> But outside these few cases we don't know. One factor is reporting. Do poor kids in India suffer fewer allergies or are they simply dwarfed by the other problems -- or just not reported?

This is a great point that gets overlooked: lack of data is often taken as non-existence, even implicitly. When the average life expectancy is under 50 and there's fewer than one doctor for every 10,000 people, sneezing and occasional sinus pain doesn't even warrant a mention.