"Diesel vehicles produce 20x the harmful pollution of petrol vehicles." Is this true?

First you need to understand pollution. There are polluting gases, there are precursors to pollutions and there are particulate matter in exhaust. There may also be pollution generated by the manufacture and transportation of a product along with its use. The statement would be true if either any one of the types of pollution were 20 x greater for diesel than gasoline or if the combination of all pollutants were 20x greater.

The most obvious choice to look would be in particulate matter which is often assumed to be greater for diesel than petrol. This study refutes those claims and casts the proposition in doubt: http://webpages.charter.net/lmarz/Diesel.pdf

You also have to consider that the statement might just be hyperbole or the study might be done by a self interested party.

Don't worry it is just the greenies going boo!

Ever notice how easy it is to frighten some people with the unknown? Where they might be a lion in the face of a gun which will kill you real fast and in a hurry, now as Cecil discovered. They crap their pants over pollution, cancer or global warming which might or might not even happen someday maybe.

Even when they are frightened by a gun it is usually not when they have one stuck in their face. It is when one exist somewhere and might someday be a threat maybe, if it should fall into the wrong hands or if a child should get a hold of it or if ad infinitum. It isn't the thing it's self, it is the uncertainty they fear.

Come on people grow a pair. It is embarrassing to me to be the same species.