Prevalence of Aeroallergic Disease

Asthma and other IgE-mediated diseases represent significant health problems throughout the world. The World Health Organization estimates that nearly 300 million people worldwide have asthma. In the United States, the prevalence of asthma is approximately 7% of the population. A reliable estimate of the prevalence of AR is difficult to obtain due to frequent misclassification bias and varied research study design. AR is more prevalent than asthma, however, with estimates of prevalence ranging from 10% to 20% (500 million) worldwide and in the United States ranging from 10% to 40%. A national survey of 35,757 households in the United States identified 500 children who were diagnosed with AR by a health care provider found the prevalence of AR to be 13% in children. Another comprehensive national survey that included 2500 adults diagnosed with AR and 400 health care practitioners who treat AR found the prevalence in adults to be 14%. Asthma and respiratory allergies often coexist in the same patients (the united airway) with approximately 19% to 38% of those with AR having asthma. Of those with asthma, between 50% and 85% of patient have some form of rhinitis.

The prevalence of atopic disease has reportedly been on the rise in the United States and around the world, although rates may have plateaued recently, according to the International Study of Asthma and Allergies in Childhood, which reported that the lifetime prevalence rates of atopic disease stabilized at 20%. This plateau is a mixture of a substantial increase in prevalence of atopic disease in Mexico, Chile, Kenya, Algeria, and Southeast Asia, with decreases in New Zealand and the United Kingdom, which previously had high rates.

Asthma and AR have also begun to demonstrate changes in prevalence patterns, with lower rates in many English-language and Western European countries and with concomitant higher rates in the developing world—Africa, Latin America, and parts of Asia. The cause of these variations is not fully understood. Increased urbanization, however, in parts of the developing world may contribute to these trends due to the higher prevalence of aeroallergic disease in urban settings compared with rural environments.

The hygiene hypothesis is often used to explain these disparities. This hypothesis holds that modifications in the environment by industrialization/urbanization have led to reduced microbial exposure in early life in contrast to farming/rural societies. It is thought that increasing urbanization is associated with improved hygiene, a reduced exposure to infectious agents (biasing the immune system away from a helper T cell type 1 immune-mediated response), and thus higher prevalence of allergy.

The hygiene hypothesis, however, does not fully explain the observed increase in allergic disease in certain areas. For example, in the inner cities of the United States, children have high rates of allergy and a very high prevalence of asthma (39.8%—nearly 6 times the national rate for children). Similarly, African children who move to large cities with supposed improved hygiene have experienced increases in infections, wheezing, and diagnosed asthma.

The increased severity of asthma in North American cities could be explained by several factors, including prolonged time indoors; high exposure to dust mite, cockroach, or rodent allergens; and extreme sedentary lifestyle/obesity. Other factors, such as exposure to elevated concentrations of fossil fuel–generated air pollution, increasing dust mite populations, and decreased home/workplace ventilation as well as the development of sensitization to a variety of novel cross-reacting exotic food and pet allergens, may result in a lower tolerance to pollens: a reverse case of immunotherapy.

Climate change/global warming may also contribute to prevalence changes in aeroallergic disease. A recent review of climate change and allergic disease by Shea and colleagues reported that climate change measurably affects “the timing, distribution, quantity, and quality of aeroallergens,” thereby altering the distribution and severity of allergic disease. It was anticipated that worsening pollution and altered locoregional pollen production would lead to worsening of aeroallergic disease with more symptomatic days and reduced quality of life if trends of climate change continue.

Burden of Aeroallergic Disease

Secondary to the high prevalence of aeroallergic disease, there is a significant economic burden of these disease processes on health care systems and society as a whole. In 1994, the total cost for the care of asthma in the United States was $10.7 billion. Direct medical costs of asthma care (which included hospitalizations, doctors’ visits, and medications) comprised $6 billion and indirect costs (workdays lost, time lost from school, and costs attributed to asthma deaths) were $4.64 billion. In 1996, the cost of allergic rhinitis, in terms of direct medical expenditures, exceeded $3 billion and an additional $4 billion was ascribed to exacerbations of concomitant conditions, such as asthma and otitis media. In addition, more than $6 billion was spent on prescription medications for the treatment of AR in 2000. Indirect costs associated with AR were reported to include 3.5 million lost workdays and 2 million missed school days each year. Nathan and colleagues also noted that traumatic injuries in the workplace were connected with the use of sedating antihistamines for self-treatment of AR symptoms.

Druss and colleagues reported that health care costs of persons with asthma in the United States were more than $27 billion (1996 dollars), whereas total societal costs (including work loss) of persons with asthma exceeded $31.2 billion. It also seems that lower socioeconomic groups may have a higher prevalence and incidence of asthma than other groups, which places further strain on the health care system.

The enormous economic burden is not limited to the United States. The Global Initiative for Asthma reviewed cost-of-illness studies on asthma and found that asthma costs in developed countries averaged an annual societal burden ranging from $326 to $1315 (1991 US dollars) per afflicted person. Furthermore, approximately 40% to 50% of the total asthma costs were attributed to direct medical expenditures. These cost studies of aeroallergic disease demonstrate the enormous economic burden in both the United States and around the world.

In addition, allergic disease exerts significant indirect costs on society. For example, Bousquet and colleagues reported that increased severity of AR had adverse affects on quality of life—sleep, activities of daily living, and professional performance. Camelo-Nunes and Sole reported that the symptoms of nasal obstruction that occur with AR result in poor sleep quality, which ultimately results in poor cognition, daytime somnolence, reduction in professional performance, and risk for work-related accidents. In a survey involving individuals with AR, 68% of those with perennial AR and 48% of those with seasonal AR reported that the disease interfered with their sleep. The impact of allergic disease would dramatically increase when the disruptions of daily life caused by associated conditions, such as asthma, sinusitis, laryngitis, otitis media, are included in the equation (see Fig. 1 ).

Burden of Aeroallergic Disease

Secondary to the high prevalence of aeroallergic disease, there is a significant economic burden of these disease processes on health care systems and society as a whole. In 1994, the total cost for the care of asthma in the United States was $10.7 billion. Direct medical costs of asthma care (which included hospitalizations, doctors’ visits, and medications) comprised $6 billion and indirect costs (workdays lost, time lost from school, and costs attributed to asthma deaths) were $4.64 billion. In 1996, the cost of allergic rhinitis, in terms of direct medical expenditures, exceeded $3 billion and an additional $4 billion was ascribed to exacerbations of concomitant conditions, such as asthma and otitis media. In addition, more than $6 billion was spent on prescription medications for the treatment of AR in 2000. Indirect costs associated with AR were reported to include 3.5 million lost workdays and 2 million missed school days each year. Nathan and colleagues also noted that traumatic injuries in the workplace were connected with the use of sedating antihistamines for self-treatment of AR symptoms.

Druss and colleagues reported that health care costs of persons with asthma in the United States were more than $27 billion (1996 dollars), whereas total societal costs (including work loss) of persons with asthma exceeded $31.2 billion. It also seems that lower socioeconomic groups may have a higher prevalence and incidence of asthma than other groups, which places further strain on the health care system.

The enormous economic burden is not limited to the United States. The Global Initiative for Asthma reviewed cost-of-illness studies on asthma and found that asthma costs in developed countries averaged an annual societal burden ranging from $326 to $1315 (1991 US dollars) per afflicted person. Furthermore, approximately 40% to 50% of the total asthma costs were attributed to direct medical expenditures. These cost studies of aeroallergic disease demonstrate the enormous economic burden in both the United States and around the world.

In addition, allergic disease exerts significant indirect costs on society. For example, Bousquet and colleagues reported that increased severity of AR had adverse affects on quality of life—sleep, activities of daily living, and professional performance. Camelo-Nunes and Sole reported that the symptoms of nasal obstruction that occur with AR result in poor sleep quality, which ultimately results in poor cognition, daytime somnolence, reduction in professional performance, and risk for work-related accidents. In a survey involving individuals with AR, 68% of those with perennial AR and 48% of those with seasonal AR reported that the disease interfered with their sleep. The impact of allergic disease would dramatically increase when the disruptions of daily life caused by associated conditions, such as asthma, sinusitis, laryngitis, otitis media, are included in the equation (see Fig. 1 ).

Gender and Age Distribution of Aeroallergic Disease

Although men and women share identical mechanisms that activate the immune system, there is a clinical preponderance of women affected with atopic disease. In the National Health Interview Survey, Pleis and colleagues found that women were more likely to have been told they had asthma, hay fever, sinusitis, or chronic bronchitis than were men. In addition, the survey of the burden of allergic disease in the United States found that the prevalence of physician-diagnosed allergic disease was more common in women (34.3%) compared with men (27%). In another large cohort of patients with difficult-to-treat asthma, women had more asthma control problems and lower asthma-related quality of life compared with men ( P <.0001). Asthma triggers, allergic rhinitis, and atopic dermatitis were also more frequently found in female subjects. These gender-related differences have several explanations. One explanation includes the estradiol-receptor–dependent mast-cell activation. Perimenopausal and postmenopausal women have been reported to increasing rates of asthma, wheeze, and AR. This association may indicate that women ultimately become hypersensitive to their own sex hormones. Other explanations include misclassification bias and female preponderance to self-reporting disease.

In contrast to adults, male children seem more frequently affected with asthma or atopic disease than female children. The male-to-female ratio of children who have atopic disease is approximately 1.8:1. In the National Health Interview Survey, boys (17%) were more likely than girls (11%) to have ever been diagnosed with asthma.

It also seems that respiratory allergic disease varies with age, with symptoms peaking in childhood and adolescence with the mean age of onset between the years 8 and 11. A nested case-control study from Tucson, Arizona, found annual incidence of asthma was 1.4% for boys and 0.9% for girls in the 0-year to 4-year age group, 1.0% for boys and 0.7% for girls in the 5-year to 9-year age group, and 0.2% for boys and 0.3% for girls in the 10-year to 14-year age group. Although the incidence of asthma in this study declined with age, the prevalence rates were comparatively constant. The prevalence of AR tends to be lower in children than adults and lower after 65 years of age than in younger adults.

An early age at onset of symptoms may ultimately result in a gradual progression of atopic disease. This phenomenon is referred to as the atopic march. The presumed method of sensitization is via the skin where possible defects in the epidermal barrier lead to later sensitization in the airways. This march is characterized by the development of atopic dermatitis followed by a sequence of food allergy, rhinitis, and/or asthma, where symptoms may develop, subsequently fade away, or persist for several years.

There is some conflicting evidence as to whether or not race plays a significant role in allergic disease. In a study of children without personal or family history of allergic disease and a low socioeconomic class who underwent skin testing, the investigators reported an increased risk among African American children for sensitization to any allergen (odds ratio [OR] 2.17; 95% CI, 1.23–3.84) and sensitization to outdoor allergens (OR 2.96; 95% CI, 1.52–5.74]). In another study, white children were found more likely to have had hay fever (10%) than African American children (8%). In a population-based cohort study, Yang and colleagues concluded that the disparity in allergic sensitization by race could be due to environmental factors rather than genetic differences. Overall, the influence of racial and/or genetic factors is complex and difficult to separate from environmental influences and changes due to emigration.

Despite the impact of environmental influences, there are some genetic links to allergic diseases. Chromosome 5 has been thought to be involved in the regulation of total IgE concentration, whereas chromosome 11q has been associated with the high total IgE concentration or specific IgE. In addition, chromosome 14 has been linked to eczema and specific HLA haplotypes are linked to the development of IgE. Despite the evidence for genetic propensity to allergic disease, it is unlikely that geographic disparities in disease prevalence between those of similar genetic background or the increase in allergic disease over the previous decades can be explained by solely a genetic cause.