He is probably referring to other pollutants such as NOx.  It is unavoidable to produce some amount of these pollutants in an oil combustion process.  But California has shown that it&#x27;s possible to get this down to a low level.  Smog used to be really bad in LA.  But a combination of catalytic converters, strict emissions testing, and oil refinery requirements to make cleaner burning gas has dramatically reduced smog.  The oil refinery requirements make CA gas very expensive compared to the rest of the nation.  But I think it&#x27;s worth it for the air pollution improvements.