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If you’ve ever smelled that sharp, chemical odor coming from a newly re-roofed building … or noticed fumes during a roofing project on your own home — you’re not imagining things.
Modified bitumen and asphalt roofing materials off-gas volatile organic compounds (VOCs) throughout their lifespan. The smell is a signal worth taking seriously.
This guide breaks down what the science actually says about modified bitumen fumes, what your real health risk is as a homeowner, and what roofing alternatives exist that don’t come with these concerns.
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If your home currently has a modified bitumen or built-up tar roof, here’s what the research tells us in plain terms:
The honest takeaway: modified bitumen fumes aren’t classified as acutely dangerous for homeowners the way they are for roofing workers. But the materials are not benign, and the research makes that clear.
If you’re planning a re-roofing project, the type of roofing system you choose matters — not just for durability and energy savings, but for what it puts into the air around your home.
Most of us who have lived or worked in a building that was re-roofed with tar, or passed by a road-crew installing asphalt are aware of the extremely pungent (smelly) odors emitted by bitumen.
The noxious fumes you smell from tars being applied to roads or roofs are volatile organic compounds, emitted from these substances.
Bitumen, asphalt and tars are based on crude oil waste.
After the gasoline, kerosene, or other more volatile fuels are removed from crude oil, the leftovers can be converted to tar. Many roofs and most roads contain tars; so the question of whether or not these materials are toxic is perhaps an important one.
While researching the subject of asphalt and modified bitumen off-gassing, I found a severe lack of good, scientific, easily palatable materials.
My goal was to find whether or not the fumes emitted by these materials are safe to breath.
Most authoritative articles were very laborious and verbose for those of us who don’t work daily in a laboratory.
I have done my best to complete some solid groundwork and give an authoritative, and hopefully enlightening perspective on the subject of asphalt fumes.
Image: Modified bitumen roofing membranes, manufactured by SBS in China .
Spray polyurethane foam roofing — what Dura-Foam has specialized in since 1981 — takes a fundamentally different approach.
Unlike modified bitumen, which is petroleum-derived and off-gases throughout its lifespan, foam roofing systems are applied as a liquid that expands and cures into a solid, seamless barrier. Once cured, the material is chemically stable and does not continue to off-gas VOCs or PAHs.
Here’s how the two systems compare for homeowners who care about what’s above their heads:
| Modified Bitumen | Foam Roofing | |
|---|---|---|
| Off-gassing VOCs | Yes — throughout lifespan | Minimal — none once cured |
| Installation fumes | Strong / hazardous | Low odor, no open flame required |
| Lifespan | 10–20 years | 20–30+ years with recoating |
| Energy efficiency | Moderate | Superior insulation (R-value) |
| Leak resistance | Seams are vulnerable | Seamless — no joints or seams |
| Environmental impact | Petroleum-based | Lower long-term footprint |
Foam roofing doesn’t just address the off-gassing concern — it solves the underlying problems that bring most homeowners to this page in the first place: aging roofs, energy bills, and leaks.
Although recent advances allow bitumen (or tar) to be made from corn, rice, or molasses, for most of the last 100 years, bitumen was made from the refinement of crude oil or coal.
After more valuable (and more volatile) fuels such as gasoline, kerosene or diesel are removed, the left-over, and thick brown viscous material is known as bitumen.
Image: Solid coal tar pitch must be heated in order to create a waterproof roofing seal. Once coal tar is applied to a roof it is extremely weather resistant. Coal tars have become unpopular due to health hazards, and they have been mostly replaced by modified bitumen.
As far back as 5,000 BC, there are records of humans using ‘pitch’ (a form of tar or bitumen) based on tree resins.
Bitumen is used as an essential binding element in many applications (such as asphalt) because it has excellent adhesive properties and takes an extremely long time to wear away. Bitumen is a semi-solid, although it is often heated to create a more viscous liquid material.
For example, in BUR roofing applications a giant kettle is heated to 200C or more to keep the bitumen in a semi-liquid state.
After the tar is hot-mopped onto the roof, it cools back into a solid form, creating a waterproof seal. For roadway applications, crushed stone, sand, and gravel are mixed with the semi-solid bitumen to create solid Asphalt.
Caulkings based on asphalt are often used to repair leaks in boats and ships; in fact, this is one of the oldest uses of the material (pitch was used in ancient times to waterproof ships).
Most of the roads and many jogging paths throughout the USA are made with Asphalt (a form of bitumen). Bitumen is also a key component in many roofing materials such as asphalt shingles and tar-paper.
Decayed plants, algae, bacteria and other organic matter collected in places, such as the ocean floor or other high pressure areas over 100 million years ago.
Layers of sediment dirt, rock and mud covered and trapped the organic matter forcing anaerobic decay under increasing pressure and heat.
Extremely mature, decayed materials, in oxygen deprived, high pressure, and high heat environments result in fossil fuels, such as coal, crude oil, or natural gas.
Since the whole process takes millions of years, fossil fuels are a limited resource and cannot easily be replaced.
The most refined fossil fuels are also the most unstable and contain the highest levels of energy. Gasoline, a very pure form of fossil fuel (with very high concentrations of stored energy) is highly volatile and will explode when placed near a match.
On the other hand, coal, a much less refined fossil fuel is less volatile, and contains less energy; thus explaining why coal only burns slowly when placed in an open flame.
Tars like modified bitumen and asphalt are based on further processing the sticky brown leftover waste after crude oil is refined. Depending on the application, tars must go through further refinement before they can be used as asphalt or bitumen for applications on roofing, ships or roads.
The long term negative health impacts of fossil fuel burn-off is well documented and widely accepted. Generally, when fuel such as gasoline or diesel is burned, various byproducts are released into the atmosphere such as CO2, heavy metals, and various toxins.
However, there are some fossil fuel based materials, such as Modified Bitumen and Asphalt which most government agencies remain undecided in terms of health impact for the general public; at least when in reference to already constructed roads and roofs. These materials are only considered ‘possible’ carcinogens for the general public.
In contrast, these same materials are known carcinogens for roof workers, or road construction workers.
Just like paint thinner, solvents, or other volatile compounds which we are exposed to in our day to day lives, the level of health risks involved scale up with increased exposure levels.
According to The National Institute for Occupational Safety and Health (NIOSH / CDC) “Studies of roofers [and workers with high exposure to modified bitumen or asphalt] show an excess of lung, bladder, brain, liver, and digestive system cancers”.
The CDC and other agencies maintain very conservative positions and will not officially declare these materials are carcinogenic unless the evidence is overwhelming.
For now, these tars remain ‘suspected’ carcinogens.
Cigarettes provide an excellent example of the governments refusal to accept ‘the writing on the wall’. For many years, studies showed a link between cigarette smoking and lung cancer, yet for decades the government refused to officially recognize this.
Generally, by the time the government is willing to declare any product as harmful, the evidence is absolutely overwhelming and indisputable.
In defense of the government, its important we understand that they will not shut down a multi-billion dollar industry, and cause millions of Americans to lose jobs unless it is absolutely clear that industry is harming consumers and/or the environment.
As a side note, consumers should not completely abdicate their own responsibilities to research purchasing decisions and come to their own conclusions.
Even those who believe the government alone should tell us what is safe and what is not must realize that at least to some extent this is not practical.
There are many sources which claim that modified bitumen is not carcinogenic (cancer causing), while in contrast, coal tar is widely accepted as carcinogenic. Some sources claim that the process of making modified bitumen, which is done at lower temperatures (less than 500C) does not release the same harmful byproducts into the material in contrast to tars made at higher temperatures.
Various tars such as coal tars, lignite tars, wood tars, peat tars and oil shale tars are usually made at temperatures around 700C although there are high temperature variations made at 900 to 1300C.
Tars made at temperatures over 1000C, which have been subsequently tested on animals, are highly carcinogenic.
Tars in general contain known cancer causing agents, and PAH is the worst: “At least 20 different powerful carcinogens have already been detected in tars and pitches.
The most powerful carcinogens from among the polycyclic aromatic hydrocarbons [PAH] have been found in the case of compounds with 5 or more benzene rings” (G. Collin, et al).
Modified bitumen, like other tars contains Polycyclic aromatic hydrocarbons (PAH), although it appears to off-gas substantially less than coal tar. Although some sources claim the substance is completely benign, there is a substantial body of research which says otherwise.
“Most concentrations of individual PAHs in roofing (232°C or 316°C) and all concentrations in paving (163°C, except for one sample at 221°C) asphalts, whole or fumes [contained] 10 ppm and 2 ppm, respectively.” (ASPHALT FUMES LITERATURE REVIEW by NIOSH)
Studies show that PAH is present in modified bitumen, just as in other tars. The key difference is that modified bitumen contains substantially less mutagenic VOCs in contrast to coal tar.
However, to say it is completely benign is to ignore the fact that polycyclic aromatic hydrocarbons are found in modified bitumen.
Studies performed by Niemeier et al.[1988] and Sivak et al. [1989] showed that incidence of cancer in coal tar pitch applied to the skin of mice was 10 to 100 times more frequent than with asphalt based tars.
In other words, various forms of tar have been tested and there is already clear evidence that modified bitumen has the same, but admittedly lower health risks verse other tars.
Although modified bitumen is substantially less carcinogenic than coal tar, it still contains polycyclic aromatic hydrocarbons (PAH). Several studies have reported carcinogenicity in mice following applications of laboratory-generated asphalt roofing fume condensates (Thayer et al., 1981; Niemeier et al., 1988; Sivak et al., 1989, 1997).
Both modified bitumen and asphalt off-gas substantially less VOCs than coal tar; enough so that the government agencies such as the CDC remain undecided and simply list these materials as ‘possible carcinogens’.
This is despite studies which show otherwise: “Experimental studies using animal and in vitro models indicate that condensates from asphalt fumes are genotoxic and can promote skin tumorigenesis [carcinogenic]” (Ma-C, et al.).
Studies have shown that rats exposed to asphalt fumes through their skin, lungs, and stomach developed cancerous tumors in each of these respective areas.
Carcinogenesis = Carcinogenesis (the creation of cancer), is the process by which normal cells are transformed into cancer cells.
Tumorigenesis = (oncology) Production of a new tumor or tumors
As stated earlier, for roof and road workers, the evidence is overwhelming; these materials are highly hazardous with long term exposures. Complications such as tumors, liver and lung problems are well documented for those who work with these hazardous materials daily.
Although studies with laboratory animals decisively show that both asphalt and modified bitumen cause cancerous tumors at high dosage levels over short time periods, the government remains unconvinced the risk levels are high enough to merit banning these materials.
There are dissenting studies as well, which claim asphalt does not cause cancer (consider there were also studies that claimed cigarette smoking was perfectly safe long term, which we know to be completely false).
In one study, roofing asphalt applied dermally to mice was not decisively carcinogenic (Emmett et al., 1981). However, we must question the methodology used since this is an exception to the rule.
For example, several studies found asphalt fumes created carcinogenic responses in mice with just 10 days or less of heavy exposure to asphalt fumes: “Asphalt fume as well as the vapor and aerosol components of asphalt were determined to be immunosuppressive following respiratory and systemic exposure. (3.5 hours per day for up to 10 days)” (Diotte-NM, et al).
Further studies showed that asphalt fumes can create tumorigenesis and carcinogenesis: “These results demonstrate that exposure to road paving-like asphalt fumes is immunosuppressive through systemic, respiratory, and dermal routes of exposure in a murine model and raise concerns regarding the potential for adverse immunological effects.” (Stacey E. Anderson, et al.)
There are many other studies, perhaps thousands or more which show similar results. Here is yet another example: “The raw roofing asphalt and neat asphalt fumes induced carcinomas (local skin cancers) in 3 of 30 and 20 of 30 C3H/HeJ mice, respectively.” (Wess-JA, et al).
Generally, the manufacturers and other purveyors of modified bitumen, asphalt, and other related substances claim what they use is ‘different’ from the materials used in the many studies performed.
Their attempts to discredit the large existing body of research which indicates these materials are carcinogenic is simply by using blanket claims that the studies used the wrong materials.
I find this hard to believe, since the vast majority of bitumens in use today are based on the exact same material: refined crude oil.
While there is truth in the statement that modified bitumen or asphalt emit less toxic fumes because they are refined at lower temperatures, to claim they are completely safe is simply not true.
Some people who have touched (skin absorption) or breathed mixtures of PAHs emitted from modified bitumen, and asphalt have developed various forms of cancer.
The same bitumen substances caused cancer in laboratory animals by breathing (lung cancer), ingesting them with food (stomach cancer), or by absorption through the skin (skin cancer).
Modified bitumen, asphalt, coal tars and all related substances are made from refined crude oil and all emit substantial levels of polycyclic aromatic hydrocarbons (PAH), which are known carcinogens.
Roofers and road workers both have a high risk of contracting cancer or other health problems when exposed to the PAHs emitted from asphalts and tars daily.
For consumers, the risk levels are unclear, although it is safe to say the fumes emitted by off-gassing tars and bitumens are truly not good for your health.
If the research on this page raised concerns about your current roofing system — or if you’re already dealing with leaks, high energy bills, or an aging roof — we’re here to help you make the right call.
Dura-Foam has been installing foam roofing systems in the Bay Area since 1981. We’ll walk you through your options with no pressure, just an honest assessment of what your roof needs.
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