GENEVA, May 7 -- The World Health Organization (WHO) on Wednesday unveiled its new urban air quality database, which indicating that air quality in most cities worldwide monitoring outdoor air pollution fails to meet WHO guidelines for safe levels.

The urban air quality database, which covers 1,600 cities across 91 counties, showed that about half of the urban population being monitored is exposed to air pollution that is at least 2.5 times higher than the levels WHO recommends.

According to WHO, air pollution is getting worse in most cities where there are enough data to compare the situation today with previous years.

"Too many urban centers today are so enveloped in dirty air that their skylines are invisible," Flavia Bustreo, WHO assistant director-general said in a statement.

WHO noted that individual cities could take local action to improve air quality, including improving hose energy efficiency, encouraging public transportation, and carrying out compact urban development plan.

The health organization called for greater awareness of implementing effective air pollution mitigation policies and close monitoring the situation in cities worldwide.

WHO last month estimated that outdoor air pollution was responsible for the deaths of some 3.7 million people under the age of 60 in 2012.