(24)The Fitness Check of the Ambient Air Quality Directives, covering Directives 2004/107/EC and 2008/50/EC, has shown that limit values are more effective in bringing down pollutant concentrations than other types of air quality standards, such as target values. With the aim of minimising harmful effects on human health, paying particular attention to vulnerable groups and sensitive populations, and the environment, limit values should be set for the concentration of sulphur dioxide, nitrogen dioxide, particulate matter (PM 10 and PM 2,5 ), benzene, carbon monoxide, arsenic, cadmium, lead, nickel and polycyclic aromatic hydrocarbons in ambient air. Benzo(a)pyrene should be used as a marker for the carcinogenic risk of polycyclic aromatic hydrocarbons in ambient air.