The source of asthma is not fully understood. Healthcare professionals believe a combination of genetic and environmental factors determine whether or not a person becomes asthmatic. The American Academy of Allergy, Asthma and Immunology estimates that 40 percent of children who have asthmatic parents will develop asthma.

One theory of asthma development involves immune system cells called T helper cells. It is known that T helper 2 cells (T_h2) release chemicals that activate other cells known to contribute to inflammation. In a normally functioning immune system, this process is balanced by T helper 1 cells (T_h1), which reduce inflammation. Evidence suggests that the T helper cells may be out of balance in asthmatics, preventing the body from controlling excess inflammation.

Immune system reactions are at the heart of another asthma theory, which states that the growing number of children with asthma stems from children having less exposure to certain types of bacteria, and infections than in the past. As a result, children have not developed the types of immune cells necessary to prevent asthma, according to this theory. Yet other research suggests that exposure to infections and allergens at an early age can increase the risk for developing asthma.  

Physicians have noted that about 70 percent of people diagnosed as asthmatic also suffer from allergies. There also appears to be a strong association between a condition called sinusitis and asthma. Sinusitis, which is inflammation of the sinus cavities, occurs in 15 percent of people with asthma and 75 percent of those with severe asthma. By contrast, sinusitis occurs in about 5 percent of the general population.

This might occur because fluid from the sinuses may drain into the bronchial tubes, causing bronchitis. In turn, this might worsen asthma. It may also occur because the tissues in the sinuses and the airway linings are similar, so they may react to similar triggers.

In addition, all airway tissues are connected, and the inflammatory response may extend from the sinuses to the lower airways (bronchi).

Additional factors which may increase a person’s risk for asthma include:

• Living in the inner city, or another large urban area
  
  
• Exposure to secondhand smoke
  
  
• Exposure to occupational irritants, such a chemicals used in farming and hairdressing, and in the manufacturing of paint, steel, plastic and electronics

A person with asthma is more likely to experience symptoms or full–blown asthma attacks when exposed to certain allergens and stimuli, which are classified as asthma triggers. These include:

• Allergens, such as:
• • Pollen
  • Mold
  • Dust
  • Cockroaches
  • Pet dander
    
    
• Irritants, such as:
• • Air pollution
  • Tobacco smoke
  • Cold air
  • Perfume
  • Spray-on deodorants
  • Strong odors from household chemicals, sprays or paints and other irritants

Other potential asthma triggers include:

• Certain foods
  
  
• Exercise
  
  
• Feathers in pillows
  
  
• Aspirin or ibuprofen
  
  
• Menstrual cycles (when hormonal changes may trigger asthma)
  
  
• Pregnancy
  
  
• Viral respiratory infections (including colds, bronchitis and flu)
  
  
• Stress
  
  
• Temperature changes
  
  
• Sinus infections
  
  
• Heartburn
  
  
• Sulfites (a preservative used in salad bars, dehydrated soups, alcohol and other foods)
  
  
• Gastroesophageal reflux disease (GERD)

Allergens are the most common trigger of asthma in those under the age of 30. Although allergens trigger the condition in many people over the age of 30, older adults tend to experience symptoms after exposure to irritants.

Physicians can help patients identify which factors trigger their asthma, and recommend steps the patients can take to avoid the triggers.

There also appears to be a link between obesity and asthma that is not yet understood. One study found that children who are obese at the time of puberty are three times more likely than their slimmer peers to continue to suffer from asthma into their teens. Other studies have found overweight school-age children are 50 percent more likely than those of normal weight to develop asthma, and those who had a high birthweight were 20 percent more likely to develop asthma later in childhood.

The association between asthma and obesity is also apparent in adults. Research has suggested that obese adults are three times more likely to develop asthma than thinner adults. There also appears to be a direct relationship between the amount of weight gained and the risk of developing asthma. Research suggests that an overweight person’s risk for asthma increases further as they gain more and more weight.

The mechanism behind the asthma-obesity link remains a mystery, though some research indicates that the two conditions share a gene that might link them together.