If Mazda is correct, synthetic urine could help save the planet. The new CX-7 uses an additive called AdBlue - 33 per cent pure urea diluted in water - to reduce harmful NOx exhaust emissions. Mazda's faux pee causes ‘selective catalytic reduction' that turns NOx into nitrogen and water when squirted into the exhaust. A $140 refill buys you 15.5 litres - enough for 20,000km - which is significantly more than the non-synthesised stuff. The car won't start when the AdBlue tank runs dry, and there is no organic roadside refill option. At least, not yet.