Many options are available to clear the air.

By Roberta Baxter

"It's just dust; why should I worry about it? Dust shows that work is going on." This has been the attitude of some in the development and construction industries, but tougher air-quality regulations, bigger fines for violations, and increasing public pressure to curb fugitive dust are changing that attitude.

Each year, an estimated 1,000-1,500 tons of soil enter the air as dust. Fugitive dust, especially particles smaller than 10 microns–known as PM₁₀–cause health problems, reduce visibility, and increase the cost of vehicle and road maintenance.

People in communities around the world are becoming more aware of the health hazards of dust. PM₁₀ particles have been linked to increased asthma attacks, allergies, and lung problems. USEPA has enacted regulations to control particles even smaller than PM₁₀.

In addition to health hazards, on unpaved roads a major concern is the reduced visibility dust causes. On highways in California and Arizona, for example, multicar pileups have occurred when the wind has kicked up a dust storm.

You know what particles of dirt do to the lubricants and moving parts in your vehicle. Controlling that dirt can extend the lifetime of many vehicle parts. Likewise, a developer can experience significant cost savings by taking steps to control dust on-site. "Dust suppressants can make an immediate difference in repeated applications by 50%," observes Lou Snow, president of Dust Pro Inc. in Phoenix, AZ. Over time, he explains, the savings accelerate because the developer is paying less rent for water trucks and fewer labor costs. Add to that less risk of a hefty fine for noncompliance with air-quality standards, and the effort becomes even more worthwhile.

A gravel road is made up of gravel mixed with tiny particles, known as fines. The fines help hold the road together. When the dust blows, fines are lost, compromising the stability of the roadbed. The dust you see from an unpaved road is made up of the fines that help bind coarse aggregate particles together to stabilize the road surface. By controlling dust, you're also helping to preserve the binder that holds the road together.

Why Not Water?

Many people immediately think of dampening dirt with water to keep dust down. That works–but only for about five minutes in most conditions. And in many parts of the United States, especially the West, water is too precious to waste by spraying it on dirt that will quickly dry out. So it's much more efficient to use an additive in the water that greatly extends the dust-control time.

Selection Factors

To select a dust suppressant, you should consider the following factors:

• Will it provide needed dust control?
• Is it environmentally compatible?
• Can it be easily applied with road maintenance equipment?
• Is it cost-effective?
• What is the soil type?
• What is the climate?
• What is the traffic volume?

Soil Seal helps prevent wind erosion on hundreds of acres of disturbed ground at the Playa Vista development.

Before you apply dust suppressants, you might need to perform some basic road maintenance. The surface must have the right contour and enough gravel and fines to stabilize the roadbed with the help of the chemicals.

Dust-control products come in two main categories: chloride-based products and resins of various types. Each category works in a different way.

One increasingly important selection factor is a product's effect in the area where it will be applied. "People want to use a product that's effective and that is also safe for the environment," states John Leslie of SynTech Products Corporation in Toledo, OH, makers of PetroTac emulsion. He notes that when choosing a product, users pay attention to the data: "Aquatic toxicity and those types of data are important to people to make themselves comfortable they aren't exchanging one problem for another."

The Chlorides Are Absorbing

Chloride-based products are used predominantly on unpaved roads or tailings piles. Calcium chloride is hygroscopic, meaning it absorbs water. As it absorbs water, the calcium chloride also dissolves into the water, forming a clear liquid that coats the gravel and fines, binding them together.

Application of calcium chloride should follow a rain or a wet-down by a water truck. The chemical absorbs moisture from the air at 29% relative humidity and a temperature of about 75ºF. The chemical also slows evaporation of moisture and, after a few months, can be revitalized by another spray of water.

As a dust suppressant, calcium chloride can be applied in a water solution or as dry flakes. When applied dry, it is usually mixed into the top 2-4 in. of roadbed material. After application, the roadbed is compacted by a roller. In liquid form, calcium chloride is sprayed on the roadbed.

Calcium chloride has a few disadvantages: It can be mildly corrosive to metals in its undiluted form–corrosion inhibitors can be added for use in areas where repeated application is necessary–and it does not perform as well in dry climates without added water. Advantages include reasonable cost compared to other products and being environmentally safe. Another benefit is that it lowers the freezing point of the road moisture and helps prevent frost heaving, thus further reducing road maintenance.

General Chemical Corporation of New Jersey manufactures calcium chloride for use as a deicer and as a dust suppressant. The company ships the chemical as dry flakes or as a liquid in strengths of 77%, 90%, and 94% by weight dry and 32%, 35%, and 38% by weight liquid. Richard Jenkins, liquid industry manager for the company's Canadian subsidiary General Chemical Canada Ltd., states that most users apply calcium chloride once a season for low traffic use and twice for heavier traffic loads. His company typically sells to governmental entities, mining and logging companies, and quarries.

Paul Boudreau with the City of Lewiston (ME) Highway Division, states, "We have saved up to 25% on road maintenance with the liquid calcium chloride because it reduces the number of times we must regrade our roads."

Magnesium chloride works in much the same manner as calcium chloride, but it results in an even harder road. Magnesium chloride's method of action is similar to that of calcium chloride in that it also absorbs water from the air. The July/August 2000 issue of Erosion Control included a Project Profile on dust control and the use of magnesium chloride. The article mentioned that BHP Copper in San Manuel, AZ, had a 3,000-ac. tailings pond that was creating a huge dust problem. South Western Sealcoating Inc. of Murrieta, CA, devised a plan to apply Dust-Off, a magnesium-chloride product, from an air tanker. Using the plane allowed a lot of dust suppressant to be applied in a very short time.

Resins Keep It Together

Resins control dust by "gluing" the particles together. Resins can be manufactured from petroleum, wood residue, or other chemicals. They can be applied as an erosion control measure for stockpiles, haul roads, or bare slopes or as a tackifier in a hydroseeding application. Rain glides off resin-treated soil without taking the soil with it. Resins also make the soil less vulnerable to wind erosion, and environmental impact is very low after application. Care must be taken with all resin products, however, to avoid introducing them into water bodies during application.

Resins are more useful in dry climates than the chlorides are; they also are noncorrosive. A resin-treated soil surface can eventually become brittle, requiring reapplication. Although resins generally cost a little more than the chlorides, they might require less frequent application.

Polymer Products

A variety of acrylic polymers and polymer emulsions are also available. These products are usually sold in concentrated form and mixed with water before being applied with a water truck or a hydroseeding machine. The solution penetrates about 0.5—0.75 in. into the soil and binds together the soil particles. Temporary or permanent dyes added to some products allow the operator to see what ground has been covered. These products include Soil Seal and Soil Sement.

Soil Seal is a latex acrylic polymer that is diluted about 30:1 for application. One advantage is its low-temperature cure; Soil Seal continues to harden even at temperatures as low as 36ºF. The usual curing time at 72ºF is eight to 12 hours. The product includes an antifoam ingredient because–as Guy Nishida, operations manager for Soil Seal, explains–"It can be a real time-waster to have the ‘root beer' foam effect during the dilution process." Nishida notes that the company is receiving more requests for dust control, not only from large commercial developers but also from smaller developers and even personal users.

Soil Seal is in use on a huge project near Marina del Rey, CA. The largest tract of undeveloped land in the area, the property is where Howard Hughes built the "Spruce Goose" airplane. Now known as Playa Vista, the 1,087-ac. land parcel is being developed by a consortium of The Moote Group and others. The project includes residential, commercial, retail, and high-density areas. A 300-ac. wetland and open space is being restored in the tract, so the product had to be environmentally safe for these areas.

Paul Pegg, senior project manager for infrastructure at The Moote Group, says 300-500 ac. of ground may be disturbed at any one time on this project. Neighbors on a nearby bluff complained because ocean winds were kicking up dust from piles that had not even been disturbed recently. Workers now spray the piles with Soil Seal for long-term dust control. "Some piles are still there after about eight months," Pegg adds.

Soil Seal is applied at the beginning of the rainy season and can be reapplied to steep slopes later in the year. It is a sandy, granular mix that requires heavy application of sealant for slope stabilization. Pegg states that the Soil Seal purchased in bulk is cheaper than hydroseeding, which would run 3-6 cents per square foot. Another advantage of using a sealant rather than hydroseeding the soon-to-be-developed areas where the piles sit is that the land does not have to be recleared.

Midwest Industrial Supply of Canton, OH, has been involved in dust-control projects for 25 years. The company produces a resin known as Soil Sement, a polymer emulsion diluted with water. According to Bob Vitale, founder and president, "We design an application program targeted to reach the dust-control measure needed." He says soil stabilization assists in meeting not only air-quality standards but also stormwater runoff requirements. Correct application can greatly save on grading costs. "We determine traffic load on haul roads: frequency, weight of vehicles, and the speed they travel. Then we set up a whole program for the customer."

Soil Sement is the product of choice for Sparks Construction of La Quinta, CA. Superintendent of Land Development Chris King describes a 343-ac. project that will eventually be divided into 500 residential lots. Up to 50 lots at a time are cleared, and Soil Sement is applied to the lot pads. The area contains sandy soil and is surrounded by residences, and the company wants to avoid creating a dust nuisance for the already-occupied homes. Moreover, the regional Air Quality Board has been vigorously fining noncompliant sites in the region. King maintains that Soil Sement has provided good, long-term stabilization: "In a lot of places, it's lasted over a year."

Soil Sement is also in use at a US Air Force base to reduce dust during jet takeoff, improve visibility for pilots, and reduce plane maintenance costs.

Ligninsulfonates

Dust Pro's Snow states that his company chooses a variety of products based on soil analysis and such factors as traffic load and length of use. Increasingly, customers lean toward the most environmentally friendly product, regardless of cost. In his opinion, the best for the environment is a resin derived from wood–ligninsulfonates. Georgia Pacific was the main US supplier for years but recently closed the plant that made the product. Snow says he now orders the product from Canada.

Ligninsulfonates bind the dirt particles in the same manner as the other resins. They are very effective in dry climates and also retain some plasticity. One disadvantage, however, is that the solids are water soluble to a certain extent and could lose binding action in a heavy rain. Snow calls the ligninsulfonates the "natural superglue" of dust-control agents. He says that the acrylic resins do not penetrate as far and calls them the "Elmer's Glue."

Ligninsulfonates are in use on the haul roads of several power plants in the construction process in southern Arizona. Snow points out that the roads carry major traffic, and an application every six months is handling the load.

Depending on soil type and traffic load, it might be necessary to experiment with more than one type of product at a location to get the desired results. One such place is the Pikes Peak Highway in Colorado. Jack Glavan, capital projects manager for the highway, says that the 12 mi. of unpaved road up America's mountain has seen several types of dust-control products. For a couple of years, a petroleum product was used and can still be seen in some places on the roadbed. Calcium chloride is currently applied for dust control. Neither product, however, has lasted long over the whole road. Glavan states that the problem is the decomposed granite soil, which has very little fines. "There's just not much for these products to hold together." Thus, stabilizing the entire 12-mi. stretch to a depth of 4-6 in. has been cost-prohibitive. "For now we aim for 60 to 90 days of dust control," Glavan reports. That covers the summer tourist season and the annual Pikes Peak International Hill Climb: a race from bottom to top of the famous mountain.

Controlling the fugitive dust on your project can decrease health problems, improve your vehicle and road maintenance, and protect you from noncompliance fines. And it's not just the permitting and regulatory agencies who are keeping a close eye on dust–it's the neighbors too. "Awareness is heightened, and you have a chain reaction as far as the visibility of the issue," says SynTech's Leslie. "Because construction sites are required by permit to actually control the dust, people are looking at how well it's being controlled and what's being used to control it."

So it's time to arrest that fugitive dust! 

Author Roberta Baxter specializes in science and technology topics.