I’ve read conflicting arguments (which range from pretty bad to incredibly bad)… The Guardian estimates 230-950 thousand tons of NOx per year for seven years, “equivalent to the emissions of all power stations, vehicles, industry, and agriculture in the United Kingdom over a one-year period.”

Not a percentage, which would be more useful, but even the lowball estimates indicate a measurable change in pollution levels.

http://www.theguardian.com/business/2015/sep/22/vw-scandal-c...