Airborne brake wear debris: size distributions, composition, and a comparison of dynamometer and vehicle tests

Our plain-English summary of Airborne brake wear debris: size distributions, composition, and a comparison of dynamometer and vehicle tests, by P.G. Sanders, N. Xu, T.M. Dalka, et al (Environmental Science & Technology, 2003).

The study

• Authors: P.G. Sanders, N. Xu, T.M. Dalka, M.M. Maricq
• Journal: Environmental Science & Technology
• Published: 2003;37(18):4060-9
• Cited by: 116 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Particle size distributions of light-duty vehicle brake wear debris are reported with careful attention paid to avoid sampling biases. Electrical low-pressure impactor and micro-orifice uniform deposit impactor measurements yield consistent size distributions, and the net particulate matter mass from each method is in good agreement with gravimetric filter measurements. The mass mean diameter of wear debris from braking events representative of urban driving is 6 microm, and the number-weighted mean is 1-2 microm for three currently used classes of lining materials: low metallic, semimetallic, and non-asbestos organic (NAO). In contrast, the wear rates are very material dependent, both in number and mass of particles, with 3-4 times higher emissions observed from the low metallic linings as compared to the semimetallic and NAO linings. Wind tunnel and test track measurements demonstrate the appearance of micron size particles that correlate with braking events, with approximately 50% of the wear debris being airborne for the test vehicle in this study. Elemental analysis of the wear debris reveals a consistent presence of the elements Fe, Cu, and Ba in both dynamometer and test track samples.

In plain English

This laboratory study measured the airborne dust given off by car and truck brakes. It tested three lining types in current use: low metallic, semimetallic and non-asbestos organic (NAO). No asbestos-containing linings were tested. The article describes NAO linings as descendants of asbestos formulations that substitute potassium titanate fibers. Brakes were run on a laboratory brake dynamometer through 24 stops typical of urban driving and through a harsh 10-stop test. A car was also tested in a wind tunnel and on a test track. In the urban stops, debris mass peaked at a particle size of about 6 micrometers for all three linings. Low metallic linings produced about 8 mg per stop per brake, against about 2 mg for the other two. In the harsh test, debris rose from about 3 mg per stop per brake to over 3 g per stop by the last two stops. About 50% of brake wear escaped the wind-tunnel car. The authors reported that emissions varied by up to a factor of 4 between lining types.

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 on a brake dynamometer, with comparison measurements on a vehicle in a wind tunnel and on a test track; particle size, mass and elemental composition were measured (no human or health outcomes)
• Who was studied: Three brake lining/vehicle combinations representing current light-duty vehicles (low metallic/mid-size car, semimetallic/full-size truck, NAO/full-size car), tested at the Ford Motor Co. Research Laboratory, Dearborn, Michigan; a full-size car was used for wind tunnel tests, and a vehicle was driven on the Dearborn Proving Grounds test track and on public roads
• Exposure: Wear debris from low metallic, semimetallic and non-asbestos organic (NAO) brake linings with cast iron rotors; the NAO linings substitute potassium titanate fibers for asbestos. Braking was a 24-stop urban driving program, a harsh 10-stop Auto Motor und Sport (AMS) test, and a series of 1.8 m/s2 stops from 96 km/h
• Main results:
  • In the urban driving stops, the mass of wear debris peaked at a particle size of about 6 micrometers for all three lining types
  • In the urban driving stops, debris mass fell 4-fold, from about 8 mg per stop per brake for low metallic linings to about 2 mg per stop per brake for semimetallic and NAO linings
  • Low metallic linings generated 2 to 3 times the number of wear particles of semimetallic and NAO linings
  • In the harsh AMS test, debris mass rose from about 3 mg per stop per brake in the first few stops to over 3 g per stop in the final two stops, and particle numbers were dominated by particles under 0.3 micrometers
  • In the wind tunnel, about 21 mg per stop per brake of wear debris was measured behind the full-size car with low metallic linings, which the authors estimated as about 50% of brake wear escaping the vehicle
  • On the test track, micron-size particle emissions were observed and correlated with braking events
  • The elements found in highest concentration in the wear debris were iron (Fe), copper (Cu), silicon (Si), barium (Ba), potassium (K) and titanium (Ti)
• The authors' conclusion: The authors concluded that their results differed from the EPA MOBILE6 brake emission description, including that emission levels from different brake formulations varied by up to a factor of 4, that particle composition varied with brake type, and that the size distribution differed from the model's assumption
• Limitations the authors state:
  • The airborne fractions are rough estimates; actual values depend on vehicle and wheel design and operating conditions
  • The wind tunnel tests differed from real driving because the floor was stationary relative to the vehicle and the wind speed was held constant
  • Test track measurements were affected by non-isokinetic sampling and transport losses, which lowered the measured mean particle size to about 3 micrometers
  • Roadway samples were complicated by re-entrained road dust
  • The elemental analysis for NAO linings may be less accurate because they produced the least wear debris
  • Organic material was not measured directly

Topics

Medical terms in this study, explained in our Lexicon: Selection Bias, Elemental Analysis.

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