Asthma is characterized by spasmotic contraction of the smooth muscle of the airways, by increased production of an abnormally viscous mucus by bronchial mucous glands, and, in severe attacks, by airway obstruction from mucus that has accumulated in the bronchial tree. This results in a greater or lesser degree of difficulty in breathing. One approach to classifying asthma differentiates cases that occur with an identifiable antigen, in which antigens affect tissue cells sensitized by a specific antibody, and cases that occur without an identifiable antigen or specific antibody. The former condition is known as extrinsic asthma and the latter as intrinsic asthma. Extrinsic asthma commonly manifests first in childhood because the subject inherits an atopic characteristic: the serum contains specific antigens to pollens, mold spores, animal proteins of different kinds, and substances from a variety of insects, particularly cockroaches and mites that occur in house dust. Exacerbation of extrinsic asthma is precipitated by contact with any of the substances to which sensitization has occurred; airway obstruction is often worse in the early hours of the morning. The other form of asthma, intrinsic, may develop at any age, and there may be no evidence of specific antigens. Persons with intrinsic asthma experience attacks of airway obstruction unrelated to seasonal changes, although it seems likely that the airway obstruction may be triggered by infections, which are assumed to be viral in many cases. Asthma attacks may be precipitated by food. For example, in small children, sensitivity to milk may stimulate an attack, and, in some adults, sensitivity to sulfite compounds found in some foods and wine may stimulate an attack. A subgroup of asthmatics are so sensitive to aspirin (acetylsalicylic acid) that ingestion of this chemical may lead to a life-threatening attack.

In some people, asthma is acquired as the result of occupational exposure (a special form of intrinsic asthma). Exposure to solder resin used in the electronics industry, to toluene diisocyanate (used in many processes as a solvent), to the dust of the western red cedar (in which plicatic acid is the responsible agent), and to many other substances can initiate an asthmatic state, with profound airflow obstruction developing when the subject is challenged by the agent.

Individuals affected by any type of asthma may exhibit airflow obstruction when given aerosols of histamine or methylcholine at much lower concentrations than provoke airflow obstruction in healthy people. Individuals with asthma may also develop airflow obstruction while breathing cold air or when exercising. These characteristics are used in the laboratory setting to study the airway status of patients.

The severe acute asthmatic attack is characterized by acute difficulty in breathing in which mucus clogs the air passages in such a way that air can be inspired but not expired. Despite the severe respiratory difficulty, the patient remains fully conscious. The most dangerous form of the condition is known as status asthmaticus. The bronchial spasm worsens over several hours or over the course of an entire day, during which the bronchi become plugged with thick mucus and airflow is progressively more obstructed. The affected person becomes fatigued, arterial oxygen tension falls, and carbon dioxide accumulates in the blood (leading to drowsiness). As a result, the acidity of the arterial blood increases to dangerous levels that could lead to cardiac arrest. Prompt treatment with intravenous corticosteroids and bronchodilators is usually sufficient to relieve the attack, but in some cases ventilatory assistance is required. In a few cases, death from asthma is remarkably rapid. The exact mechanism of death in these cases is not completely understood.

Asthma may interrupt normal activities and schooling and provide intense or sustained anxiety, especially in individuals who have experienced one or more severe attacks. It tends to diminish in severity with age, and people who had quite severe asthma in childhood may lead normal lives after the age of 20.

Developed countries around the world are reporting a disturbing increase in the prevalence, severity, and mortality of asthma. For example, today more than 7 percent of children and about 9 percent of adults in the United States suffer from asthma. The reason for this increase is not known; however, evidence points to greater control of ordinary infections during early childhood than was achievable in the 1960s and ’70s. For example, children who regularly attend day care or preschool programs acquire more respiratory infections but are less likely to develop asthma later in life. It is possible that frequent infections in early childhood are necessary to “prime” the developing immune system in a way that protects against subsequent development of asthma.

Hay fever

Hay fever is a common seasonal condition caused by allergy to grasses and pollens. It is frequently familial, and the sensitivity is often to ragweed pollen. Conjunctival infection and edema of the nasal mucosa lead to attacks of sneezing. Allergic inflammation and the development of polyps in the nasal passages represent a severer form of hay fever that is often associated with asthma.

Hypersensitivity pneumonitis

Hypersensitivity pneumonitis is an important group of conditions in which the lung is sensitized by contact with a variety of agents and in which the response to reexposure consists of an acute pneumonitis, with inflammation of the smaller bronchioles, alveolar wall edema, and a greater or lesser degree of airflow obstruction due to smooth muscle contraction. In more chronic forms of the condition, granulomas, or aggregations of giant cells, may be found in the lung. Inflammation can lead to widespread lung fibrosis and chronic respiratory impairment.

One of these illnesses is the so-called farmer’s lung, caused by the inhalation of spores from moldy hay (thermophilic Actinomyces). This causes an acute febrile illness with a characteristically fine opacification (clouding, or becoming opaque) in the basal regions of the lung on the chest radiograph. Airflow obstruction in small airways is present, and there may be measurable interference with diffusion of gases across the alveolar wall. If untreated, the condition may become chronic, with shortness of breath persisting after the radiographic changes have disappeared. Education of farmers and their families and the wearing of a simple mask can completely prevent the condition.

A similar group of diseases occurs in those with close contact with birds. Variously known as pigeon breeder’s lung or bird fancier’s lung, these represent different kinds of allergic responses to proteins from birds, particularly proteins contained in the excreta of pigeons, budgerigars (parakeets), and canaries.

An acute hypersensitivity pneumonitis may also occur in those cultivating mushrooms (particularly where this is done below ground), after exposure to redwood sawdust, or in response to a variety of other agents. An influenza-like illness resulting from exposure to molds growing in humidifier systems in office buildings (“humidifier fever”) has been well documented. It is occasionally attributable to Aspergillus, but sometimes the precise agent cannot be identified. The disease may present as an atypical nonbacterial pneumonia and may be labeled a viral pneumonia if careful inquiry about possible contacts with known agents is not made.