Green Vehicle Guide

Greenhouse Gas Versus Smog Forming Emissions

Why are there different ratings?

GHG and Smog ratings on fuel economy label

We rate these emissions separately because they represent different pollutants that affect different layers in our atmosphere.

  emissions from vehicle

Greenhouse Gas Emissions

  • Greenhouse gases (GHGs) are emitted from the tailpipes of cars and trucks that combust fuel. 
  • Once GHGs are released, they can stay in the atmosphere for 100 years or more.
  • GHGs act like a blanket around Earth, trapping energy in the atmosphere and causing it to warm.  This can change Earth's climate, raise sea levels, and result in dangerous effects to human health and welfare, and to ecosystems.
  • The effects are global.

Smog Forming Emissions

  • Cars and trucks that combust fuel also emit smog forming emissions, such as nitrogen oxide, non-methane organic gases, carbon monoxide, particulate matter, and formaldehyde.
  • These emissions are usually trapped close to the ground, and can form a brownish haze that pollutes our air, particularly over cities in the summertime.
  • Smog can make it difficult for some people to breathe, triggering lung diseases such as asthma, emphysema, and chronic bronchitis.
  • The effects are local, regional, and national.

The Good News...


For more information, visit EPA's sites on: