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Ozone and Your Patients' Health

Health Effects of Ozone in the General Population

Review Key Points

Introduction

Breathing ground-level ozone can result in a number of health effects that are observed in broad segments of the population. Some of these effects include:

  • Induction of respiratory symptoms
  • Decrements in lung function
  • Inflammation of airways

Respiratory symptoms can include:

  • Coughing
  • Throat irritation
  • Pain, burning, or discomfort in the chest when taking a deep breath
  • Chest tightness, wheezing, or shortness of breath

In addition to these effects, evidence from observational studies strongly indicates that higher daily ozone concentrations are associated with increased asthma attacks, increased hospital admissions, increased daily mortality, and other markers of morbidity.  The consistency and coherence of the evidence for effects upon asthmatics suggests that ozone can make asthma symptoms worse and can increase sensitivity to asthma triggers.

Figure2: Pyramid of effects caused by ozoneFigure 2: Pyramid of effects caused by ozone
The relationship between the severity of the effect and the proportion of the population experiencing the effect can be presented as a pyramid.   Many individuals experience the least serious, most common effects shown at the bottom of the pyramid. Fewer individuals experience the more severe effects such as hospitalization or death.

 

This section of the course addresses exposure and health effects issues common to all people.  The next section of the course, Health Effects in Patients with Asthma and Other Chronic Respiratory Disease, addresses those issues specific to people with asthma and other chronic lung disease.

How are people exposed to ozone?

Primary exposure occurs when people breathe ambient air containing ozone. The rate of exposure for a given individual is related to the concentration of ozone in the surrounding air and the amount of air the individual is breathing per minute (minute ventilation).  The cumulative amount of exposure is a function of both the rate and duration of exposure.   

Although ozone concentrations in the outside (ambient) air are generally similar across many locations in a particular airshed, a number of factors can affect ozone concentration in "microenvironments" within the larger airshed (e.g., inside a residence, inside a vehicle, along a roadway). Ozone concentrations indoors typically vary between 20% and 80% of outdoor levels depending upon whether windows are open or closed, air conditioning is used, or other factors such as indoor sources.  People with the greatest cumulative exposure are those heavily exercising outdoors for long periods of time when ozone concentrations are high.  In addition, during exercise people breathe more deeply, and ozone uptake may shift from the upper airways to deeper areas of the respiratory tract, increasing the possibility of adverse health effects.  People with the lowest cumulative exposure are those resting for most of the day in an air-conditioned building with little air turnover. 

Ozone levels may also affect indoor levels of some aldehydes formed as reaction products of ozone with indoor substances (Apte et al 2008).  This provides a potential pathway for people indoors to experience respiratory effects mediated by ozone reaction products.  Further research is needed to test the importance of these exposures on health effects. 

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How does ozone react in the respiratory tract?

Because ozone has limited solubility in water, the upper respiratory tract is not as effective in scrubbing ozone from inhaled air as it is for more water soluble pollutants such as sulfur dioxide (SO2) or chlorine gas (Cl2).  Consequently, the majority of inhaled ozone reaches the lower respiratory tract and dissolves in the thin layer of epithelial lining fluid (ELF) throughout the conducting airways of the lung.

In the lungs, ozone reacts rapidly with a number of biomolecules, particularly those containing thiol or amine groups or unsaturated carbon-carbon bonds.  These reactions and their products are poorly characterized, but it is thought that the ultimate effects of ozone exposure are mediated by free radicals and other oxidant species in the ELF that then react with underlying epithelial cells, with immune cells, and with neural receptors in the airway wall.  In some cases, ozone itself may react directly with these structures.  Several effects with distinct mechanisms occur simultaneously following a short-term ozone exposure and will be described below.

Schematic illustration showing how ozone injures the lining of the respiratory tract.Figure 3: Ozone is highly reactive in the respiratory tract
When breathed into the airways, ozone interacts with proteins and lipids on the surface of cells or present in the lung lining fluid, which decreases in depth from 10 µm in the large airways to 0.2 µm in the alveolar region. Epithelial cells lining the respiratory tract are the main target of ozone and its products. These cells become injured and leak intracellular enzymes such as lactate dehydrogenase into the airway lumen, as well as plasma components. Epithelial cells also release a variety of inflammatory mediators that can attract polymorphonuclear leukocytes (PMNs) into the lung, activate alveolar macrophages, and initiate a train of events leading to lung inflammation. Antioxidants present in cells and lining fluid may protect the epithelial barrier against damage by ozone or its reaction products.
Source: Devlin et al., (1997)

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What are ozone's acute physiological and symptom effects?

The predominant physiological effect of short-term ozone exposure is being unable to inhale to total lung capacity.  Contr