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.

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