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Vehicle emissions are the pollutants and greenhouse gases that road vehicles release, mainly through the exhaust, and the US EPA names nitrogen oxides (NOx), particulate matter (PM), volatile organic compounds (VOCs) and several air toxics as the main concerns. In EPA’s 2020 National Emissions Inventory, transportation accounted for about 45% of total US NOx emissions and less than 10% each of VOC and PM emissions. Regulatory emissions are measured on a laboratory chassis dynamometer using fixed speed-versus-time drive schedules, not on the open road.
Definition and scope
Vehicle emissions, in the sense used by regulators, are substances discharged by motor vehicles that affect air quality or climate. EPA’s transportation page groups the air-quality side into smog-forming pollutants, soot (particulate matter) and air toxics. Greenhouse gases such as carbon dioxide are a separate category, tracked in national greenhouse gas inventories rather than in the air-pollutant inventory. For research administrators, the distinction matters because projects, permits and reporting lines treat these categories differently; see carbon accounting in research and Scope 1, 2 and 3 emissions in a research organisation for the organisational-footprint view, which is a different question from vehicle-level emissions.
Pollutants: what EPA names and what it says about them
| Pollutant (EPA wording) | Category | What the EPA page states |
|---|---|---|
| Nitrogen oxides (NOx) | Smog-forming pollutant | Transportation contributed about 45% of total US NOx in the 2020 National Emissions Inventory. |
| Volatile organic compounds (VOCs) | Smog-forming pollutant | Transportation contributed less than 10% of US VOC emissions in the same inventory. |
| Particulate matter (PM2.5, PM10) | Soot | Transportation contributed less than 10% of directly emitted PM2.5 and PM10; this excludes PM formed in the atmosphere from vehicle gases. |
| Benzene, formaldehyde, diesel particulate matter | Air toxics | EPA describes air toxics as known or suspected to cause cancer or other serious health and environmental effects. |
Source: US EPA, “Smog, Soot, and Other Air Pollution from Transportation” (epa.gov), read 2026-11-08. The table reproduces the page’s categories and percentages; it does not rank pollutants by harm.
EPA also states a projection that its air quality emissions standards for vehicles will prevent, by 2030, about 40,000 premature deaths, 34,000 hospitalizations and 4.8 million lost work days per year. This is an EPA projection, not a measured outcome, and the page does not tie it to a single named rule.
How vehicle emissions are measured: EPA drive schedules
For certification, EPA tests a vehicle on a chassis dynamometer: the vehicle is held in a laboratory while rollers present the load, and a driver follows a published speed-versus-time trace called a drive schedule. Because every laboratory follows the same trace, results are comparable between vehicles. The schedules EPA lists on its “Dynamometer Drive Schedules” page include:
| Schedule | What EPA says it represents |
|---|---|
| FTP-75 (EPA75) | The test used for EPA emissions and fuel economy; the Urban Dynamometer Driving Schedule (UDDS), a 10-minute soak, then a repeat of the first 505 seconds of the UDDS. |
| UDDS (LA4) | City driving. |
| HWFET | Highway driving below 60 mph. |
| NYCC | Low-speed, stop-and-go driving. |
| US06 | Aggressive, high-acceleration driving. |
| SC03 | Driving with the air conditioning operating. |
| LA-92 | Used for Class 3 heavy-duty vehicles under the Tier 3 rules (the “Hot LA-92” is the first 1,435 seconds). |
| IM240 | Inspection and maintenance testing; no longer required for EPA certification. |
Source: US EPA, “Dynamometer Drive Schedules” (epa.gov), read 2026-11-08. The same page lists the UN/ECE Elementary Urban and Extra-Urban cycles (the Type 1 test) and Japan’s 10.15 Mode as examples of non-US schedules, which is why a result quoted from one schedule cannot be compared directly with a result from another.
Types of standards and related concepts
Three ideas are often confused in reports and grant documents:
- Criteria and toxic pollutants (NOx, PM, VOCs, air toxics): local air-quality and health effects; regulated per vehicle on the schedules above.
- Greenhouse gases (for example carbon dioxide): climate effects; counted in national inventories. EPA’s inventory of US greenhouse gas emissions and sinks reports a 2022 total of 6,343 million metric tons of CO2 equivalent (5,489 net of sinks) for all sources combined, not for vehicles alone.
- Fuel economy: measured from the same FTP-75 test, but it is a performance figure, not a pollutant.
Life-cycle questions, such as emissions from manufacturing and fuel production, are a separate method; see life cycle inventory.
Where it matters for research and reporting
- Method statements. A paper or report that quotes a vehicle emissions figure should name the drive schedule, because the schedules above represent different driving conditions.
- Inventory use. Transportation shares quoted from the National Emissions Inventory are specific to a pollutant and an inventory year; a share for NOx cannot be applied to PM or to greenhouse gases.
- Organisational reporting. Staff travel and fleet vehicles feed an institution’s footprint; see academic travel emissions. Facility-level releases to air are tracked separately, for example in the Toxics Release Inventory.
Common misunderstandings
- “Transportation is the largest source of every pollutant.” EPA’s 2020 inventory figures show about 45% for NOx but less than 10% for VOC and for directly emitted PM.
- “Laboratory results equal on-road results.” Certification schedules are fixed traces on a dynamometer; they are standardised so vehicles can be compared, not recordings of any one driver’s journey.
- “Emissions means carbon dioxide.” Air pollutants and greenhouse gases are tracked and regulated as different categories.
- “A health projection is a measurement.” The 2030 figures quoted above are EPA projections.
Frequently asked questions
What are the main pollutants in vehicle emissions?
EPA names nitrogen oxides, particulate matter and volatile organic compounds, plus air toxics including benzene, formaldehyde and diesel particulate matter.
How are vehicle emissions tested?
EPA certification uses a chassis dynamometer and published drive schedules, with FTP-75 the test used for EPA emissions and fuel economy.
How much of US air pollution comes from vehicles?
In EPA’s 2020 National Emissions Inventory, transportation was about 45% of NOx and less than 10% of VOC and of directly emitted PM2.5 and PM10.
Is IM240 still used for certification?
EPA states that IM240, an inspection and maintenance cycle, is no longer required for EPA certification.
Sources
- US EPA, Smog, Soot, and Other Air Pollution from Transportation, read 2026-11-08.
- US EPA, Dynamometer Drive Schedules, read 2026-11-08.
- US EPA, Inventory of US Greenhouse Gas Emissions and Sinks, read 2026-11-08.
