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Most of the world's 1.8 billion urban dwellers breathe air of unacceptable quality, according to a study released Thursday and billed as the first-ever comprehensive global assessment of air pollution.

Prepared by the World Health Organization and the U.N. Environment Program, the study says many countries have made great progress in reducing air pollution but the situation is still very serious, especially in Third World nations.The 100-page survey is based on data obtained during up to 15 years of monitoring in more than 60 countries.

Dr. Michael Gwynne, head of the environment program's Global Monitoring System, said the survey represents the "most comprehensive, detailed and informative study ever carried out."

It focuses on pollution by sulfur dioxide, of which man-made emissions worldwide are estimated at up to 180 million tons per year, and suspended particulate matter, of which more than 100 million tons - chiefly dust and smoke - are assumed to be produced annually by human activities.

Continuous exposure, especially of children, to the two pollutants can result in acute or chronic respiratory problems.

Gwynne said 625 million people, mostly in developing countries, are exposed to "unacceptable levels of sulfur dioxide" and that another 550 million live in conditions just within the range of tolerable levels set by the World Health Organization.

He said the situation is worse with respect to dust and smoke, estimating that about 1.25 billion city dwellers live in "unacceptable conditions" and another 200 million live in conditions that are "only marginal."

Data from 54 cities suggest that the average annual sulfur dioxide level during the 1980-84 monitoring period was highest in Milan, Italy.

But Seoul, Rio de Janeiro and Paris were also among the cities where the average range was above WHO's guidelines. Melbourne, Tel Aviv, Toronto, Chicago and Munich were among those with the lowest averages.