What are the principal outdoor factors that affect asthma?
Certain outdoor environmental factors are thought to contribute to making asthma worse in people who already have the disease. Sometimes this is called exacerbation of asthma. The term "trigger" refers to something that contributes to an asthma attack in a person who already has asthma. Not all asthmatics respond to all triggers.
In the outdoor environment, both air pollutants (1) and biological allergens (pollens, fungus, molds) can make asthma worse. Pollutants include ozone, particulate matter, sulfur dioxide, carbon monoxide, and nitrogen dioxide. In addition, the World Health Organization identified traffic pollution as a causative influence for asthma (2), and diesel exhaust has also been identified as a possible concern (1, 3).
Biological allergens include pollens, molds, and fungi (4). Weather conditions including thunderstorms may particularly affect asthmatics (5, 6).
It is less certain what causes the onset of asthma in the first place. There is some evidence that exposure to ozone and biological allergens (particularly certain types of pollens) may contribute to the initial development of asthma, but there is no consensus in the scientific community about this. Environmental factors may contribute to the onset of asthma, probably in conjunction with other factors including genetic factors.
What Air Pollutants Are Thought to Exacerbate Asthma?
Many studies have reported that higher concentrations of air pollution are associated with higher frequency of asthma reported several ways -- as emergency room visits (7), hospitalizations or hospital discharges, use of medication, increased frequency of symptoms, increased visits to doctors' offices, or increased number of days of school or work missed. Decreased lung function is also reported in some studies.
The results of the studies are not completely consistent, in that not all studies find that the same pollutants are associated with the same effects. Some studies find effects of particulate matter; while other find effects of ozone. Moreover, some studies find no effect. Below, only positive studies are cited.
Particles in the air are often referred to as particulate matter or as total suspended particulates. Different studies measure studies of different sizes. "PM 10" refers to particles 10 microns or less in diameter, while "PM 2.5" refers to particles 2.5 microns or less.
Times Series Studies -- Many studies have reported higher frequency of asthma on or shortly after days when particulate matter in the air is high, including increased emergency room visits for adults in Seattle (8, 9) and California (10) and for children (11-13);
Panel Studies -- Studies that follow groups of people over time have reported increased symptoms or severity of asthma to be associated with higher exposures to particulate matter in adults (14), African-American children in Los Angeles (15), children in Southern California (16), fifth and sixth grade students in Utah (17), children in Vancouver (18), children in Seattle (19), and children in Mexico City (20). Some studies have found reduced lung function in children (20, 21) and in children over 15 (22), and particular susceptibility for asthmatic children (18).
Long-term Studies -- A few studies have reported an association between long-term exposures to PM 2.5 and increased severity or symptoms of asthma in adults (23) and in children (24).
Ozone is a gas that is formed from chemical reactions in the lower atmosphere in the presence of light. It is the pollutant most associated with "smog." Ozone at ground level causes adverse health effects, while ozone in the upper atmosphere protects the earth from radiation from the sun.
Times series studies -- Many studies have reported higher frequency of asthma on or shortly after days when ozone concentrations in the air are high, including increased emergency room visits for adults in Seattle (9) and for children (13, 25, 26) ; increased medication use for more severely asthmatic children (15); and increased visits to doctors' offices (27) .
Panel studies -- have reported higher exposures of ozone to be associated with diminished lung function in children over 15 (22) and increased severity of asthma symptoms for children in the US (16, 28), African-American children in Atlanta (29), children in Australia (30), and children in Mexico City (20). Effects in older children in San Diego were seen particularly when personal exposure to ozone was measured (21, 31). Asthmatics may be considered a susceptible population for effects of ozone (32)
Sulfur dioxide is a gaseous pollutant for which the primary sources are related to combustion.
On or shortly after days when particulate matter in the air was high, studies reported increased emergency room visits (11) ; and increased visits to doctors' offices (27) for asthma.
On or shortly after days when nitrogen dioxide in the air is high, studies have reported increased hospitalization or hospital discharges (33) and increased symptoms in African-American children (15). Some studies have looked at lung function and found that reduced lung function is associated with exposures to higher concentrations of particulate matter. Studies looking at long-term exposures to nitrogen dioxide have reported an association with increased severity or symptoms of asthma in adults (23) and children (24).
On or shortly after days when carbon monoxide in the air is high, studies have reported increased emergency room visits for adults (9) and children (12) and increased asthma symptoms in children (19). A study of long-term exposures reported an association with increased severity of asthma (23).
What Biological Allergens Exacerbate Asthma?
The outdoor biological allergens that are thought to exacerbate asthma are pollens, molds, and fungus
Pollens are released by plants and vary geographically according to the type of vegetation. Pollens are typically monitored by capture in traps placed on roofs, then identification and counting of pollen particles under a microscope (34). Improvements in both collection methods to collect a higher percentage of particles and of identification and counting methods, to better ascertain the amount of allergen contained in the sample, would improve the value of monitoring for prediction of impacts on asthmatics (34). Studies of the relationship between pollen exposures and asthma generally look at either the relationship between sensitization as determined by a skin test and the presence of asthma or exposure to pollen and asthma symptoms (34). The pollens typically identified may not represent those most important to exacerbation of asthma (35)
Higher exposure to certain molds (Alternaria and Cladosporium) was associated with increased symptoms in a panel of African-American children in Los Angeles (15)
A large study of outdoor concentrations of pollens and fungus, as well as air pollutants, found that emergency room visits were associated with higher concentrations of certain kinds of fungus but not pollens (36). A panel study of a few children found that higher concentrations of fungus, but not pollens, were associated with increases severity of symptoms in asthmatic children (4, 31), increased use of medication (4), and reduced lung function (4)
There is some evidence that the combination of air pollutants and bio allergens may interact to increase the effect on asthmatics. One study reported that exposure to pollens and air pollutants led to heightened response for ozone but not nitrogen dioxide (37).
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