Tyre..

 Electric vehicles might reduce carbon emissions (though far from eliminating them – indeed their manufacture involves carbon emissions). They might not have exhaust pipes spewing out nitrogen oxides. 

But growing attention has been paid in recent years to pollution from tiny particulate matter, which can penetrate deep into human lungs. Long-term exposure has been linked to an increased risk of heart disease and lung cancer. Trouble is that a fair amount of these emissions from cars emanate from tyres, not engines, and electric vehicles could possibly emit more because their heavier weight causes greater tyre wear.

While huge attention has been paid to emissions from exhausts, which quite rightly have been cleaned up over the years thanks to progressively tougher regulations, rather less attention has been paid to tyres. The Euro 4 regulations for petrol engines and Euro 6 regulations for diesel engines, take little account of emissions from tyres; they are based on emissions from exhausts.  Just a "Tire" thought.