Honolulu has slipped a little in a ranking of cities that have the best quality of life, though still tops all other places to live in the United States.

Hawai'i's capital city ranked 31st of 221 cities worldwide in terms of quality of life in the ranking done by management consulting company Mercer Consulting.

That's a tad lower than the 29th-best ranking Honolulu warranted last year, though Mercer noted it added six cities to the listing this year, making direct compari-sons difficult. In three prior years, Honolulu had ranked as high as 27th best.

The annual review of cities is done to help multinational companies considering where to base employees, including how much salary they'll have to pay. Mercer looks at 39 factors that affect quality of life, including political stability, personal freedom, schools, recreation, consumer goods, the environment and housing.

Most of the top 25 cities were in Europe, with Vienna, Zurich and Geneva occupying the top three spots.

Mercer said the quality of life was stable for North American cities.

Honolulu placed ahead of San Francisco (32), Boston (37) and Chicago and Washington (tied at 45) in the U.S.

New York came in at 49, while Philadelphia and Dallas were at 55 and 61, respectively.

Honolulu also got a high score in an "eco-ranking" component that scored cities on water and air quality, waste removal, sewage system quality and traffic congestion. It ranked No. 2 in this aspect, behind Calgary.

"A high-ranking eco-city optimizes its use of renewable energy sources and generates the lowest possible quantity of pollution," Siagin Parakatil, Mercer senior researcher, said in a media statement.

"A city's eco-status or attitude toward sustainability can have significant impact on the quality of living of its inhabitants."

Baghdad occupied the last place on the list.