In 2001-2002 the UK began taxing vehicles according to CO 2 emission rates. Since then, there has been a significant increase in consumer choice of small cars and diesel engines. We estimate CO 2 reductions and air quality impacts resulting from UK consumers switching from petrol to diesel cars from 2001 to 2020. Annual reductions of 0.4 megatons (Mt) of CO 2 and 1 million barrels of oil are estimated from switching to diesels. However, diesels emit higher levels of particulate matter estimated to result in 90 deaths annually (range 20-300). We estimate 570, 460, and 0 additional deaths per Mt of CO 2 abated, for Euro III, Euro IV, and post-Euro IV emission class vehicles, respectively. CO 2 policies are suspected to have contributed substantially to diesel growth, but the magnitude of impact has yet to be quantified rigorously. To the extent that CO 2 policies contribute to diesel growth, coordinating CO 2 controls with tightening of emission standards would save lives. This research shows that climate policy, while reducing fuel use and CO 2 , does not always ensure ancillary health benefits. Lessons from the UK can help inform policies designed elsewhere which strive to balance near-term ambient air quality and health with long-term climate mitigation.