In a study of the health effects of air pollution, it was found that the ' proportion of the total population of city A that suffered from respiratory diseases was larger than the proportion for city B. Since city A was generally regarded as being less polluted and more healthful than city B, this result was considered surprising. Therefore, separate investigations were made for the younger population (under age 40) and for the older population (age 40 or older). It was found that the proportion of the younger population suffering from respiratory diseases was smaller for city A than for city B; and also that the proportion of the older population suffering from respiratory diseases was smaller for city A than for city B. Discuss and explain these results.