London is a large congested city with lots of vehicles and a high density population. There is no escape from the pollution - even if the wind blows the particulates and gases simply end up with neighbouring boroughs.

Little Snoring (or wherever) in rural Dorset, Northumberland or the Highlands is quite different. The main victims of a 40 year old diesel tractor emitting large amounts of soot are mainly cows , sheep, hedgehogs and vermin.

I know cows, sheep etc are all Gods creatures and were they able to protest they might. Life is tough!!

Because the MOT is testing whether a single vehicle is putting out a reasonable amount of toxic emissions. Multiply all of those reasonable emissions by 3.7 million car trips per day in London and you get an awful lot of pollution in one place.

And it's not just London, it's also (to one extent or another) Birmingham, Bristol, Oxford, Glasgow, Bath, Bradford and Portsmouth and probably soon Manchester, Aberdeen, Dundee, Edinburgh, Newcastle and Sheffield.

If the MOT emissions test is clean enough for most of Britain why is it it not good enough for London with their ULEZ? Are Londoners more important than the rest of us?

An MOT test is a quick check of the general condition and safety of a vehicle. The MOT emissions test has nothing in common with the comprehensive emissions testing carried out by vehicle manufacturers to meet the requirements for vehicle type approval.

Think about it. At the MOT the vehicle is not driving a rolling road to enable the engine to produce a power output, and the ambient conditions (temperature, pressure etc.) are not controlled. It is a simple "rough and ready" check on just a couple of the many constituents of the products of combustion in the exhaust. With no rolling road the engine is only producing enough power to keep itself running (ie. just enough to overcome internal friction and to suck in the air/fuel mixture and push the exhaust out). Its actual power output is zero hence the amount of fuel burnt is tiny.

An MOT emissions pass does not prove a vehicle meets its type approval emissions standards but indicates that it is not a hopeless "shed".