The Bharat Stage Emission Standards (BSES) are a set of legally enforced regulations instituted by the Government of India to control the output of air pollutants from motor vehicles, including those with compression-ignition and spark-ignition engines. These standards, which are based on the European (Euro) emission norms, were first introduced as "India 2000" (equivalent to Euro I) in the year 2000 to address the problem of rising vehicular pollution. The standards and their implementation timeline are set by the Central Pollution Control Board (CPCB) under the Ministry of Environment, Forest and Climate Change (MoEF&CC).
The mechanism works by setting progressively stricter limits on pollutants like Carbon Monoxide (CO), Hydrocarbons (HC), Nitrogen Oxides ($\text{NO}_{\text{x}}$), and Particulate Matter (PM), which necessitates cleaner fuel and advanced engine technology. A major recent change was the decision in 2016 to skip the BS-V stage entirely and move directly from BS-IV to BS-VI norms. This transition was enforced nationwide from April 1, 2020, following a decisive judgment by the Supreme Court of India in the case of M.C. Mehta v. Union Of India (2018 INSC 982), which mandated that no BS-IV compliant vehicle could be sold or registered after that date. The ratio of the judgment was that the right to a clean environment is an inherent part of the fundamental right to life under Article 21 of the Constitution, justifying the stringent norms to safeguard public health.
The BS-VI standard significantly reduced the maximum permissible sulphur content in fuel from 50 parts per million (ppm) under BS-IV to 10 ppm, which is crucial for the effective functioning of advanced emission-control systems like the Diesel Particulate Filter (DPF) and Selective Catalytic Reduction (SCR). Furthermore, the BS-VI Phase-II norms, implemented from April 2023, introduced Real Driving Emissions (RDE) testing to ensure vehicles comply with emission limits under actual on-road conditions, not just in a laboratory. The BSES framework is closely connected to the National Auto Fuel Policy, which laid out the roadmap for its phased implementation.