Emission Standard Level
(listed from least clean to cleanest)
T1
Tier 1 The least stringent emission standards
TLEV
Transitional Low Emission Vehicle More stringent than Tier 1 standards for hydrocarbons (HC)
LEV
Low Emission Vehicle More stringent than TLEV standards for both hydrocarbons (HC) and nitrogen oxides (NOx)
ULEV
Ultra Low Emission Vehicle More stringent than LEV standards for HC
SULEV
Super Ultra Low Emission Vehicle Even more stringent standards than ULEV for both HC and NOx
ZEV
Zero Emission Vehicle The strictest emission standard, permitting no emissions
Graphical representations of the emission standards for hydrocarbons and nitrogen oxides are shown below:
Emission Standard for Nitrogen Oxides.

Emission Standard for Hydrocarbons

For more information about emission levels and standards, visit EPA's Green Vehicle Guide