In India, the ambient air quality has been degrading from past few decades especially in urban areas. Vehicular emissions are amongst one of the major reasons for the deterioration of ambient air quality in such areas. This article is an effort to review the vehicular emission standards of the major countries (USA, Europe, Japan and Australia) and compare with Indian emission standards. However, there exists several differences in present emission standards followed by different countries. For instance, emission standards in USA are fuel neutral, while no separate weight categorization exists among light-duty vehicles (LDVs). In Europe, Japan, Australia and India, separate weight categorization and emission limits for both petrol and diesel vehicles are provided. It was observed that different driving test cycles used by different countries are the reasons for numerical differences in vehicular emission standards. To rectify this, a worldwide harmonized test cycle (WHTC) is introduced by United Nations Economic Committee for Europe (UNECE) that would represent real-world driving and verifies that the statutory emission limits are not exceeded during actual driving. Countries like Japan and Europe have already been following the WHTC, while some other countries may introduce the same in near future. Yet the real-world emissions across the world keeps on diverging. In such situation, harmonization of vehicle emission standards worldwide is a bit challenging, based on dynamics of road conditions, driving patterns, environmental conditions, etc. Thus, more concerted research is needed for evolution of a common universal emission standards implementable worldwide so that the uniformity in information and policies available to the common public could be maintained.

在印度，环境空气质量在过去几十年中一直在下降，尤其是在城市地区。车辆排放是这些地区环境空气质量恶化的主要原因之一。本文旨在回顾主要国家（美国、欧洲、日本和澳大利亚）的车辆排放标准，并与印度的排放标准进行比较。但是，不同国家现行的排放标准存在一些差异。例如，美国的排放标准是燃料中性的，而轻型车辆 (LDV) 之间没有单独的重量分类。在欧洲、日本、澳大利亚和印度，为汽油和柴油车辆提供了单独的重量分类和排放限制。据观察，不同国家使用的不同驾驶测试周期是造成车辆排放标准数值差异的原因。为了纠正这一问题，联合国欧洲经济委员会 (UNECE) 引入了一个全球统一测试循环 (WHTC)，该循环将代表真实世界的驾驶，并验证在实际驾驶期间没有超过法定排放限制。日本和欧洲等国家已经在关注 WHTC，而其他一些国家可能会在不久的将来推出相同的产品。然而，世界各地的实际排放量却在不断分化。在这种情况下，基于道路条件、驾驶模式、环境条件等的动态，全球车辆排放标准的统一具有一定的挑战性。因此，