A considerable proportion of the exposure to ambient air pollution and of the related health effects occurs in urbanized areas of Europe, mainly from emissions from motor vehicle transport. Improving the urban infrastructure and reducing transport-related emissions are therefore priority areas for action to reduce the health effects of ambient air pollution in cities.
In 1999 the US Environmental Protection Agency (EPA) is replacing its previous 1-hour ozone standard with a new 8-hour standard. The National Ambient Air Quality Standard (NAAQS) has three parts: the concentration or level, the measurement period and the "form" of the standard. The new standard is set at a concentration of 0.08 parts per million (ppm). The measurement period is 8 hours. Under the form adopted by the EPA, areas are allowed to disregard their three worst measurements every year and average performance over three years to determine if they meet the standard.