At present the compliance level for clean air zones is Euro 6 for diesel cars (generally vehicles registered from September 2015 onwards) and Euro 4 for petrol cars (January 2006) onwards. The reason for the stricter limits for diesel engines is because of particulate matter, which as a general rule are greater from diesels than from petrol engines. However, although it is difficult to predict how legislation will change in the future, Euro 7 emission limits will be introduced in July this year, and each level of Euro standard tends to be superseded at around five years, so we could be seeing Euro 8 introduced at the end of the decade. Inevitably older cars will eventually fall foul of clean air zone limits, but when this will happen is hard to predict.