Are environmental exposures related to asthma?

The air pollutants related to asthma can be divided into indoor air pollutants and outdoor air pollutants. Each will be discussed separately here; they are also discussed in more depth elsewhere in this web page. Click here for more information on indoor air pollutants, and on outdoor air pollutants.

Indoor Air pollutants

Indoor air may contain a complex mixture of potential pollutants. The most important of these pollutants include:
  • environmental tobacco smoke
  • combustion products
  • volatile organic compounds
  • bioallergens
To learn more about indoor air pollutants, click here. Indoor air pollutants have been increasingly recognized as important contributors to childhood asthma. Each pollutant (environmental tobacco smoke, bioallergens, and combustion products) will be discussed separately.

  • Environmental tobacco smoke
    Environmental tobacco smoke (ETS) is also known as second-hand smoke. Any time someone smokes indoors, ETS is produced.

    Chronic ETS exposure worsens asthma symptoms in children with asthma. [11-14]
  • Bioallergens
    Bioallergens are small proteins from a variety of sources that can trigger an immune response in at-risk individuals. The most important bioallergens include:
    • cockroach antigen
    • dust mite antigen
    • molds
    • cat antigen
Bioallergens can exert their effects through a process known as allergic sensitization. After an individual is exposed to sufficient amounts of the allergen over a sufficient period of time, the person’s immune system may react to very small amounts of the antigen or nonspecific stimuli (cold weather, exercise, air pollutants, etc.).

If this exposure continues over time in the lung tissue, this may lead to asthma. The relationship of each important bioallergen with asthma will be discussed separately.
  • Cockroach antigen:
    Acute exposure to cockroach antigen can trigger asthma attacks in sensitive individuals. [15]

    Chronic exposure to this antigen likely raises the risk of allergic sensitization. This in turn may worsen existing asthma [16][17]
  • Dust-mite antigen:
    In sensitized individuals, dust-mite antigen may trigger an asthma attack.

    Several human studies strongly suggest that chronic exposure to dust-mites can cause a person to become sensitized to the antigen. This may then worsen asthma. [16][17]
  • Molds:
    In sensitized children, exposure to molds can trigger asthma attacks. Some of these may be life-threatening. [18][19]

    Some human studies suggest that chronic mold exposure may aggravate asthma symptoms and increase rates of allergic sensitization. [20][21]
  • Cat antigen:
    Acute exposure to cat antigen may trigger an asthma attack in a susceptible individual.

    There is limited human evidence that exposure to cats may increase rates of sensitization to cat antigen in susceptible individuals. [22] However, not all studies support this relationship. [23]
Combustion products

Combustion, or burning, is a process that occurs when a fuel source (wood, gas, coal) is burned. This process uses oxygen and creates various combustion products. Some important combustion products include nitrogen dioxide (NO2) and particulate matter. The relationship between each combustion and asthma will be discussed separately:
  • Nitrogen dioxide:
    Acute, high-level exposure to NO2 may trigger an asthma attack.

    One study suggests that chronic NO2 exposure may increase rates of asthma among school age girls. [24] However, other human studies argue against this association. [25]
  • Particulate matter:
    Acute, high-level exposure to indoor particulate matter may trigger an asthma attack in a susceptible individual.
  • Volatile Organic Compounds
    VOCs are chemicals likely to evaporate into the air at room temperature. Examples include formaldehyde, certain hydrocarbons (compounds made of hydrogen and carbon) and others.

    Acute exposure to volatile organic compounds (VOCs) may trigger an asthma attack in a susceptible individual.

    Several human studies have suggested that chronic exposure to VOCs may cause allergic sensitization. This may then worsen asthma symptoms [26-27]
Outdoor air pollutants

Outdoor air pollution is a complex mixture of several pollutants. The most important of these outdoor air pollutants in the U.S. today are ozone, PM, and SO2. To learn more about outdoor air pollutants, click here. The relationship between each outdoor air pollutant (ozone, particulates, sulfur dioxide [SO2], )and asthma will be discussed separately.
  • Ozone
    Ozone is formed from a chemical reaction between certain volatile organic compounds (VOCs) and vehicle exhaust emissions (nitrogen oxides) in the presence of sunlight and moisture.

    Acute exposure to ground level ozone can trigger an asthma attack in sensitive individuals.

    Increased ozone levels are associated with increased number of children hospitalized for severe asthma attacks. [28]

    Children exposed chronically to ozone also have higher rates of asthma. [29]

    Some studies suggest that chronic exposure to ozone can cause new cases of asthma in children who did not have it previously. [32][34]
  • Respirable particle matter (PM)
    Airborne PM are small particles resulting from incomplete combustion of various fuels as well as physical breakdown of rock, soil, and dust.

    Acute exposure to PM can trigger asthma attacks in sensitive individuals.

    Increased PM levels are associated with increased number of children hospitalized for severe asthma attacks. [28]

    Some studies link chronic PM exposure with worsened asthma symptoms in susceptible individuals. [31-33][38]
  • SO2
    SO2 is a gas released from various industrial processes.

    Acute exposure to SO2 can trigger an asthma attack in sensitive individuals.

    Increased levels of SO2 are associated with increased number of ER visits for asthma. [35]

    One study suggests that chronic exposure to SO2 may cause new cases of asthma in children who did not have it previously. [34]