When It’s Life in the Big City, Best Bet Is to Hold Your Breath : Contaminants: While Los Angeles has the dirtiest air in the United States, most polluted cities are in the developing world.

Five hundred people with heart and respiratory ailments landed in hospitals in Athens, Greece, one weekend last May. What put them there was nitrogen oxide from car and truck exhausts.

Athens is one of the most polluted capitals, but it finds plenty of company in the world’s other great old cities, including Baghdad, Beijing, Istanbul, Leningrad and Calcutta.

The best way to survive there--and in many other large cities--is to hold your breath.

Generally, experts say, the bigger the city, the worse the pollution. The world’s two largest metropolitan regions--Tokyo- Yokohama, with nearly 29 million people, and Mexico City, with more than 19 million--are among the most contaminated.

A decade ago, 35 cities had populations of more than 4 million people. By 2000 the number of such cities will nearly double.

Most polluted cities are in the developing world.

“Large cities in developing countries are growing much faster than cities in the industrialized world ever have,” said Sharon L. Camp, vice president of the Population Crisis Committee, a nonprofit organization advocating population stabilization.

By the end of the 1990s, the urban population of the developing world will be almost double that of the industrialized world. By 2025 it will be four times larger, and Africa’s urban population will be three times as great as North America’s.

Los Angeles, almost synonymous with smog, is indisputably the most polluted U.S. city.

In daily air-pollution monitoring from 1987 to 1989, the Los Angeles-Anaheim-Riverside area violated acceptable standards for both ground-level ozone and carbon monoxide more days than any other U.S. city, the federal Environmental Protection Agency reports.

The 17 other cities that violated both standards include the largest U.S. urban areas. Ozone, an ingredient of smog, is usually a greater threat in the summer. Concentrations of carbon monoxide, a problem in cities such as mile-high Denver, are more hazardous in winter.

Worldwide, the cities suffering the severest sulfur-dioxide pollution are Milan, Italy; Shenyang, China; Tehran, Iran; Seoul, South Korea; and Rio de Janeiro and Sao Paulo, Brazil, according to a 1988 air-quality report of the Global Environment Monitoring System, which is sponsored by the World Health Organization and the United Nations Environment Program. Airborne sulfur dioxide comes chiefly from burning coal.

The joint project monitors more than 40 geographically selected cities for five contaminants: sulfur dioxide, dust, nitrogen oxides, carbon monoxide and lead. More than half of the cities regularly exceed WHO standards for these pollutants.

Global emissions of carbon monoxide are probably greater than those of all other major pollutants combined.

The study estimates that people living in one-third of the earth’s cities are exposed to marginal or unacceptable concentrations of lead in the air, despite worldwide efforts to reduce lead in gasoline. The highest lead levels in gasoline were detected in Mexico City.

Dust-filled air, technically clogged with “suspended particulate matter,” is worst in Kuwait city; Beijing and Xian, China, and New Delhi and Calcutta, India.

“Many of the major cities of Eastern Europe have the kind of pollution the United States had 30 to 50 years ago--sooty, smoky, dirty air from economies driven by coal,” says Rudolf Husar, director of the Center for Air Pollution Impact and Trend Analysis at Washington University in St. Louis.

“Because more coal is consumed for heating, air pollution is heaviest in the winter,” he tells National Geographic. Sulfur dioxide is a dominant pollutant in Beijing, where coal is the principal power source.

“We should see the biggest changes in Eastern Europe as cities become more involved in getting pollution data and cleaning up the air,” says Gerald Akland of the EPA. Air quality is expected to deteriorate in Asian cities as industrial activity intensifies there.

Today the world’s five most unlivable cities are Lagos, Nigeria; Kinshasa, Zaire; Kanpur, India; Dhaka, Bangladesh, and Recife, Brazil, according to a study of the world’s 100 largest metropolitan areas released by the Population Crisis Committee in November. About 60% of the largest metropolitan areas are in the developing world.

The committee ranked the cities using 10 criteria, including the murder rate, food prices, traffic congestion and air quality.

Only three cities, Melbourne and Sydney in Australia and Taipei, Taiwan, scored perfectly for all clean-air indicators. The poorest air hovers over Calcutta, New Delhi, Beijing, Shenyang, Tehran, Milan, and Jakarta, Indonesia. Air-quality data was unavailable for one-third of the cities.

By contrast, the study found the world’s five most livable cities to be Melbourne, Montreal, Seattle-Tacoma, Atlanta and Essen-Dortmund-Duisburg, Germany.