Light-Duty

For more than two decades, diesel particulate filters have been used to control particle mass and particle number for diesel passenger cars. In recent years, Europe (with Euro 6 emission standards) and China (with China 6 emission standards) have taken bold steps in emissions regulations for light-duty vehicles, tackling particulate emissions from fuel efficient gasoline-direct-injection (GDI) vehicles. Not only have limits been placed on the particle mass emitted from vehicles, but the regulatory additions also regulate the number of soot particles allowed in vehicle exhaust to reduce the emission of ultra-fine particulates (larger than 23 nanometers in size).

To reach the defined limits of ultra-fine particulates, automakers have added gasoline particulate filters (GPFs) to their emissions control systems in China and Europe. These highly efficient exhaust filters trap most of a vehicle’s particulate emissions, including ultrafine particles, preventing them from leaving tailpipes or eventually entering the atmosphere.

In addition, Europe and China also instituted real-world driving emissions norms, known as real-world driving emissions (RDE). To put it simply, RDE is a regulatory compliance protocol that ensures vehicles meet emissions limits not only under controlled conditions but also during every day on-road driving. This includes a wide range of different driving conditions such as driving uphill, rapid accelerations, driving on motorways as well as in cities, and in different outdoor temperatures including cold weather. Portable emissions measurement systems (PEMS) are used to monitor the tailpipe in real-time, making sure that vehicles stay within emissions limits.

European EU6e regulations have already resulted in a significant reductions in the emissions of gaseous pollutants when compared with vehicles developed to meet prior regulations. 

In 2024, the European Council adopted new rules on emission limits for cars and trucks with latest EU7 norms. For cars, the regulation maintains the current Euro 6 exhaust emission limits while implementing stricter requirements for solid particles, which now include ultrafine particles down to 10nm (vs. 23nm in current EU6 regulations).

Future regulations in China (CN7), which are expected to be implemented before 2030, are likely to tighten gaseous and particulate emissions, requiring advanced emissions-control technologies, too.

Heavy-Duty

For heavy-duty vehicles, EU VII standards set tighter limits on a range of pollutants, including some that were previously unregulated like nitrous oxide (N₂O). Overall, compliance is being strengthened to include all normal driving conditions as well as cold start emissions. New advanced technologies will also be needed to meet these regulations as well.    

In the United States, emission standards are regulated by the EPA (United States Environmental Protection Agency). In parallel, CARB (California Air Resources Board) has provided even tighter regulations for California, and several other states decided to follow the tighter CARB requirements as well. 

Light-Duty

As of 2025, Tier 3 standards have been fully phased in for light-duty vehicles (2017-2025). These standards aim to reduce emissions of harmful pollutants significantly. The EPA provides a summary of the standards here.

Though the U.S. Clean Air Act of 1970 and its amendments have long served as a model for emissions regulations around the world, the US has chosen to use only a particle mass (PM) regulation instead of both a mass-based and a particle number-based regulation. Current Tier 3 PM regulations are less stringent when compared to particle number limits in Europe, China and even India. As a result, particulate filters are currently not being used on U.S. gasoline cars as they are in Europe and China. However, with the anticipated implementation of upcoming U.S. Tier 4 regulations, which target much lower PM limit that is now roughly equivalent to the particulate number regulations in Europe and China, gasoline particulate filters (GPF) may become a requirement.

Heavy-Duty

California has proposed significant reductions in NOx and particulates from internal combustion engines while extending the full useful life requirements. This includes mandating a subset of their fleet to transition to zero-emission vehicles. The U.S. EPA Cleaner Trucks Plan has also been finalized, requiring significant NOx reductions, likely necessitating the adoption of advanced technologies.

Learn more about the U.S. Clean Air Act and its success in cleaning up ambient air nationwide.

In 2018, Delhi was ranked the most polluted capital in the world and, in total, 22 out of 30 most polluted cities were to be found in India. As a result, the Indian government has been taking strong regulatory action to limit tailpipe emissions. In April 2020, India skipped a regulatory phase and moved from Bharat Stage (BS) 4 in light-duty and BS IV in heavy-duty, to BS 6/VI norms, which closely reflect European vehicle emissions regulations.

Since the adoption of BS 6/VI norms, India has continued to push for stricter enforcement and compliance and the official announcement of even tighter BS7/VII standards will probably be announced in the course of 2025.