In the U.S. allergens are widespread, but extent can vary depending on the environment. According to the nation's largest indoor allergen study to date, over 90% of homes have three or more detectable allergens, with 73% having at least one allergen at elevated levels. These elevated allergen levels are known to exacerbate symptoms in people who suffer from asthma and allergies. There are several factors that affect elevation of allergens, of which include pets and pests or even the type of housing. Mobile homes, older homes, rental homes, and homes located in rural areas were found to have higher levels of allergens. Differences in elevation levels were also found in geographic locations and climatic conditions.