Asthma seems to be caused by both hereditary (genetic) and environmental factors. While asthma can develop at any age, it tends to most commonly start in children age 2-6 years. At this age, asthma is triggered by allergens such as dust mites, tobacco smoke, and viral upper respiratory infections
Adults can also develop asthma, and only about 30% of the triggers are related to allergies such as to pets, mold, dust, or perfumes. Risk factors for adult-onset asthma include being female, obesity, hormonal fluctuations such as those experienced during or after pregnancy or menopause, and viral or other infections.