Catalytic converters have proven to be reliable devices and have
been successful in reducing noxious tailpipe emissions. However,
they may have some adverse environmental impacts in use:|
The requirement for a rich burn engine to run at the
stoichiometric point means it uses more fuel than a "lean burn"
engine running at a mixture of 20:1 or less. This increases the
amount of fossil fuel consumed and the carbon dioxide emissions of
the vehicle. However NOx control on lean burn engines is
problematic at best, and many lean burn engine manufacturers are
considering rich burn variations. Another solution is to increase
the amount of biofuels used - if 100% biofuel was used the engines
would be CO2 neutral, presuming no fossil fuels were consumed in
production of the biofuels, which currently is far from the case
(see energy balance of biofuels).
Catalytic converters are "estimated" to account for 50% of total
nitrous oxide (dinitrogen oxide, 'laughing gas') emissions to
atmosphere. While N2O emissions in these concentrations are not
harmful to human health, it is a potent greenhouse gas, accounting
for around 7% of the overall greenhouse effect despite its small
concentration in the atmosphere. The California Air Resources
Board is investigating this issue, and will introduce legislation
to address it if necessary.
The manufacturing of catalytic converters requires palladium
and/or platinum; a portion of the world supply of these precious
metals is produced near the Russian city of Norilsk (about 15%),
with significant negative environmental effects due to the lack of
environmental protection legislation. 
It can be argued that catalytic converters have reduced toxic
emissions and smog at the expense of increased greenhouse gases,
however anyone making this argument should consider the California
Air Resources Board reports on improvements in Air Quality that
have been achieved over the last 30 years.