Asphalt, often referred to as bitumen, is a dark, petroleum-based material primarily used as a binder in road paving and roofing applications. It is a highly viscous liquid or semi-solid residue created during the distillation of crude oil. Public concern often arises about the material’s potential to cause harm due to its complex chemical makeup. The risk to human health depends heavily on the material’s temperature and the extent of human contact. This article examines the conditions under which asphalt poses a risk, detailing its chemical nature, exposure pathways, and documented health consequences.
Primary Chemical Composition and Volatile Release
Asphalt is a complex mixture composed mainly of hydrocarbons, along with smaller amounts of nitrogen, sulfur, and oxygen. When the material is cold and cured, such as on a finished road surface, it is considered chemically stable and largely inert. Toxicity emerges when asphalt is heated for application, typically between \(150^\circ\text{C}\) and \(190^\circ\text{C}\) for hot-mix asphalt (HMA) paving.
This intense heating releases asphalt fumes—a complex mixture of vapors, aerosols, and particulate matter. These fumes are the primary source of concern because they contain Polycyclic Aromatic Hydrocarbons (PAHs) and Volatile Organic Compounds (VOCs). PAHs are known irritants and potential carcinogens. The concentration of these toxic constituents can increase significantly with temperature, sometimes rising three to seven times for every \(20^\circ\text{C}\) increase.
Routes of Human Exposure
Potential human exposure to asphalt primarily occurs through occupational activities. Workers involved in road paving, roofing, and asphalt manufacturing are the most likely to encounter harmful levels of exposure. The most significant pathway is the inhalation of asphalt fumes and aerosols, particularly by paver and screed operators working directly above the hot material.
Dermal contact is another route. Contact with hot material can cause severe thermal burns, and solvents used in cut-back asphalts can be absorbed through the skin, potentially affecting various organs. Accidental ingestion, though rare, is also a possible exposure route. For the general public, exposure is typically limited to transient inhalation near construction sites or contact with cured asphalt, presenting a significantly lower risk than occupational exposure.
Documented Health Impacts
Exposure to asphalt fumes is associated with acute and chronic health consequences. Acute exposure, often experienced by workers during hot-mix applications, can cause irritation of the eyes, nose, and throat, headaches, dizziness, and nausea. These symptoms resolve quickly once the individual is removed from the source of the fumes.
Chronic exposure, involving repeated contact over many years, includes respiratory damage. Workers report lower respiratory tract effects such as coughing, wheezing, and decreased lung function, which can lead to conditions like bronchitis. Long-term skin contact with asphalt and its byproducts may lead to irritation, dermatitis, and changes in skin pigmentation, especially when combined with sunlight exposure.
Regarding cancer risk, the International Agency for Research on Cancer (IARC) classifies occupational exposure to straight-run asphalt emissions (road paving) as Group 2B, meaning “possibly carcinogenic to humans.” This classification is based on limited evidence in humans and less than sufficient evidence in experimental animals. However, IARC classifies oxidized asphalt emissions (roofing) as Group 2A, or “probably carcinogenic to humans,” citing stronger evidence from studies on roofing workers and experimental animals. The overall carcinogenic risk is highly dependent on the concentration and duration of exposure, with the greatest risk confined to occupational settings involving hot asphalt fumes.
Regulatory Status and Worker Safety Measures
Regulatory bodies manage asphalt exposure risk by setting occupational health and safety standards. In the United States, the Occupational Safety and Health Administration (OSHA) addresses the chemical components of asphalt fumes through general industry standards, although there is no specific permissible exposure limit (PEL) exclusively for asphalt fumes. The American Conference of Governmental Industrial Hygienists (ACGIH) recommends a Threshold Limit Value (TLV) of \(0.5 \text{ mg}/\text{m}^3\) as an eight-hour time-weighted average.
Employers must implement safety protocols to minimize worker contact with the material and its fumes. Engineering controls, such as adequate ventilation and working upwind of the application area, are the preferred methods for controlling exposure. When engineering controls are insufficient, workers must use personal protective equipment (PPE).
Personal Protective Equipment (PPE)
- Thermally insulated gloves to protect against burns and chemical absorption.
- Safety glasses and face shields.
- Respirators to prevent inhalation of fumes and vapors.
Good hygiene practices, such as frequent hand washing and prompt changing of contaminated clothing, are also mandated to reduce dermal exposure. These measures target the high-exposure environments of construction and manufacturing, where the health risk is concentrated.