Asphalt Fume Exposures During the Application of Hot Asphalt to Roofs

The primary purpose of this document is to increase the awareness of roofing contractors, safety and health professionals, and engineers about current practices used to reduce occupational exposure to asphalt and asphalt fumes during the application of hot asphalt to roofs. This document represents a collaborative effort of 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 (UURWAW); and the National Institute for Occupational Safety and Health (NIOSH). During public meetings held in Cincinnati, Ohio (July 22–23, 1996), attendees agreed to develop a series of technical and educational documents that (1) describe the extent of asphalt exposure during the application of hot asphalt to roofs and (2) provide information about measures to reduce asphalt exposures.

This technical document identifies work practices and other control measures that may be effective in reducing worker exposures to asphalt fumes during the application of hot asphalt to roofs. Furthermore, this document is designed to be part of a comprehensive information and education program to be offered by the NRCA and the UURWAW in cooperation with NIOSH.

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

NIOSH, labor, and industry are working together to better characterize and quantify the health risks from asphalt exposure. Representatives of industry, labor, government, and academia met in Cincinnati, Ohio (September 11–12, 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 developed and implemented to reduce worker exposure to asphalt fumes.

Asphalt is a dark brown to black, cementitious, thermoplastic material 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 roofing material used in the United States. However, coal tar is still used in some roofing work, usually to conform to government building specifications that require its use [Freese and Nichols, Inc. 1994].

Most of the asphalt used in the United States is in paving (87%) and roofing (11%) operations. The remaining uses include waterproofing, dampproofing, insulation, and paints [AI 1990a]. Asphalt roofing products and systems include shingles and roll roofing, ply felt, built-up roofing (BUR), polyisobutylene (PIB) single-ply systems, and modified bitumen systems made from straight-run or oxidized asphalts modified with polymers, including styrene-butadiene-
styrene (SBS) and atactic polypropylene (APP).

Approximately 46,000 contractors are in the U.S. roofing business today [NRCA 2000]. The industry consists overwhelmingly of small businesses that specialize primarily in residential roofing. This sector of the roofing industry is characterized by relatively high rates of turnover, both in the contractor population and in the workforce. However, the commercial/industrial segment of the industry generally includes larger firms with comparatively greater commercial longevity and relatively lower rates of worker turnover. These differences are due primarily to the significantly higher capital startup costs and technical sophistication required for commercial/industrial roofing systems. In this sector, where work frequently involves hot asphalt, it is common to find workers with 20 to 30 years of experience in the industry. Some of these workers have been employed by the same contractor throughout their careers. The low-slope commercial/industrial sector accounts for 69% of the industry (measured in revenue dollars), according to the most recent NRCA market survey data [NRCA 2000]. In the low-slope roofing sector (primarily commercial, industrial, and multiunit residential buildings), asphalt BUR systems, modified bitumen membrane systems, and asphalt shingles account for 46% of sales in new construction and 53% of reroofing jobs [NRCA 2000].

Currently, the industry estimates that about 50,000 on-roof workers are exposed to asphalt fumes during approximately 40% of their working hours [AREC 1999].

The four basic grades of roofing asphalt are (1) coating-grade asphalt, an oxidized asphalt used to make shingles and roll roofing; (2) mopping-grade asphalt, an oxidized asphalt that is melted and used in the construction of BUR and modified bitumen systems; (3) modified bitumen-based asphalt, a lightly oxidized or nonoxidized asphalt used in saturated felt plies for the construction of BUR systems and in organic felt shingles or organic roll roofing; and (4) saturant-grade asphalt, a lightly oxidized or nonoxidized asphalt used in saturated felt plies for the construction of BUR systems and in organic felt shingles or roll roofing.

The principal physical 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 246 ?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 1997].

The four types of mopping-grade asphalt are described in Table 2–1. The viscosity of mopping grade asphalts differs among the four types that are produced (see Table 2–1). 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,” may also be a blend of residue 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].

Table 2-1. Types of mopping-grade asphalt

Asphalt type Susceptibility to flow at room temperatures (viscosity) Highest % slope
suitable for use Softening point 
°F °C 
I, dead level Relatively susceptible 2 135–151 57–66 
II, flat Moderately susceptible 4 158–176 70–80 
III, steep Relatively susceptible 25 185–205 85–96 
IV, special steep Relatively susceptible 10 210–225 99–107 

Adapted from ARMA [1996].

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 about the same proportion of saturates* [Corbett 1975; Boduszynski 1981; Moschopedis and Speight 1973]. The process effluent contains water, carbon dioxide, 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 includes ketones, acids, acid anhydrides, and esters) [Campbell and Wright 1966; Petersen et al. 1975; Goppel and Knotnerus 1955]. 

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 that are sealed, adhered, and surfaced with hot mopping asphalt. The systems are used in low-slope commercial roofing.

• Modified bitumen roofing systems are used in low-slope systems with BUR; or they are used by themselves and adhered with hot asphalt, heat, or adhesives to make the waterproof roofing system.

*To determine gross composition, asphalt is frequently fractionated by treatment with heptane or a similar hydrocarbon solvent to precipitate the asphaltenes. This step 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].

2.4.1 Asphalt Shingles and Roll Roofing
Asphalt shingles introduced in the early 1900s account today for about 75% of new construction and re-roofing in steep-slope residential and some commercial roofing applications [NRCA 1996]. 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, both products are typically installed over an underlayment felt that has been impregnated with coating asphalt during manufacture [NRCA 1996].

2.4.2 BUR Systems
BUR systems were introduced in the late 1800s and remain the most popular roofing system for commercial and industrial buildings. These systems account for about 20% of the new and retrofit markets for low-slope roofs [NRCA 2000]. 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, a weatherproofing top layer is applied—either in the form of (1) roll roofing made from organic or inorganic materials or (2) a flood coat† of mopping asphalt (usually Type I).

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].

2.4.3 Modified Bitumen Roofing Systems
Polymer-modified bitumen roofing systems were introduced in the 1970s and today account for about 18% of the new construction market and about 23% of the re-roofing market for low-slope (i.e., primarily commercial and industrial) roofs [NRCA 2000]. Modified bitumen products are of two types: (1) those made primarily with APP and (2) those made primarily with SBS as the polymer modifier.

†Flood coat is the surfacing layer of asphalt into which surfacing aggregate is embedded on an aggregate-surfaced BUR. A flood coat is generally thicker and heavier than a glaze coat and is applied at approximately 45 to 60 lb/100 ft2 (2 to 3 kg/m2). APP membranes are primarily torch-applied—that is, they are made to adhere to an underlying base sheet onto 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 also 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].