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Study finds automobile tires are a potential source of carcinogenic dibenzopyrenes to the environment

A new study by researchers from Stockholm University concludes that automobile tires may be a potential previously unknown source of carcinogenic dibenzopyrenes—a type of high molecular weight polycyclic aromatic hydrocarbon (PAH)—to the environment. Their findings are published in the ACS journal Environmental Science & Technology.

Automobile tire rubber in general consists of approximately 40−60% rubber polymers; 20−35% reinforcing agents/fillers such as carbon black or silica; and 15−20% mineral oils such as extender oils/ softeners. Precise compositions vary depending upon where in the tire the rubber is used. Highly aromatic (HA) oils used as extender oils or softeners in the tire-manufacturing process have concentrations of PAHs ranging between 10 to 30%.

PAHs are a class of organic compounds produced by incomplete combustion or high-pressure processes—i.e., when complex organic substances are exposed to high temperatures or pressures. There are approximately 100 different known PAHs in air, soil, food, and water, although the number of chemicals that comprise the PAH class is not fully known.

PAHs primarily occur in complex mixtures; many are classified as carcinogenic/mutagenic environmental toxins with the potential to cause negative long-term effects both in humans and in the aquatic environment.

Owing to the potential hazard of HA oil usage, their use has been regulated by EU Directive 2005/69/EC, which forbids HA oils in rubber manufacturing as a measure to phase out these PAH-rich oils and replace them with low-PAH alternatives such as mild extracted solvates (MES) and treated distillate aromatic extracts (TDAEs). Any new tire or tire tread used for retreading manufactured after first January 2010 may not contain any extender oil with more than 1 mg kg benzo[a]pyrene (B[a]P) or more than 10 mg kg−1 of the sum of the eight PAHs: B[a]P, benzo[e]pyrene (B[e]P), benzo[a]anthracene (B[a]A), chrysene (CHR), benzo[b]-fluoranthene (B[b]F), benzo[j]fluoranthene (B[j]F), benzo-[k]fluoranthene (B[k]F), and dibenz[a,h]anthracene (DB[a,h]- A).

Tires that were manufactured prior to this date are not covered by these threshold values and may still be sold on the market. It has been estimated that the replacement of HA-oils will reduce the PAH emission originating from tire-tread-wear by 98%. Another material known to contain PAHs used in tire manufacturing is carbon black. Carbon black is a form of elemental carbon used as reinforcement material in the tire rubber to give the tire its desired properties in abrasion resistance and tensile strength.

...There has not so far, to our knowledge, been any data reported on the mutagenic/carcinogenic dibenzopyrenes; dibenzo[a,l]pyrene (DB[a,l]P), dibenzo[a,i]pyrene (DB[a,i]P) and dibenzo[a,h]pyrene (DB[a,h]P) in automobile tires, and the literature regarding B[a]P is scarce.

...Dibenzopyrene isomers have been suggested to be even more mutagenic/carcinogenic than B[a]P, which might lead to an underestimation of the carcinogenicity of a PAH-containing matrix if solely the B[a]P concentration is taken into account. Data available of dibenzopyrenes in other urban emission sources are diesel and gasoline particulate emissions. The reported concentrations vary, however, depending on vehicle and driving conditions.

—Sadiktsis et al.

Non-exhaust traffic sources of particles
New technology and regulatory requirements have resulted in a significant decrease in particles from tail pipe emissions, the authors note. However, traffic-related non-exhaust particles remain largely unchanged, making these emissions increasingly significant contributors to the environmental burden.
These non-exhaust vehicle sources include: brake lining, tire and road-surface wear, and resuspension of road dust. Using emission-factor models, 50−85% of traffic-generated PM10 has been attributed to non-exhaust sources, they said.
It has been suggested that airborne tire-debris particles have a bimodal size- distribution that could constitute as much as >90% by weight of particles with an aerodynamic diameter below 1 μm, with the remaining part made up by particles with an aerodynamic diameter larger than 10 μm. This would mean that a significant fraction could be deposited deep into the respiratory system when inhaled and that tire-tread-wear particles could be transported far from their sources of emission.
—Sadiktsis et al.

In addition to the release of PAH from tread-wear, the release of PAHs from stockpiled scrap tires, PAH emissions from pyrolysis of scrap tires, or leaching of PAHs from recycled tire rubber material make tires a source of environmental pollution of PAHs throughout their entire lifecycle, the researchers note.

The goal of the Stockhold University study was to determine the level of dibenzopyrenes in tires in order to evaluate the importance of automobile tires as a source of dibenzopyrenes to the environment. The researchers analyzed 8 different types of tires for 15 high molecular weight (HMW) polycyclic aromatic hydrocarbons (PAH), using pressurized fluid extraction. The variability of the PAH concentrations determined between different tires was large—a factor of 22.6 between the lowest and the highest. The relative abundance of the analytes was quite similar regardless of tire.

Almost all (92.3%) of the total extractable PAH content was attributed to five PAHs:

  • benzo[ghi]perylene
  • coronene
  • indeno[1,2,3-cd]pyrene
  • benzo[e]pyrene
  • benzo[a]pyrene

The mean concentration of the four dibenzopyrene isomers—dibenzo[a,l]pyrene, dibenzo[a,e]pyrene, dibenzo[a,i]pyrene, and dibenzo[a,h]pyrene—for all tires was 0.53 μg g−1, corresponding to less than 2% of the total PAH content (range: 0.4−1.9%).

Using the Swedish Chemical Inspectorate estimate for tire-tread-wear emission of 10 × 106 kg annually (valid for 1994) in Sweden and the mean values, calculated from all tires, of B[a]P and dibenzopyrenes, the researchers calculated an annual emission of 49 kg B[a]P and 5.3 kg dibenzopyrenes, respectively. Using the median values, the corresponding annual emissions are estimated to be 50 kg B[a]P and 3.6 kg dibenzopyrenes, respectively.

This suggests that tire-tread-wear could be a larger contributor of B[a]P to the environment than diesel vehicle exhaust and residential oil heating. An annual emission of dibenzopyrenes from these sources is currently not available in the literature. However, the total mean dibenzopyrene content measured in the tires is low compared to reported concentrations in other urban sources i.e., diesel and gasoline particulate exhausts. The mean sum of the dibenzopyrene content in diesel exhaust particles varies between a factor of 6.6, and 0.90−15.6 (mean of two vehicles, urban driving cycle and motorway driving cycle) higher than the determined content in tires. Gasoline vehicle exhaust particles contain a factor of 2.3−285 (mean of two vehicles, urban driving cycle, and motorway driving cycle), more dibenzopyrenes than tires.

But it should be noted that the particle size of the emissions differ drastically between the sources and this is crucial for the impact on the environment, population exposure, and potential health effects. Particles emitted from incomplete combustion are <1 μm, whereas the mass particle size distribution of tire-tread-wear may be much larger, but is much less known.

The tires that were analyzed in this present study were all manufactured prior to the ban of HA-oils in tire rubber, which makes it reasonable to anticipate that there will be a decline of PAH emissions originating from tire-tread-wear in the future. It is however necessary to investigate what significance carbon black has to the PAH content in a tire. By replacing HA-oils with alternative extender oils, carbon black may well become a relatively more important source to PAHs than the extender oil used in the tire manufacturing.

Because of the substantial variation in the dibenzopyrene content across the analyzed tires, further research is necessary to determine the distribution of tires in a real-world vehicle fleet, taking into consideration the market share of tires from different manufacturers and the age of the tires in operation. It is also necessary to fill the gaps in our knowledge regarding tire-particle generation and distribution in the environment in order to assess the implications to human health and our environment.

—Sadiktsis et al.

Resources

  • Ioannis Sadiktsis, Christoffer Bergvall, Christer Johansson, and Roger Westerholm (2012) Automobile Tires—A Potential Source of Highly Carcinogenic Dibenzopyrenes to the Environment. Environmental Science & Technology. doi: 10.1021/es204257d

Comments

HarveyD

This is just one more harmful byproduct from our gas guzzlers and large highway trucks. However, this is a very well known source of harmful pollution and many EU countries have already moved schools 2+Km away from major highways for that very same reason. We often do the opposite to ease access for large school buses and parents driving large vehicles. etc.

The news didn't come out in North America because of the political influence of the Big 3, Tire makers and (R) groups.

Peter_XX

@HarveyD
Sad enough, we see the same development in the EU as in the USA; just a couple of years later. The health of our children suffers not only from pollution but also from lack of exercise, since parents give them a ride to school.

The problem of PAH in tires was noted in Sweden more than 10 years ago. Calculations were made but no dedicated research was conducted to actually quantify the problem. Thus, it is nice to see that the group at Stockholm Uni. finally could receive funding to start to investigate this problem (there is obviously more work to do in this field…). I do not believe in the theory of conspiracy that the auto industry would block this research in the USA. It may just be lack of imagination that has prevented US research groups from entering this field.

A D

Stop doing donut tire burnouts after races and stop doing burnouts to heat-up the tires before 1/4 mile acceleration races. Also put low resistance hard tires in nascar and formula one races so there will be less tire wear.

Peter_XX

Burnouts? Well, maybe we could do this on new tires in the EU. Note that we have the EU Directive 2005/69/EC that bans HA oils. I usually follow emission regulations in the USA but I do not have a clue if you have something similar in the USA for tires. I believe not… Usually (as for e.g., particle number emissions), when USA finds that EU leads in the environmental field, the “not invented here” syndrome appears to apply. I frankly have the feeling that US EPA do whatever they can not to copy any regulation from the EU, no matter how big the improvement for environment and health would be.

Note that HEVs or EVs would not help in cleaning up these emissions. Maybe they could have lower power (less burnouts) but higher tire wear due to greater mass (than conventional drivetrains).

Engineer-Poet

Without knowing the particle size and fate in the environment, we don't know whether to rate this as a major, minor, or non-issue.  If these substances aren't turning up in large quantities in runoff water, they're probably well down on the list of urgent concerns.

SimonDM

I don't see why people don't give more funds to the development of polyurethane tires, specifically improving traction properties. No more filth in those tires; half the rolling resistance and no more flats.

SJC

If you have ever walked into a tire shop where they have stacks of tires on the floor, you can smell what they are made of.

I am a believer in alternative tires, the pneumatic tire has been around 100 years and has served its purpose. It is time to think outside the box and save some energy and lives.

Peter_XX

@Engineer-Poet
What’s wrong with trying to find out if this is an environmental/health problem or not? Large quantities or not, many of these substances cause cancer or mutations (which can cause cancer), so there is no “safe” threshold level. Are you paid by the tire lobby to defend their position?

Engineer-Poet

Where did I say I didn't want to find out?

Go fight your strawman on some other website.

HarveyD

Peter XX...studies carried out in France arrived at the same conclusions many years ago.

Peter_XX

@Engineer-Poet
Big words for a poet.


HarveyD
As I said, this problem was noted in Sweden many years ago. Why don´t you recognize this problem in the USA?

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