Asbestos brake emissions

Our plain-English summary of Asbestos brake emissions, by R.L. Williams, J.L. Muhlbaier (Environmental Research, 1982).

The study

• Authors: R.L. Williams, J.L. Muhlbaier
• Journal: Environmental Research
• Published: 1982;29(1):70-82
• Cited by: 31 articles, as counted by NIH iCite
• Read it: PubMed · Publisher
• In J-M's library: Johns-Manville's company medical library held a copy (J-M no. 004981).

Abstract

An enclosed brake-testing machine was built to determine the importance of asbestos emissions from automobile brakes. Higher emissions resulted from disc brakes than from drum brakes due to the smaller surface area contacting the wheel in the former case. Median emissions from one disc brake were 3.3 mg particulate/stop (55% airborne. 12% sedimentary, 34% brake-entrained), 0.6 mg C/stop as hydrocarbons, 0.7 mg C/stop as CO2, and 0.1 mg C/stop as CO. Higher emissions were observed with increases in velocity and deceleration rate. The mass median aerodynamic diameter of the emitted particles was 3.1 µm. Although the original brake material was about 50% asbestos, the emitted particles averaged only 0.029% asbestos. Therefore, it appears that over 99.9% of the mass of original asbestos fibers was broken down into nonfibrous magnesium silicates. In addition, asbestos fibers which are on the order of millimeters in length in the brake had a median length of 0.5 µm in the emitted debris. Based on the mass emission results and a stopping rate of 1.2 stops/km, it can be estimated that 2.6 µg of asbestos/km/vehicle is emitted into the air, 0.76 µg/km settles to the roadway, and 2.2 µg/km is entrained in the wheel. Although brake emissions are responsible for a minor fraction of ambient asbestos levels, there may be a noticeable increase in asbestos levels near high-braking areas such as tollbooths.

PubMed has no abstract for this study. This is the authors' abstract, copied from the article.

In plain English

This laboratory study measured what comes off automobile brakes during braking. The authors built an enclosed brake-testing machine. They ran new and used disc brakes and new drum brakes through repeated stops at different speeds and deceleration rates. The friction material was about 60% asbestos by weight. They collected airborne dust in all tests, but settled dust and dust trapped in the wheel only in the disc-brake tests, and examined filters for asbestos fibers using optical and transmission electron microscopy. Disc brakes gave off more dust than drum brakes. Median emissions from one disc brake were 3.3 mg of particles per stop. Emissions rose with speed and, much less, with deceleration rate. Airborne dust averaged 0.027% asbestos from disc brakes and 0.031% from drum brakes, and the airborne asbestos fibers had a median length of 0.5 micrometers. Assuming the brakes started at 50% asbestos, the authors calculated that over 99.9% of the asbestos was broken down into nonfibrous material. They estimated 2.6 micrograms of airborne asbestos per kilometer per vehicle, a minor share of ambient asbestos. They also estimated that braking could account for a significant fraction of the asbestos measured near a Connecticut tollbooth.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory experiment using an enclosed brake dynamometer, with mass balance of emissions and electron microscope analysis of the dust, plus calculated estimates of environmental contribution
• Who was studied: Automobile brake assemblies: new original-equipment disc-brake sets and drum linings for standard size vehicles, plus two used disc-brake sets (8000 km and 60,000 km of use)
• Exposure: Brake friction material of about 60% asbestos, 22% inorganic modifiers and 18% organic binder by weight. Tested at simulated speeds of 40, 64 and 88 km/hr and deceleration rates of 1.2, 1.8, 2.5 and 4.9 m/sec squared. The tested material's fiber type is not stated; the introduction says brake linings in general are mainly chrysotile asbestos.
• Main results:
  • Emissions were generally higher from disc brakes than from drum brakes under the same conditions
  • Median emissions from one disc brake were 3.3 mg particulate per stop, 0.6 mg carbon per stop as hydrocarbons, 0.7 mg carbon per stop as CO2, and 0.1 mg carbon per stop as CO
  • Disc-brake particles were approximately 55% airborne, 12% sedimentary and 34% brake-entrained
  • At a deceleration rate of 1.85 m/sec squared, doubling velocity caused a 12-fold increase in particulate mass, a 31-fold increase in hydrocarbons, a 31-fold increase in CO2 and a 68-fold increase in CO (disc brake)
  • Quadrupling the deceleration rate at 64 km/hr increased particle emissions only 1.4 times and hydrocarbon emissions 1.7 times
  • The mass median aerodynamic diameter of airborne particles averaged 3.1 micrometers in 10 impactor tests, with submicrometer mass only 10% of the total
  • The ratio of small fibers under 5 micrometers long (seen by electron microscope) to large fibers over 5 micrometers long (seen by optical microscope) was about 460 for disc brakes and 230 for drum brakes. The few large fibers contributed almost twice as much mass as all the small fibers.
  • Airborne dust from disc-brake tests ranged from 0.00095 to 0.18% asbestos, average 0.027% (23 filters analyzed)
  • Airborne dust from drum-brake tests ranged from 0.0020 to 0.19% asbestos, average 0.031% (24 filters analyzed)
  • There was no significant difference in asbestos fraction between drum and disc brakes, and no significant trend in asbestos content with deceleration rate or velocity. Total asbestos emissions still rose with speed and deceleration because total particle emissions rose.
  • Assuming an initial asbestos content of 50% and a final content of 0.029%, over 99.9% of the asbestos was broken down to nonfibrous material during braking
  • Wheel-entrained and sedimentary particles had a slightly higher asbestos content (0.04%) than airborne particles
  • The median length of airborne asbestos fibers was 0.5 micrometers, compared with fibers on the order of millimeters in the brakes
  • Estimated total asbestos emission per vehicle was 5.5 micrograms per km: 2.6 airborne, 0.76 settling to the roadway and 2.2 entrained in the wheel. This assumed four brakes per vehicle and 1.2 stops per km, a stopping rate taken from Jacko and DuCharme.
  • The authors estimated nationwide airborne asbestos emissions from autos at 4.7 metric tons per year, approximately 0.2% of total asbestos emissions to air
  • The authors predicted an urban asbestos concentration from braking of 0.063 ng/m3, 0.06 to 0.6% of measured urban asbestos concentrations of 10 to 100 ng/m3
  • For a tollbooth with 2300 cars per hour, the authors estimated a maximum asbestos concentration from braking of 19 ng/m3, compared with 25 ng/m3 measured near a Connecticut tollbooth
• The authors' conclusion: The authors concluded that brake emissions are responsible for a minor fraction of ambient asbestos levels, but that there may be a noticeable increase in asbestos levels near high-braking areas such as tollbooths
• Limitations the authors state:
  • The test schedule may have overestimated brake-entrained material because the brake was cleaned out after each test
  • Sedimentary and entrained dust were collected only for disc brakes, because material was lost when drum brakes were disassembled
  • Asbestos results varied widely within a test condition because only 0, 1 or 2 large fibers were typically seen, yet they greatly affected the calculated mass
  • Repeatability between the two electron microscope grids averaged 32%
  • The urban estimate assumed that leaded gasoline is the main source of atmospheric lead and that lead particles and asbestos fibers behave the same in the environment

Topics

Medical terms in this study, explained in our Lexicon: Silicate.

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