From the introduction of Euro 6 in 2015 vehicles fitted with CAT, combines oxygen with carbon monoxide to form non-poisonous carbon dioxide (CO2) reducing the high concentrations in the exhaust manifold (typically 30,000 ppm or more) to low concentrations (typically below 1,000 ppm after the catalytic converter). Most cars are now also fitted with a PPF reducing the ppm to around half again.
Personally I don’t think Euro 6(d) cars are that bad to be behind or near, however get around an older petrol or diesel it stinks and finds its way into the car even if all vent are closed..