynamic diameter of 10 µm or less (PM-10). Subsequent epidemiological studies found that smaller particles of 1 to 2 µm in diameter are an optimal size for reaching the alveoli, or air sacs, in the deepest region of the lungs. EPA further refined the NAAQS standard for particulates in 1997 by including a new standard for fine particles having an aerodynamic diameter of 2.5 µm or less (PM-2.5).
Airborne particulate matter tends to cluster into two distinct size ranges: coarse particles greater than 2 µm in diameter and fine particles smaller than 2 µm in diameter. This particle size division is also the same separation between silt and clay particles of earth soils, so that the fine fraction of soil-derived dust is comprised of colloidal clay particles that behave differently from larger silt particles, ranging from 2 to 75 µm in diameter, that form coarse anthropodust.
Colloidal clay particles range from 1 to 0.001 µm in diameter; their physical behavior is controlled by interfacial forces rather than gravitational forces, which govern silt behavior. Of more significance is the difference in surface chemical behavior between relatively inert silt particles and colloidal clay particles having an unbalanced, typically negative, electrical charge, with the result that cations of hazardous toxic substances can be adsorbed onto the surface of clay particles.
Outdoor air pollution from colloidal clay particles is significant. According to an EPA emissions estimate, about 70 percent of its PM-2.5 inventory is comprised of soil-derived anthropodust. While people cannot choose the air they breathe, the source of fine anthropodust can usually be controlled, or the anthropogenic pollution be removed through an air-purifying respirator before breathing. In the South Coast Air Basin in California, which has the dubious honor of being among the worst places on the North American continent for fine particulate pollution, environmental regulations mandate reasonably available control measures (RACMs) must be upgraded to best available control measures (BACMs).
Generation of anthropodust
Outside of rural desert environments, most wind-driven fugitive dust is soil-derived anthropogenic emissions from natural atmospheric winds sweeping across disturbed earth surfaces, or over open storage piles of bulk granular materials. Much of the earth surface disturbance is caused by earthwork construction. Moving transportation vehicles over roads and railroad tracts and at airport runways, including helipads, create turbulent wakes that suspend particles off pavements and shoulders similar to natural winds.
Non wind-driven anthropodust is produced by mechanical activities, such as drilling, blasting, crushing, grinding, shoveling, sweeping and tilling in connection with construction, mining, waste disposal and agricultural operations. For example, demolition last year of the J. L. Hudson department store by implosion resulted in downtown Detroit being covered in a billowing cloud of dust.
Anthropodust from unpaved and paved roads is the principal source of PM-10 and PM-2.5 anthropodust pollution, according to EPA emissions estimates. This soil-derived dust includes resuspension of road dirt, grinding of pavement particles by vehicle tires and wind erosion of shoulders. Other major sources of anthropodust, in descending order of severity, are agriculture, construction, mining and quarrying, and waste disposal activities. These dispersed sources of solid particulate matter are a contrast to exhaust-stack and tailpipe emissions from energy, transportation, manufacturing and processing industries.
Awareness of outdoor air pollution from anthropodust is not readily apparent because particles finer than 60 µm - the diameter of a human hair or coarse silt - cannot be seen by the naked eye. Regular air sampling is necessary to meaningfully monitor concentrations of inhalable PM-10 particles and the more health-damaging particles finer than 2 µm.
Anthropodust is generated from both wind- and non wind-driven events, but when airborne emissions become visible, anthropodust concentrations generally exceed standards for particulate matter under the Clean Air Act. The current EPA standard for particles 10 µm in diameter mandates the daily maximum concentration not exceed 150 micrograms per cubic meter (µg/m3) more than once per year, and for fine dust particles less than 2.5 µm in diameter the 24-hour concentration not exceed 65 µg/m3.
Controlling wind-driven anthropodust is most effectively accomplished by establishing a durable protective cover to prevent wind erosion of exposed soil surfaces and open storage piles. Dust control measures depend principally upon the material to be protected and its moisture content, but also on the planned duration of protection, which may range from a day on active land development earthworks to decades at shoulders along railroad tracks. Dust from roads, particularly unpaved, is only partially wind driven, since the grinding action of vehicle tires on the road surface is another anthropodust source.
Watering and vegetative cover are the oldest forms of dust suppression and are still in use today, despite some shortcomings. Water evaporates quickly in hot weather, requiring several applications a day, but is nevertheless convenient in earthwork construction where surfaces experience continual disturbances. However, use of water is limited in many western states.
Vegetative cover provides good permanent protection against wind erosion but requires several months to become established, as well as periodic waterings to maintain growth. Another technique to suppress emissions at source in non-traffic areas is to place a clean gravel cover about 4 to 6 inches thick over exposed earth surfaces.
Chemical non-toxic dust suppressants have become environmentally acceptable for both short- and long-term surface stabilization of anthropodust sources, and there are more than 60 trade-name dust suppressants commercially available that may be categorized broadly into six groups based on composition and suppressant mechanism. They are:
- Hydroscopic salts;
- Wood fibers and mulches;
- Lignin sulfonate;
- Asphalt or petroleum emulsions;
- Polymers; and
- Miscellaneous unclassified.
EPA lists the use of chemical dust suppressants as one of the most effective BACMs for long-term stabilization of fugitive dust sources. In general, the greater the suppressant's concentration at application, the longer the suppressant will maintain a stabilized surface. Liquid chemical stabilizers are commonly applied from a tanker truck's spray bar or by spray hose as shown in Figure 1
A unique wind-driven source of anthropodust is mobile mechanical blowers used to clean not only city street and parking area pavements but also home gardens and drainage gutters. The rate anthropodust settles through air can range from five minutes/m for coarse particles larger than 10 µm in diameter, to 10 days/m for ultrafine particles smaller than 0.1 µm.
Airborne silt particles travel short distances within the ground-based air layer, whereas clay particles float in the atmosphere and travel greater distances by advection, dispersion and diffusion. For example, fine dust was blown more than 100 miles from the dry bed of Owens Lake, Calif. Such mechanical sources of anthropodust can be essentially eliminated by replacing leaf and debris blowers with vacuum devices.
Anthropodust resulting from mechanical activities, such as crushing, grinding, sanding, drilling, blasting and demolishing, involves active operations for which controlling dust at the source is awkward and frequently impossible. When anthropodust concentrations in the ambient air exceed any NAAQS standard for solid particulate matter, the ambient air should be cleaned by filtration or particle agglomeration and precipitation before being inhaled by humans.
A common protection from anthropodust-contaminated air is an air-purifying respirator, such as a quarter-facepiece model to remove nuisance dusts or a half-facepiece or full-facepiece model with interchangeable cartridges for filtering out different contaminants. To utilize electrostatic effects, air cleaning with a respirator can be enhanced by placing an electrostatically charged perforated plastic sheet between a high-efficiency particulate air (HEPA) filter and a conventional fiber filter, as shown in Figure 2
, to remove ultrafine particles smaller than 0.1 µm.
The fiber filter can trap the coarse dust particles, which carry an extremely weak electrostatic charge, and the HEPA filter can remove inhalable fine particles larger than 0.1 µm. With a positive electrostatic charge on the perforated plastic sheet, colloidal clay particles smaller than 1.0 µm, which typically carry a significant negative electrostatic charge, will adhere to the perforated plastic.
Anthropodust generated within a restricted space, such as an underground excavation or enclosed building, can be controlled by fogging the air before release into the atmosphere. A fog generator breaks up water into fine droplets that agglomerate with the floating dust particles and settle to ground surface. The agglomeration process can be hastened by ionizing the fine water droplets with a positive electric charge before the fog generator blows the droplets into the space containing the negatively charged dust particles.
For more information
"From dirt to toxic dust," Environmental Protection, May 1997, p.12.
"Dustborne infectious diseases," Environmental Protection, September 1998, p. 28.
This article originally appeared in the 03/01/1999 issue of Environmental Protection.