COPENHAGEN (Reuters) - The United States remains the fourth best country in the world to live in, but adjustment for inequality drops it into 23rd place, according to an annual U.N. ranking of nations' development released on Wednesday.

The ranking came in a human development index, a gauge of well-being published by the U.N. Development Program for the past 21 years that combines economic prosperity with education levels and life expectancy.

The UNDP published the inequality-adjusted index for the second time this year after introducing it last year, and aims to make it a standard component of its annual Human Development Report alongside its main, unadjusted index.

A number of other rich countries also scored considerably lower when adjusted for equality.

Canada, which was sixth in the overall human development index, was 12th in the inequality-adjusted measure.

South Korea, 15th overall, came 28th out of 187 nations when scored for equality.

The inequality-adjusted index is adjusted for inequalities in the three areas of human development covered by the UNDP's human development index -- life expectancy, education and standard of living in terms of income.

The discrepancies in some countries' adjusted and unadjusted index rankings were highlighted by some other nations whose rankings did not differ much or at all on the different scales because they were deemed to have a high degree of equality.

Oil-producing Norway, which again grabbed the top rank in the overall index -- its ninth top placement in the past 11 years -- was also No. 1 in the adjusted index.

Australia, second in the overall human development index (HDI), was also second in the inequality-adjusted index. The Netherlands ranked third and fourth in the different measures.

Some nations fare better when the equality of their society is taken into account. Sweden, which ranked 10th in the overall development index, was third in the adjusted index, and Denmark climbs from 16th to eighth place with such adjustment.

The Democratic Republic of Congo (DRC), torn and impoverished by more than a decade of war, was ranked worst in 187th place in the overall development index and 134th in the adjusted index.

African nations Niger, Burundi, Mozambique and Chad were just above the DRC at the bottom of the overall index and somewhat higher when adjusted for inequality except for Burundi on which adjustment data were missing.

The UNDP's new Human Development Report focused on the relationships between environmental sustainability and equity, a term that the agency uses for fairness and social justice and access to a good quality of life.

Distribution of income has grown more unequal over the past several decades at the country level in much of the world though gaps in health and education have narrowed, it said.

Increasing evidence points to widespread environmental degradation around the world and potential future deterioration, the UNDP said.

"Forecasts suggest that continuing failure to reduce the grave environmental risks and deepening social inequalities threatens to slow decades of sustained progress by the world's poor majority -- and even to reverse the global convergence in human development," UNDP administrator Helen Clark warned in the report.

(Reporting by John Acher Editing by Maria Golovnina)

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