The following information about asthma and air pollution was originally developed for a healthcare professional audience and was published in Allergy, Rheumatology, Immunology and Asthma (ARIA), a journal produced by National Jewish Health and The Children's Hospital of Denver.
Asthma and Air Pollution: Clearing the Air
By Nathan Rabinovitch MD
What is air pollution?
Air pollution is a mixture of particles and gases in the ambient air,which have been associated with adverse health effects. The main sources of air pollution are fuel combustion by automobiles, powerplants and homes. Because it is difficult to quantify this pollution mixture as a whole, selected components are measured to investigate health effects and for regulatory monitoring. The Environmental Protection Agency (EPA) has designated some of these components as "criteria pollutants" to be regulated under the Clean Air Act. These include gases such as sulfur and nitrogen oxides, carbon monoxide and ozone and respirable particles (known as PM, which stands for particulate mass).
These criteria pollutants have seasonal and geographic tendencies.Because cold air tends to trap air pollutants in the atmosphere and heating increases in the winter months, certain components of air pollution such as PM and sulfur or nitrogen oxides are elevated in the winter months. Ozone, which is formed when sunlight and high temperatures interact with nitrogen oxides in the presence of hydrocarbons, is elevated in the summer months. High levels of sulfate and sulfur dioxide occur principally in the northeastern portion of the United States where there are many coal-burning power plants. Ozone is a particular problem in temperate parts of the US, which receive abundant sunlight (e.g. Los Angeles and its vicinity).
Air pollution and health: Mechanisms
The mechanism involved in air pollution's health effects is unknown.Most of the large epidemiological studies point to PM as the component of air pollution best associated with health effects. Yet chamber studies in which individuals inhale particulates in a closed chamber show little if any changes in any health outcomes. It is unclear if this is because PM is acting as a marker for some other bioactive component of air pollution or if the interaction of some or all of the components of air pollution is crucial. Although most studies point to PM alone, some studies have pointed to sulfates in conjunction with PM as being an essential component involved in adverse health effects. Because asthmatics may be more susceptible to the well-described bronchoconstrictive effects of SO2, this may partly explain why asthmatics are a susceptible population.
Ozone is the only component of air pollution for which a pathophysiological and inflammatory mechanism has been well described. Ozone at ambient levels can result in bronchoconstiction and chest tightness although this response tends to decrease with repeated exposures. Ozone has also been shown to increase nonspecific bronchial hyperactivity and exposure to ozone increases allergen responsiveness in the lungs of asthmatics. Ozone exposure results in neutrophil migration into the airways although it is unclear if this inflammation is involved in the pathophysiological response.
Asthma and health effects from air pollution
An important question under investigation in the asthma community is why asthma prevalence is higher in industrially advanced societies and urbanized settings. One hypothesis is that air pollution is the cause. In recent years, published studies are not definitive regarding this association. For example, at the time of reunification, the asthma prevalence in East Germany was significantly less than in West Germany even though East Germans were exposed to higher levels of air pollution through industrial sources. This study suggested that air pollution was not associated with asthma prevalence. Although others have commented that the particular type of air pollution mixturein East Germany was not the same as "Western Air Pollution" (because it was not principally derived from automobiles), there continues to be little evidence that asthma prevalence is due to air pollution. On the other hand, there are a number of studies that demonstrate lower mean pulmonary function in populations residing in high pollution areas compared to low pollution areas. Additionally there have been multiple studies that have demonstrated acute increases in asthma symptoms in association with spikes in air pollution. Therefore, although air pollution has not been shown to play a role in increased asthma prevalence, it may play a role in asthma morbidity in those that already have the disease. Thus, air pollution may be implicated in the increased asthma severity found among asthmatic children in urban settings where air pollution exposure is high. Further investigations among this population are needed to examine this possibility.
Recommendations regarding air pollution and asthma
Before making specific recommendations for an asthmatic that believes he/she is being triggered by air pollution, the clinician should be aware that simply urging these patients to stay indoors during high pollution days may be a difficult strategy to implement and in some cases may be counterproductive. First, it is difficult for the patient to know when air pollutant levels are high because few areas provide access to real-time exposure data. In Denver, the media will announce "blue" (low pollution) and "red" (high pollution) days, however, these advisories are based on meteorological forecasts that do not correlate well with actual pollution levels. Additionally, although it is true that outdoor pollutants are found at lower levels indoors, patients who have indoor sources of pollutants including pet dander and especially environmental tobacco smoke will be exposed to dramatically higher levels of respirable particulate indoors compared to outdoors. Therefore, a full environmental history including a thorough investigation of indoor pollution sources is appropriate. If the indoor environment is clean, then refraining from exercising outside on high pollution days may be indicated since this increases minute ventilation and decreases nasal filtering of polluted air. Because there is some data suggesting that asthmatics on inhaled steroids are at less risk from air pollution, effective use of inhaled steroids may be the most efficient strategy for many patients.
Conclusion: The role of air pollution in asthma
The role of air pollution in asthma is not yet clearly defined. Large epidemiological studies consistently show increased morbidity and mortality associated with exposure to air pollution. Although, air pollution probably is not a factor in asthma prevalence, it may be involved in increased asthma severity that is seen in some urban settings. This may be due to chronic exposures leading to lower pulmonary function and acute exposures that may lead to acute exacerbations in association with other triggers. Further studies are needed to identify the mechanism by which air pollution may affect asthmatics and which subsets of asthmatics are most susceptible to the effects of air pollution.
This information has been approved by Nathan Rabinovitch, MD (January 2006). Original publication Spring 1991; updated January 2006.
