A number of allergens are known to occur in indoor environments, especially in homes. These include allergens from dust mites, molds, animals such as cats or dogs, cockroaches, rodents (mice and rats), and birds. These allergens are known or suspected to cause allergic symptoms in those who are sensitized to them. These symptoms can include exacerbation of existing asthma or allergies, and a variety of upper or lower respiratory symptoms such as sneezing, congested or runny nose, cough, wheeze, difficulty breathing, or shortness of breath. Over 70% of U.S. homes have been shown to have elevated levels of at least one indoor allergen.
Increasing evidence also suggests that for at least some of these (such as dust mite, cockroach, and molds or other dampness-related agents), even occupants without specific allergic sensitization can have symptomatic responses, which may be due to inflammatory effects rather than specific allergic effects. Thus, even if you or your child had not been medically demonstrated as allergically sensitized to a specific type of mold or to cockroaches, for instance, it would be wise to prevent these exposures. In particular, there is no demonstrated advantage to testing and identifying the specific type of mold in a moldy home, as the presence of any mold increases the risk of respiratory disease, in both those allergic and non-allergic.
It is not yet clear if allergens in the home from cows or horses cause problems. In fact, there is increasing evidence that living near cows and other farm animals when very young may be protective against later development of asthma and allergy.
More information about indoor allergens can be found from the American Academy of Allergy, Asthma, and Immunology, the Allergy and Asthma Foundation of America, the US EPA, and the American Lung Association.