More than 17 million people suffer from asthma in the United States according to the American Academy of Allergy, Asthma and Immunology. Many list tobacco smoke as one of their most common triggers of asthma attacks.

Tobacco is usually smoked through one of the following:

• Cigarette
• Cigar
• Pipe

Roughly 200 of the 4,000 chemicals in tobacco smoke are considered toxic, and about 40 of the chemicals are known carcinogens (substances which are capable of causing cancer). Several of these harmful chemicals can directly trigger an asthmatic attack or increase the likelihood of an asthmatic attack occurring. These include:

• Ammonia. Can irritate the lungs when inhaled.
  
  
• Methanol. Considered toxic when inhaled or swallowed.
  
  
• Hydrogen cyanide. Causes an interference with normal respiratory function.
  
  
• Carbon monoxide. Lowers the amount of oxygen in the blood, which hampers breathing.
  
  
• Tar. Leaves a residue on the interior of the lungs, lowering lung function.

Tobacco smoke is one of the most commonly acknowledged asthma triggers. It is capable of entering the lungs in two forms: direct inhalation and second-hand smoke. Although both are very harmful, second-hand smoke can, in some circumstances, be more dangerous because it contains higher concentrations of damaging substances, such as tar and carbon monoxide.

In either form, once tobacco smoke has been inhaled, it can cause an asthma attack by settling on and irritating the smooth muscles that surround the airways, causing them to constrict. This closes off the airways, potentially causing shortness of breath or difficulty breathing.

In addition, tobacco smoke may cause the mucous membranes that line the airways to produce greater amounts of mucus than normal. Excessive mucus diminishes the diameter of the airway, reducing the amount of air that can be brought into the lungs and inhibiting normal respiration. This mucous membrane reaction alone can bring on an asthma attack in many individuals. It can also worsen attacks already in progress.

Tobacco smoke also damages the cilia that line the airways. These tiny, hair-like structures are an important mechanism for the removal of dust and excess mucus from the airways. These secretions also offer a favorable spot for infections to begin, which can further clog an already inflamed airway.

Directly inhaled smoke and second-hand smoke can cause a person to experience asthma attacks more frequently. These attacks may also be more severe. In addition, tobacco smoke can make asthma attacks harder to control with medication, by undoing the effects of controller medications and increasing the need for rescue medication. Ultimately, this may lead to more visits to the physician or the emergency room. Directly inhaled smoke and second-hand smoke can also cause death.