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New Study Unseen Emissions From Vehicle Brakes
Created on 2025.10.26
A recent study reveals that vehicle brake wear is a significant and under-explored source of non-exhaust traffic emissions, releasing a wide array of gaseous pollutants. Research investigated the real-time emissions of Volatile Organic Compounds (VOCs) from different brake materials, establishing a direct link between these gases and the creation of hazardous ultrafine particles.
Key Highlights· Nonasbestos Organic (NAO)
brake pads, commonly used in the United States, were found to emit significantly higher concentrations of Volatile Organic Compounds (VOCs) than the Low-Metallic Copper-Free (LMCF) pads typical in the European market.
· The study successfully identified over 200 different chemical sum formulas from the emissions of both brake pad types.
· A distinct increase in gaseous emissions was consistently observed just before the formation of ultrafine particles, supporting the hypothesis that these harmful particles nucleate from the emitted VOCs.
· Significant emissions were recorded under conditions simulating normal urban driving, challenging the common perception that such pollutants are only released during extreme braking events like a hill descent.
A Tale of Two Brake Pads
In a controlled laboratory setting using a pin-on-a-disc tribometer, researchers compared the emissions from two commercially prevalent brake materials: NAO and LMCF. The results demonstrated a major difference in VOC emissions between the two. Under identical mechanical stress conditions, the NAO material consistently emitted higher concentrations of VOCs than the LMCF material.
The chemical makeup of the emissions also varied. A key finding was that siloxanes, a group of silicon-based compounds, were a characteristic emission of NAO brake pads, particularly under high stress, but were almost entirely absent in LMCF emissions. While the overall range of most other VOCs was similar between the two pad types, this discovery makes siloxanes a potential tracer for NAO brake wear.
The Link Between Gases and Particles
Perhaps the most critical finding is the direct relationship between gaseous emissions and the subsequent formation of ultrafine particles, which are particles smaller than 100 nanometers. The study is the first to definitively show that the increase in gas-phase emissions precedes the formation of ultrafine particles.
During experiments, a surge in VOC concentrations consistently occurred as brake disc temperatures rose, peaking just before the detection of nanoparticles. This observation strongly supports the theory that the hazardous ultrafine particles emitted from brake wear are formed through the nucleation of these VOCs.
Environmental and Health Implications
This research raises important questions about the environmental impact of brake materials often marketed as “friendly” alternatives. The study found that NAO pads, despite a “green” reputation, emit more gaseous pollutants compared to LMCF pads. Many of these identified VOCs, such as benzene and formaldehyde, are known to be toxic to human health, casting doubt on the net benefits of these materials.
Furthermore, these emissions are not confined to extreme braking scenarios. The experiments showed significant VOC release at temperatures and speeds that correspond to typical urban driving. This suggests that mitigating gaseous emissions from brake pads could be a crucial strategy for reducing the formation of harmful ultrafine particles in cities and that emission factors measured in labs may not accurately reflect complex real-world conditions.
A recent study indicates that vehicle brake wear is an important and underexplored source of non-exhaust traffic emissions, releasing various gaseous pollutants. The research investigated the real-time emissions of volatile organic compounds (VOCs) from different brake materials, thereby establishing a direct link between these gases and the generation of harmful ultrafine particles.
Main Highlights · Research has found that the commonly used non-asbestos organic (NAO) in the United States
Brake padsThe concentration of volatile organic compounds (VOCs) emitted is significantly higher than that of typical low-metallic copper-free (LMCF) brake pads in the European market.
The study successfully identified over 200 different chemicals and formulas from the emissions of two types of brake pads.
Before the formation of ultrafine particles, a significant increase in gas emissions has been observed, supporting the hypothesis that these harmful particles nucleate from the emitted volatile organic compounds.
A large amount of emissions was recorded under conditions simulating normal city driving, challenging the common view that such pollutants are only released during extreme braking events like downhill driving.
The Story of Two Brake Pads
In a controlled laboratory environment using pin-type friction testers, researchers compared the emissions of two commercially popular brake materials: NAO and LMCF. The results indicated significant differences in VOC emissions between the two. Under the same mechanical stress conditions, NAO material consistently emitted higher concentrations of VOCs than LMCF material.
The chemical composition of emissions varies. A key finding is that siloxanes, a group of silicon-based compounds, are characteristic emissions of NAO brake pads, especially under high stress, but are almost completely absent in LMCF emissions. While the overall range of most other volatile organic compounds is similar between the two types of brake pads, this finding makes siloxanes a potential tracer for NAO brake wear.
The connection between gases and particles
Perhaps the most critical finding is the direct relationship between gas emissions and the subsequently formed ultrafine particles (particles smaller than 100 nanometers). This study clearly indicates for the first time that the increase in gas-phase emissions precedes the formation of ultrafine particles.
During the experiment, as the temperature of the brake disc increased, the VOC concentration continued to surge, peaking before the detection of nanoparticles. This observation strongly supports the theory that the harmful ultrafine particles emitted from brake wear are formed through the nucleation of these volatile organic compounds.
Environmental and Health Impacts
This study raises important questions about the environmental impact of brake materials that are commonly marketed as "friendly" alternatives. The research found that, despite having a "green" reputation, NAO pads emit more gaseous pollutants compared to LMCF pads. It is well known that many identified volatile organic compounds (such as benzene and formaldehyde) are toxic to human health, which raises doubts about the net benefits of these materials.
In addition, these emissions are not limited to extreme braking scenarios. Experiments have shown that the release of volatile organic compounds is significant at temperatures and speeds corresponding to typical urban driving. This indicates that reducing the gas emissions from brake pads may be a key strategy for reducing the formation of harmful ultrafine particles in urban areas, and the emission coefficients measured in the laboratory may not accurately reflect the complex conditions of the real world.
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