The primary purpose of this document is to increase awareness among plant managers, safety and health professionals, and engineers of the potential for occupational exposure to asphalt and asphalt fumes during the manufacture of asphalt roofing products. The document represents a collaborative effort of industry, labor, and government. During public meetings held in Cincinnati, Ohio (July 22–23, 1996), several groups agreed to develop a series of documents that (1) describe the extent of asphalt exposure during the production of asphalt roofing products and during asphalt roofing operations, and (2) provide information about measures to reduce exposures. These groups included the National Roofing Contractors Association (NRCA); the Asphalt Roofing Manufacturers' Association (ARMA); the Asphalt Institute (AI); the United Union of Roofers, Waterproofers, and Allied Workers; and the National Institute for Occupational Safety and Health (NIOSH). This document identifies work practices and other control measures that, when available, may be effective in reducing worker exposures to asphalt fumes during the manufacture of asphalt roofing products.

In 2000, NIOSH conducted a review of the health effects data on asphalt that had become available since the publication of the 1977 criteria document on asphalt [NIOSH 2000]. This review addresses acute and chronic effects and is available at the NIOSH Web site (www.cdc.gov/niosh) for readers interested in additional information.

NIOSH, labor, and industry are working together to better characterize and quantitate the health risks from asphalt exposure. Representatives of industry, labor, government, and academia met in Cincinnati, Ohio, on September 11 and 12, 2000, and identified research to assess completely the health risks associated with exposure to asphalt. Through these and other efforts of this partnership, effective workplace measures can be implemented to reduce worker exposure to asphalt fumes.

Asphalt (CAS* No. 8052–42–4) is a dark brown to black, cementitious, thermoplastic material found in a natural state or (more commonly) manufactured in petroleum refineries by atmospheric or vacuum distillation; it may also be left as residue after evaporating or otherwise processing crude oil or petroleum. Asphalt is solid or highly viscous at ambient temperatures. This material is an extremely complex mixture containing a large number of high molecular weight organic compounds [King et al. 1984]. Asphalt is now the dominant material in roofing in the United States. However, coal tar is still used in some roofing work, usually to conform to government building specifications that require the use of coal tar [Freese and Nichols, Inc. 1994].

Most of the asphalt used in the United States is used in paving (87%) and roofing (11%) operations. Only about 1% is used for waterproofing, dampproofing, insulation, paints, and other activities [AI 1990a]. Asphalt roofing products and systems include shingles and roll roofing, ply felt, built-up roofing (BUR) systems, saturated felt used as underlay for shingles, and modified bitumen systems. These products and systems are described in Section 2.4.

*Chemical Abstracts Service.

Ninety-seven asphalt roofing manufacturing plants currently operate in the United States. (The North American Industry Classification System [NAICS] code is 324122 for asphalt shingle and coating materials manufacturing.) These plants are owned by 27 companies and are located in 28 States, generally within 500 to 800 miles of their primary markets. The plants vary greatly in size, number of workers, and products manufactured. A typical asphalt roofing plant manufactures several products, including shingles, roll roofing (smooth or mineral surfaced), ply felt for use in BUR, and saturated felt used as underlay for shingles. Modified bitumen products are manufactured on machines designed for polymer-modified materials.

Approximately 3,000 to 4,000 workers are exposed to asphalt fumes in approximately 100 roofing manufacturing plants [ARMA 2001].

The three basic grades of roofing asphalt are (1) saturant-grade asphalt, a nonoxidized or oxidized asphalt used to manufacture saturated felt plies used in the construction of BUR systems, organic felt shingles, and other roofing materials such as roll roofing; (2) coating-grade asphalt, an oxidized asphalt used to manufacture roofing materials for a variety of roofing systems such as asphalt shingles, polymer-modified bitumen roofing, reinforcing and underlayment felts, and roll roofing products; and (3) mopping-grade asphalt, an oxidized asphalt that is melted and used in the construction of BUR and modified bitumen systems. Each grade of asphalt is also used to manufacture a variety of asphalt coating and sealant products.

The principal differences between saturant and coating grade asphalts are viscosity and softening point. Saturant asphalts typically have a softening point of about 120 to 140 °F (50 to 60 °C), making them less viscous than coating asphalts, which have a softening point of approximately 200 to 225 °F (95 to 105 °C). Despite their lower viscosity, saturant asphalts are processed at significantly higher temperatures (about 425 to 475 °F [218 to 245 °C]) than coating asphalts (about 380 to 460 °F [190 to 238 °C]) because of the need to ensure adequate impregnation of the organic felts that use saturant asphalts [ASTM 1995].

The four types of mopping-grade asphalt are described in Table 2–1. The viscosity of mopping-grade asphalts depends on which of the four types is being manufactured. Type I is the softest (least viscous) grade and is used on very-low-slope roofs. Type IV is the hardest (most viscous) grade and is used on the highest slope roofs suitable for BUR systems.

Petroleum refineries and independent asphalt manufacturers produce oxidized roofing asphalt by air-blowing the residuum of refinery atmospheric or vacuum distillation processes. This starting material, termed "asphalt flux," also may be a blend of residua from different sources. In the air-blowing or oxidation process, heated asphalt flux is placed into a tank known as a blowing still, and air is blown through it. The reactions that take place are exothermic, so the temperature is controlled within the range of 400 to 550 °F (204 to 288 °C). The temperature and the amount of air are varied by the manufacturer depending on the nature of the asphalt flux and the intended characteristics of the oxidized roofing asphalt being produced. This process raises the softening point and viscosity and lowers the penetration and ductility of the asphalt [King et al. 1984; IARC 1985; Corbett 1979].

At the temperatures of the air-blowing process, the oxidations and subsequent reactions ultimately yield compounds of increased polarity and higher apparent molecular weight [Boduszynski 1981; Corbett 1975; Goppel and Knotnerus 1955]. Compared with the asphalt flux, the air-blown asphalts contain an increased proportion of asphaltenes, decreased proportions of naphthene-aromatics and polar aromatics, and about the same proportion of saturates† [Corbett 1975; Boduszynski 1981; Moschopedis and Speight 1973]. The process effluent contains water, carbon dioxide, and other reaction products and small amounts of relatively volatile components of the asphalt [Corbett 1975; Goppel and Knotnerus 1955]. The oxygen added to asphalt in the air-blowing process appears to reside in hydroxyl, peroxide, and carbonyl functional groups, the latter including ketones, acids, acid anhydrides, and esters [Campbell and Wright 1966; Petersen et al. 1975; Goppel and Knotnerus 1955].

†For determination of gross composition, asphalt is frequently fractionated by treatment with heptane or a similar hydrocarbon solvent to precipitate the asphaltenes; fractionation is followed by chromatography of the maltenes (soluble portion) into three fractions, which are (in order of increasing polarity) the saturates, naphthene-aromatics, and polar aromatics [Corbett 1975; Boduszynski 1981].

Today, three commercially popular roofing products or systems are made from roofing asphalt, each with different characteristics and applications:

• Asphalt shingles and roll roofing are used in residential and steep-slope commercial roofing.
• BUR systems are asphalt-impregnated felt pieces sealed and surfaced with hot mopping asphalt; the systems are used in low-slope commercial roofing.
• Modified bitumen systems are also a low-slope commercial product using polymer-modified roofing asphalts to impregnate and coat one or more fabric plies.

 

Asphalt shingles introduced in the early 1900s account today for about 75% of new construction and reroofing in steep-slope residential and some commercial roofing applications [NRCA 1995]. Roll roofing was the mainstay of the steep-slope market until it was largely displaced by shingle products. Today, roll roofing is used mainly in BUR systems on low-slope roofs. With low-slope roofing, smooth-surface roll roofing can be used in building the BUR membrane, and mineral-surfaced roll roofing is used as a cap or "top sheet" [NRCA 1996; AI 1990a].

Asphalt shingles and roll roofing both consist of a reinforcing felt covered with coating asphalt; organic felts are impregnated with a saturant asphalt. In most cases, asphalt shingles and roll roofing contain a surfacing material, usually coarse or fine mineral. Asphalt shingles and roll roofing are installed using mechanical fasteners or cold-applied adhesives; they do not require hot mopping asphalt. In addition, they are typically installed over an underlayment felt that has been impregnated with coating asphalt during manufacture, and both are affixed to the roof substrate by mechanical means or cold adhesives [NRCA 1996].

BUR systems were introduced in the late 1800s and remain the most popular roofing system for commercial and industrial buildings, accounting for about 30% of the new and retrofit market for low-slope roofs [NRCA 1995]. The BUR membrane is composed of layers (or "moppings") of mopping asphalt between felt plies of saturant asphalt or coating asphalt reinforcing fabric, such as organic felts (e.g., cellulose), fiberglass scrim or mat, or polyester fabric. BUR membranes are installed in multiple-ply configurations that typically involve three to six interply moppings of mopping asphalt. In addition, roll roofing made from organic or inorganic materials, or a flood coat‡ of mopping asphalt (usually Type I) is applied as a weatherproofing top layer.

All three grades of roofing asphalt (coating, saturant, and mopping) may be used in the manufacture or construction of BUR systems: saturant asphalts are used to manufacture organic felts and roll roofing; coating asphalts are used for virtually all felt ply and roll goods; and heated mopping asphalts are used for the interply moppings and, in some cases, the flood coats applied in constructing the BUR membrane [NRCA 1996].

Polymer-modified bitumen roofing systems were introduced in the 1970s and today account for about 23% of the new and retrofit market for low-slope (i.e., primarily commercial and industrial) roofs [NRCA 1995]. Modified bitumen products are of two types: (1) those made primarily with atactic polypropylene (APP), and (2) those made primarily with styrene-butadiene-styrene (SBS) as the polymer modifier.

APP membranes are primarily torch-applied—that is, they are adhered to an underlying base sheet or to the manufacturer's approved substrate by heating the back side of the APP membrane and the substrate with high-intensity, propane-fired torches or specially designed hot-air welders. The heat is applied only as needed to soften the asphalt and make the modified bitumen membrane adhere to the substrate; these products can be cold-applied with adhesives. SBS membranes may be applied by adhesion in hot asphalt or in a cold-applied, solvent-based asphalt adhesive; or they may be torch-applied [NRCA 1996].

‡ Flood coat is the surfacing layer of asphalt into which surfacing aggregate is embedded on an aggregate-surfaced, built-up roof. A flood coat is generally thicker and heavier than a glaze coat and is applied at approximately 45 to 60 lb/100 ft² (a square) (2 to 3 kg/m²).