The expected growth of Third World mega-cities with populations of 10 million or more, combined with rapid industrial development and the reliance on dirty fossil fuels is creating the conditions for a huge increase in cancer and respiratory diseases.
Devra Lee Davis, of the World Resources Institute in Washington DC, said the study came to worrying conclusions about the risks facing millions of young people, who are more vulnerable to the effects of air pollution than adults.
"Most children who live in large cities in the developing world breathe air that is the equivalent of smoking two packs of cigarettes a day," she told the American Association meeting.
"Never before in history have so many children lived so closely together in so many cities where they also lack basic services, healthcare and sound nutrition.
"The rate of growth of cities in the developing world is also without precedent. Many of these cities have grown in two decades by as much as the developed world grew in a half century," she said.
Children are more vulnerable to the effects of air pollution than adults, the study found. "Children are smaller, they breathe more relative to adults and they breathe faster and they have lungs that continue to develop," Dr Davis said.
More children are dying of respiratory diseases in the world than from any other disorder. Scientists say air pollution in the largest cities of the developing world is often between two and eight times the maximum levels recommended by the World Health Organisation (WHO).
"The exposure of children to such polluted air in developing countries has sky-rocketed and is a relatively new public health issue, caused by a massive and unprecedented shift of population from rural to urban areas in the past few decades," Dr Davis said.
The study, funded by the World Health Organisation, the US Environmental Protection Agency and other groups, looked at three measurements of air pollution - total suspended particulates (TSPs), sulphur dioxide and nitrogen dioxide.
WHO guidelines say the maximum permissible amount of TSPs is 90 micrograms per cubic litre of air. The problem is that these cities have rapidly growing economies but old-fashioned technology.
"While development is welcome, the fact that it occurs in areas with technology that is 30 or 40 years old places all the populations of these cities at risk," she said.
The report says developing countries should be helped to clean up their cars, factories and other polluters before it is too late. Developed countries, although many are still polluted, have benefited from cleaner technology.
"If these pollution-reduction technologies are made more readily available to developing countries, these could help them avoid the high risks to public health that result from current patterns of growth in their industrial and urban sectors," the report says.
Pollution has been shown to damage the cilia in the airways - the tiny hairs that sweep out pollutants and invaders. Without the cilia, harmful chemicals can get deep into lung tissue and cause permanent damage, Dr Davis said.