While some cities in China were much dirtier in pollutants such as TSPs, Mexico City was worst when combining all three measurements and taking into account the overall number of children under five exposed to the foul air, the study said. Two Chinese Cities Close Behind Beijing, Shanghai, Tehran and Calcutta follow Mexico City on the list of cities whose air poses greatest risk to children. Respiratory disease is now the leading cause of death for children worldwide, according to the study. More than 80 percent of all deaths attributable to lung infections caused by air pollution occur in children younger than five in Third World countries like Mexico, it said. In the short-term, pollution strikes hardest at children, who breathe faster and whose lungs are still developing, the elderly and those with lung trouble such as asthma.
Air pollution reduces the lung's ability to defend itself.
A Generation of Increasing Filth Sadly, Mexico City's air has gone from among the world's cleanest to among the dirtiest in the span of a generation. Average visibility of some seven miles in the 1940s is down to about one mile. Snow-capped volcanoes that were once part of the landscape are now visible only rarely. And levels of almost any pollutant like nitrogen dioxide now regularly break international standards by two to three times. Levels of ozone, a pollutant that protects us from solar radiation in the upper atmosphere but is dangerous to breathe, are twice as high here as the maximum allowed limit for one hour a year.