Since it made the headlines 10 years ago as one of the most polluted cities in the world, Italy's Milan has achieved general improvement of air quality, but needs to continue with more modern socio-economic measures, local experts said on Thursday.

The positive trend over the past decade was especially evident for primary pollutants. Levels of sulfur dioxide (SO2) and carbon monoxide (CO) halved, while nitrogen dioxide (NO2) concentrations fell by more than 20 percent, according to figures released by ARPA, the environmental protection agency of Lombardy region whose capital city is Milan.

ARPA found that policies have mainly focused on replacement of polluting vehicles and tightening of traffic restrictions. Regarding energy measures, action was directed towards improvement of energy efficiency, diversification of energy production and increased use of renewable energy sources (RES).

Yet, despite the significant improvements, PM and ozone have remained the most critical pollutants whose limits have often been exceeded, said Luigi Bisanti, a noted epidemiologist and author of a study showing that fine particulate matter (PM) combined with other pollutants have been the main cause for some 550 yearly deaths in Milan over the past years and until now.

Bisanti told Xinhua that further improvements appear to be the difficult, though possible, challenge to be addressed by changing the urban structure. "What is important is to acknowledge that, for each rise of 10 micrograms per cubic meter of PM, we pay a penalty of hundreds of annual deaths," he said.

In 2012, the concentrations of PM10, which stands for particulate matter up to 10 micrometers in size, reached an average level of 43 micrograms per cubic meter (slightly above the 40-microgram-average limit set by the European legislation) against the value of 54 hit in 2002. Though the daily limit of 50 micrograms was often surpassed during the years, the annual number of exceed-ance days has decreased by more than 140 days in the early 2000s down to 107 in 2012.

The exceeding of the 25-microgram annual average limit for PM2.5, or smaller particles that can penetrate the respiratory system even more deeply, to be respected by 2015, was also widespread but fell from 41 in 2006 to 30 in 2012.

"The yearly results show a descending trend, but limits are still exceeded for too many consecutive days, which is especially due to the unfavorable weather conditions of the northern city," said an environmental chemistry professor at the University of Milano-Bicocca and one of Italy's leading experts in atmospheric particulate, Ezio Bolzacchini.

He said that, due to its position at the center of populous Po Valley which runs along the Po River and is home of intensive industry and farming, Milan is exposed to a 300-meter-high "inversion layer," a sort of blanket which prevents the smog from dissipating higher in the atmosphere, especially during the winter season. "It is for this reason that we need to adopt more stringent criteria compared to other European cities," the professor told Xinhua.

Again this January, the monthly average levels of PM10 rised to 59 micrograms per cubic meter, while PM2.5 reached the value of 52 in the same spam of time. The readings being registered for more than 10 consecutive days prompted the municipality to adopt emergency measures from further limiting circulation of most polluting vehicles to reducing heating use and forbidding shops to leave their doors open in order to avoid wasting heat.