So, what is asphalt and where does it come from? The source is petroleum, that dark viscous liquid found deep within the Earth, the product of once living organisms subjected to millennia of intense pressure and heat. It is not an understatement to say that petroleum is vital to modern life. It is of course used to produce the fuel that our cars, trucks, buses, ships and airplanes run on, but petroleum also furnishes the raw materials used to make our plastics, synthetic fibres, medications, agrochemicals and personal care products.
Chemically, petroleum is a complex mixture of thousands of compounds, the vast majority of which are hydrocarbons, meaning that they are composed only of carbon and hydrogen atoms. Refining petroleum is based on separating its components by virtue of differences in boiling point through the process of fractional distillation. The lowest boiling compounds, those that contain the fewest carbon atoms in their framework, like propane and butane, often referred to as “liquefied petroleum gas,” come off first, followed by the components of gasoline, then kerosene, diesel fuel, heating oil and lubricating oil. What is finally left behind is a thick, dark guck that presents a bit of a nomenclature problem. In the technical literature, it is either referred to as bitumen or asphalt which leads to confusion because this is not the asphalt that is used to pave our roads. That asphalt is made by using the residue from petroleum refining to bind together sand, gravel and sometimes other materials such as ground-up rubber tires. It is more correct to refer to the residue as bitumen, and mixtures that are bound together with it as asphalt.
The first uses of bitumen go back at least 6000 years and relied on natural deposits found in Mesopotamia, roughly corresponding to modern-day Iraq. The ancient city of Hit featured springs where natural bitumen rose to the surface. Mesopotamians used it to waterproof their boats. Even the Bible mentions such waterproofing, with Moses’s basket being treated with “pitch,” another term for bitumen, and Noah waterproofing his ark with the material.
Unfortunately, bitumen is loaded with polyaromatic hydrocarbons (PAHs), many of which are carcinogenic. And that’s probably why asphalt workers have a slightly increased risk of lung, stomach, bladder and skin cancer. Indeed, when blood samples are taken from workers, damage to DNA, a hallmark of initiation of cancer, is found to be related to exposure to asphalt. Such exposure can be measured in various ways. Workers can be equipped with pumps on their collars to measure inhaled asphalt fumes, or with adhesive patches on their wrist to measure skin exposure. Surprisingly, levels of polycyclic aromatics in the urine, a measure of the extent to which these compounds are absorbed into the bloodstream, are influenced much more by skin exposure than by inhalation. Eight times more of these carcinogens enter the bloodstream through the skin than through the lungs. Obviously, it is very important for workers to wear gloves and cover all exposed skin surfaces.
Does the presence of carcinogenic compounds in asphalt have any relevance for people who don’t spend their days working with pavement? Well, maybe. Pavement isn’t an inert substance, it wears away. And as it does, polycyclic aromatics get distributed into the environment. Parking lots are an especially good source. Many parking lots also use a sealant to protect the pavement from sun exposure which degrades it, as well as from the destruction wrought by freeze and thaw cycles. These sealants are often made with coal tar, the material left behind when coal is burned and converted into coke for the steel industry. Coal tar is far higher in polycylics than is asphalt. For this reason, some communities have banned coal tar sealants. There are silicone sealants which are more expensive but far more friendly to the environment. In addition to all this, barbecuing, smoking and incinerators also release PAHs, so we shouldn’t be too surprised if some end up in our water and food. How risky is this? Not very. After all, asphalt workers who are exposed to huge doses have only marginally elevated cancer rates. And some epidemiologists debate whether this is due to the asphalt or due to other lifestyle factors.
Something else you may not know about asphalt is that it is the most widely recycled material in North America. Plastic, paper, aluminum and steel are all recycled, but not to the extent of asphalt! If our potholes aren’t repaired, it is not for the lack of this underappreciated material.
