But it's the density of diesel vehicles in towns and cities that concentrates the pollution. You can literally chew the diesel fumes in traffic. Plus, there are more and more older diesels, but often only six or seven year old cars, that throw out a cloud of filth when accelerating. I draw back and often open the window or use recycle on my A/C to try and get reasonable air to breathe, but the reality is that the filthy air caused by the microscopic, and not so microscopic, particulates these vehicles pump out is saturating urban air. And it is astonishingly toxic.
Such smokey vehicles should be impounded on the spot and destroyed, with the charge being met by the owner. Any found to have had their pollution controls tampered with should incur a big fine, and this should be checked for in MOTs with any workshop carrying out such illegal work being closed down.
Sorry, but the present weak controls just aren't working.