image from Stuart B at Creative Commons
Data from the US dominate this discussion because global data are sparse and often not very recent. We acknowledge that asthma is a worldwide issue and present data that we can find from reliable sources.
Asthma is a chronic lung disease that affects hundreds of millions of people—estimates are between 223 and 334 million worldwide.1 Asthma can cause substantial burden to people, often leading to a reduced quality of life due to both physical effects and also psychological and social effects. Environment plays a significant role in both the development of asthma and as triggers of asthma episodes, sometimes called asthma attacks.
Asthma is a growing burden in low- and middle-income countries which previously had low levels, and where most asthma-related deaths occur.2 Asthma in some wealthier countries has remained static or declined, but in others including the US prevalence has been increasing for unknown reasons, although environmental factors likely play a role.3
Asthma has a global distribution with a relatively higher burden of disease in Australia
and New Zealand, some countries in Africa, the Middle East and South America, and northwestern Europe. A 2014 report concluded that asthma symptoms became more common in children from 1993 to 2003 in many low- and middle-income countries which previously had low levels. However, in most countries where the rates are highest, the prevalence of asthma changed little and even declined in a few countries.4
In the United States, about 24 million people, including about one in ten children, are affected.5 Researchers estimated that in 2007 asthma cost the US about $56 billion in medical expsneses, lost school and work days, and early deaths.6 According to the American Lung Association, asthma is one of the most common chronic disorders in childhood and a leading cause of hospitalization in children, as well as one of the leading causes of school absenteeism.7
Rates of asthma in the US are increasing, according to a 2012 report from the National Center for Health Statistics.9 In 2014, asthma affected 7.7 percent of the US population, about 24 million people. The rates were higher among children under 18 overall (8.6 percent) than in adults (7.4 percent). Females (9 percent) in the US experience asthma at greater rates than males (6.3 percent).10
In the US, racial and socioeconomic disparities are dramatic. Asthma prevalence is higher among persons with family income below the poverty level.11 Compared to White children, Black children have even higher levels of asthma (13.4 percent), worse asthma, and worse management of asthma by their physicians. Racial disparities, with increased asthma and hospital visits for asthma, are evident with increased risk for Blacks compared to Whites even after controlling for factors such as economic status. The disparities hold true across economic strata and in urban as well as rural communities.12 Higher exposures to risk factors for asthma and lack of comprehensive asthma management may contribute to disparities.
Asthma is not a single disease; its origins can differ from one person to another. Typically, people with asthma experience recurrent attacks of breathlessness, wheezing, chest tightness and coughing. The underlying process includes inflammation of the airways, reversible obstruction of the flow of air in and out of the lungs, and the tendency of the airways to overreact to stimuli. People with asthma have similar symptoms, but the origins and triggers of the disease may differ considerably from person to person.
Most commonly, asthma is a multifactorial disease, although in some individuals a single factor may be predominantly responsible for its onset—for example, an exposure to a chemical airway sensitizer like formaldehyde14 or an isocyanate.15 Secondhand smoke alone is also a risk factor for new cases of asthma in preschool-aged children. After asthma develops, various exposures can trigger or exacerbate an asthmatic episode.
When we regard environmental factors in asthma, we consider both causes of asthma and triggers for asthma episodes or attacks. Causes of asthma (substances or conditions that contribute to the development of asthma) can differ from triggers of asthma (those things that cause an asthma attack, or make it worse), but sometimes they overlap. For example, tobacco smoke and traffic-related air pollution can both cause the onset of asthma in people who previously did not have the disorder as well as trigger asthma attacks in people already diagnosed.
In addition to tobacco smoke and traffic-related air pollution as mentioned, other exposures that may cause the onset of asthma, but for which the evidence is highly suggestive but less well established, include allergens from dust mites, rodents, cockroaches, pets and mold, as well as psychosocial stress.
Many substances, called asthmagens, are generally accepted as causing asthma. Asthmagens may be encountered in the workplace, at home, at school or elsewhere. Examples:
A sensitizer is a "a chemical that causes a substantial proportion of exposed people or animals to develop an allergic reaction in normal tissue after repeated exposure to the chemical."16
These asthma allergens and sensitizers are listed in CHE's Toxicant and Disease Database:
- formaldehyde, commonly found in certain pressed-wood furnishings and building materials such as cabinets and other consumer goods17
- chlorine bleach
- certain cleaners and disinfectants
- latex rubber
- grain and flour dust
- wood dusts
A more complete list of asthmagens is available from the Association of Occupational and Environmental Clinics.
image from Joshua Miller at Creative Commons
Although many chemicals shown to cause asthma in workers have not been studied in children, it is likely that they can also cause asthma in children. For example, asthma onset was more common in children living in trailers contaminated with high levels of formaldehyde after hurricane Katrina than in children whose exposures were lower. Working parents can also bring exposures home to children on clothing and in other ways, and so parents and pediatricians of children with asthma should consider occupational exposures of parents when assessing causes and triggers.
Traffic Related Air Pollution (TRAP) and Asthma
Short-term traffic exposures thought to increase the risk of exacerbated asthma symptoms include these:
image from Luc at Creative Commons
- nitrogen dioxide
- sulfur dioxide
- particulate matter (PM2.5)
- TRAP (traffic-related air pollution, a complex mixture of particulate matter derived from combustion, non-combustion and primary gaseous sources)18
Increasing evidence suggests that long-term exposures to air pollution, especially TRAP and its surrogate, nitrogen dioxide, can contribute to new-onset asthma in both children and adults.19 Components of diesel exhaust may also cause asthma,20 shown by studies finding that children growing up along streets with heavy truck traffic are more likely to develop asthma-related respiratory symptoms.21
Climate Change, Outdoor Air Pollutants and Asthma
As carbon dioxide (CO2) levels rise and temperatures increase due to climate change, both ground-level ozone and airborne pollen levels are increasing. The combination of higher levels of asthma-related air pollutants associated with changes in atmospheric conditions is expected to continue to increase the frequency of asthma attacks in people with asthma. These conditions may also increase the prevalence of asthma.
Effects of climate change on asthma:
- Increases in levels of ozone and fine particulate matter can trigger inflammation of the lungs and reduce lung function cause chest pain and coughing.22
- Increasing carbon dioxide concentrations affect the timing of allergen distribution, amplifying the allergenicity of pollen and mold spores.23
- Longer growing seasons for allergens will produce more airborne allergens and could lead to more asthma attacks worldwide, including for 10 million Americans with allergic asthma.24
- Increasing precipitation due to climate change can increase mold spores, an asthma trigger.25
- Increasing frequency of droughts can increase dust and particulate matter.26
- Increasing wildfire activity due to global temperature changes could exacerbate asthma and increase asthma ED and hospital visits.27
Family and Community Stressors
Family and community stressors such as financial problems, divorce, and exposure to violence at home or in the community can make children more susceptible to many health problems, including asthma.
Stress can add to and even magnify the impacts of exposure to other environmental conditions that foster the onset or increase the severity of asthma. For example, children in relatively low-income families and exposed to traffic-related air pollution are at greater risk of frequent asthma symptoms than children in the same neighborhoodwhose families are financially better off.28
An attack in someone who has asthma can be triggered or set off by a number of different variables, including these:
These asthma irritants are listed in CHE's Toxicant and Disease Database unless noted otherwise:
- indoor air pollutants such as tobacco smoke
- other environmental chemicals including pesticides
- allergens including mold, pollen, cockroach droppings and pet dander
- outdoor air pollution
- exercise and cold weather.
Triggers vary from one person to another.
Interactions among these different exposures may be important in the development of asthma. For example, a 2003 study found that asthma symptoms in children ill with a respiratory virus are likely to be more severe if they are exposed to the air pollutant nitrogen dioxide, even at levels of nitrogen dioxide below current air quality standards.35 In a study from 2011, combining exposure to low levels of pollen with exposure to levels of pollutants commonly found in urban air dramatically worsened asthma symptoms.36
Gene-environment Interaction: Genetic Factors Increase Susceptibility to Air Pollution
Genes can influence individuals’ susceptibility to environmental toxicants. For example, glutathione S-transferase (GST) and epoxide hydrolase (EPHX1) are enzymes involved in detoxification and elimination of chemicals like polycyclic aromatic hydrocarbons in tobacco smoke and traffic exhaust that can play a role in the development of asthma. Certain genetic variants in GST and EPHX1 are individually associated with increased risk of developing asthma.37 A 2007 study found a nine-fold increase risk for lifetime asthma among those with in the high risk group of GST, EPHX1, and in proximity to a major roadway.38
Obesity increases the risk of developing asthma, worsens asthma symptoms and leads to poor asthma control. Maternal obesity during pregnancy is also associated with increased risk for asthma in the offspring.40
Certain environmental exposures and conditions can protect against onset or symptoms:
- Exposure to microbial diversity in the perinatal period may diminish the development of asthma symptoms.41
- Breastfed infants are less likely to develop asthma and allergies compared to those fed infant formula.42 Breastfeeding enhances immune function.
- Higher vitamin D intake during pregnancy is associated with decreased risk of wheeze in early childhood. Reduced risk of wheezing may be due to reduced frequency of respiratory infections.43
- A healthy microbiome full of good bacteria and other microbes promotes the immune system’s ability to fight off pathogens. Broad exposure to a wealth of non-pathogenic microorganisms early in life is associated with protection against many health conditions. Studies have found that first-year infants exposed to house dust high in levels of mouse, cockroach, or cat allergens along with a variety of bacteria report significantly lower rates of allergies and recurrent wheeze at age three.44
Strategies known or likely to prevent the onset of asthma (primary prevention) include these:
image from Brandon at Creative Commons
- eliminating exposure to tobacco smoke
- reducing indoor and outdoor air pollution
- reducing occupational exposures to asthmagens
- reducing obesity
- improving maternal health
- encouraging breastfeeding
- stress reduction
These are likely to be more effective when addressed in combination.
Decreasing vehicle use and utilizing alternative transportation options, such as walking and biking, will promote healthier lifestyles as well as benefit our air quality. However, with greater outdoor activity there is a potential increase of exposure to harmful outdoor air pollutants in urban areas, so reducing traffic-related and industrial air pollution is critical. If possible, major roadways with heavy traffic should be located away from walking and biking routes, homes and schools. See the Built Environment webpage for more information.
See more about environmental contributors to asthma in the list of CHE publications and Dig Deeper resources in the right sidebar.
This page was last revised by student intern Jessica Hale with Maria Valenti and Ted Schettler, MD, MPH, and editing and graphics by Nancy Hepp, in February 2017. Read more about Jessica's internship, including her review summary.
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