What are the main causes of this problem?

In the metropolitan cities, of India, many children are suffering from allergy/asthma. What are the main causes of this problem? Give some symptoms of allergic reactions.

In metropolitan cities, life style is responsible for lowering of immunity and high sensitivity to allergens. It is due to the protected environment provided early in life. Some symptoms of allergic reactions are sneezing, watery eyes, running nose ana difficulty in breathing.