Here is a map of childhood asthma prevalence by state published by CDC with the north-east of USA taking the cake.

The CDC is rather coy in the article as to why this may be but The Green Man has some interesting factoids that may shed some light.

The Australian Government Health site speculates

1. in wealthy, hygienic Western countries, most babies are not exposed to bacterial infections that "kick start" the immmune system in early life and may be important in directing the immune system away from allergic responses. They also grow up in warm, well-furnished, carpeted homes that don't allow much airflow. This encourages the rapid breeding of large numbers of house dust mites in bedding, carpets and furnishings. Many children, instead of playing outside in fresh air, spend most of their time indoors. This further increases dust mite sensitisation.

2. diet in Western countries, such as a high proportion of processed foods, a higher salt intake, a lower antioxidant intake and a lack of fresh oily fish (lower intake of omega-3 fatty acids) may contribute to the development of asthma.

3. spending more time inside in front of the television means that children get far less exercise. Reduced exercise may mean less stretching of the airways, and a greater tendency for the muscle in the airway walls to contract abnormally when exposed to minor irritants.

Could it be that children of the north east spend more time parked in front of a TV, consuming junk food, in a near sterile environment, than those of say, Arizona.