Milan is one of the most polluted cities in Europe. The air quality is extremely poor due to the volume of fine particles, pollutants that pose a risk to human health. While the maximum level considered safe by the World Health Organization (WHO) for airborne particulate pollutants does not exceed a long-term average of 5 µg/m3 (micrograms per cubic meter), levels in the city are almost four times the safe level at 19.7 µg/m3. As part of an effort to tackle dangerous emission levels, Milan authorities want to ban private car traffic in the city center. "This may seem small, but it will be a historic decision," Mayor Sala said last week at a festival to promote sustainability.