The furore over Volkswagen's cheating of US emissions tests (see Nature; 2015) prompts a reminder that pollution from diesel vehicles has long been under-reported. This includes nitrogen oxides, hydrocarbons and particulates.

Emissions can be measured across cities using instruments on aircraft and high towers, and for vehicles using number-plate recognition and remote sensing. Modern diesel engines emit roughly four times more nitrogen oxides on average than are recorded in lab tests, which use unrepresentative driving cycles and technical strategies to reduce emissions (D. C. Carslaw and G. Rhys-Tyler Atmos. Envir. 81, 339–347; 2013). Real-world emissions of diesel hydrocarbons exceed estimates used for air-quality planning by up to 70 times (R. E. Dunmore et al. Atmos. Chem. Phys. 15, 9983–9996; 2015).

Particulate matter is estimated to kill 29,000 people each year in the United Kingdom (see This figure will rise when the UK Committee on the Medical Effects of Air Pollutants, which advises government, quantifies the extra health burden associated with nitrogen dioxide.

Improvements in urban air quality stalled a decade ago in many European cities, where nitrogen dioxide often exceeds regulatory standards and global health guidelines. To tighten up diesel-emissions control, tests need to be more accurate, more transparent and regulated more rigorously (see also F. J. Kelly and J. C. Fussell Envir. Geochem. Health 37, 631–649; 2015).