In the Cincinnati study into Childhood Allergy and Air Pollution, 624 babies and toddlers of allergic parents were assessed and when exposured to a combination of high levels of indoor allergens (such as bacteria from throat infections, house dust mites and mould spores)  plus traffic air pollution, this increased their risk of developing wheezing and persistent asthma by six fold.  The well-established Hygiene Hypothesis highlights the connection between a sterile home environment in infancy and  the risk of deveoping childhood allergies.  It was the combined effect of exposure to high levels of indoor allergens (endotoxin) together with the motor car diesel exhaust fumes that seemed to be such a potent trigger for wheezing and persistent airway inflammation.  The resultant chronic airway inflammation is also known to retard long-term normal lung development.  While in children exposed to moderate levels of indoor or outdoor allergens, only 11 percent developed asthma and in those exposed to low levels of allergen but high level air pollution, 18 percent developed persistent or chronic asthma.

http://www.newswise.com/articles/view/558806/?sc=mwtn