On 27 September 1996, the European Council adopted Directive 96/62/EC, referred to as the "framework directive", on ambient air quality assessment and management. The main aims of this text are to:

• Establish the foundations of an overall strategy aiming to define and set limit values for ambient air quality in order to avoid, prevent or reduce adverse effects on human health and the environment, assess air quality in the member States and provide information to the general public.

Under this 1996 framework directive "daughter directives" set various regulatory limit values relating to monitored pollutants:

• Limit value : level set according to scientific knowledge in order to avoid, prevent or reduce adverse effects on human health and (or) on the general environment, to be reached within a given time and not to be exceeded thereafter.
• Alert threshold: level above which short-term exposure presents a risk for human health, and following which member States take immediate measures in accordance with Directive 96/62/EC,
• Target value : level set with the aim of long-term prevention of adverse effects on human health and (or) on the general environment, to be reached where possible for a given period,
• Long-term objective : concentration in ambient air below which, according to current scientific knowledge, there are few likely direct adverse effects on human health and (or) on the general environment,
• Information threshold : level above which short-term exposure presents a risk to human health for at-risk population groups, and which therefore requires regular public awareness reports.

The main EU thresholds relating to concentrations in ambient air are therefore stipulated in Directives 1999/30/EC (sulphur dioxide, nitrogen oxides, particulate matter and lead), 2000/69/EC (benzene and carbon monoxide) and 2002/3/EC (ozone).