Vehicle emissions regulations did not happen all at once.
They developed over decades as engineers, regulators, and manufacturers responded to air quality problems caused by hydrocarbons, carbon monoxide, nitrogen oxides, particulate matter, and later greenhouse gas emissions.
For diesel engines, the biggest changes came when regulations began focusing more heavily on particulate matter, NOx, fuel sulfur levels, and aftertreatment systems.
Continue reading History of Vehicle Emissions Regulations: From Smog Controls to Modern Diesel Standards
