Air Pollution

2. How does Ozone (O₃) affect human health?

2.1 Effects of long-term exposure to levels of Ozone observed currently in Europe
2.2 Is Ozone per se responsible for effects on health?
2.3 Are health effects of Ozone influenced by the presence of other air pollutants?
2.4 Characteristics of individuals may influence how Ozone affects their health
2.5 Is there a threshold below which nobody’s health is affected by Ozone?

Adverse health effects have been documented after short-term exposure to ozone (O₃) peaks, as well as following long-term exposure to relatively low concentrations.
Studies have shown that short-term exposure to peak levels of ozone can temporarily affect the lungs, the respiratory tract, and the eyes, and can also increase susceptibility to inhaled allergens. Long term exposure to ozone has primarily been found to reduce lung function. More in question 3.3...

2.1 Which effects can be expected of long-term exposure to levels of ozone observed currently in Europe?

WHO states: "There are few epidemiological studies on the chronic effects of ozone on human health. Incidence of asthma, a decreased lung function growth, lung cancer and total mortality are the main outcomes studied. At levels currently observed in Europe, the evidence linking O₃ exposure to asthma incidence and prevalence in children and adults is not consistent. Available evidence suggests that long-term O₃ exposure reduces lung function growth in children. There is little evidence for an independent long-term O₃ effect on lung cancer or total mortality.

The plausibility of chronic damage to the human lung from prolonged O₃ exposure is supported by the results of a series of chronic animal exposure studies." More...

"Long-term O₃ effects have been studied in two cohort studies. There is little evidence of an independent long-term O₃ effect on mortality so that no major loss of years of life is expected. The issue of harvesting, i.e. the advancement of mortality by only relatively few days, has not been addressed in short-term exposure studies of O₃." More...

Source & © : WHO Europe
"Health Aspects of Air Pollution" (2003) Section 6.2

2.2 Is Ozone per se responsible for effects on health?

Short-term studies have shown independent effects of ozone (O₃) especially in the summer. Independently of the effects of other pollutants, ozone exposure influences pulmonary function, lung inflammation, lung permeability, respiratory symptoms, levels of medication usage, morbidity, and mortality.

The results of epidemiological studies addressing long-term effects of ozone are not entirely consistent. Several studies have used models that take into account other pollutants and their effects. For instance, considering the effect of particle acidity has partly explained effects previously attributed to ozone. A few studies in North America found effects of ozone on asthma incidence and lung function. These effects were independent of the effects of other classical pollutants including particulate matter, but particle acidity was not considered.

Experimental studies show the potential of ozone to cause these health effects. More...

2.3 Are health effects of Ozone influenced by the presence of other air pollutants?

Epidemiological studies show that short-term effects of ozone (O₃) can be enhanced by particulate matter, and vice versa. At higher ozone concentrations, experimental studies show synergistic, additive, or antagonistic effects, depending on the experimental design, but the relevance of this evidence for ambient exposures is unclear. Ozone may facilitate responses to allergens. More...

2.4 Which characteristics of individuals may influence how Ozone affects their health?

Are effects of ozone dependent upon the subjects’ characteristics such as age, gender, underlying disease, smoking status, atopy, education etc? What are the critical characteristics?

How readily individuals respond to ozone (O₃) exposure, and hence experience different ozone related health effects, varies between persons. The reasons for this remain largely unexplained but appear to be partly linked to genetic differences.

There is some evidence that short-term ozone exposure effects on mortality and hospital admissions increase with age. Results on gender differences in responses to ozone exposure are not consistent. It appears that the effects of ozone exposure are greater in asthmatic children compared to general population children or healthy children. For asthmatic children, decreases in lung function have been associated with O₃ exposure in children with low birth weight or premature birth.

One important factor modifying the effect of ozone on lung function is ventilation rate. With deeper breaths, for instance when exercising, ozone penetrates deeper into the lungs.

Duration of exposure is also a critical factor, as ozone effects accumulate over many hours. When the respiratory system is exposed repeatedly over several days, it adapts, leading to a reduction in the functional responses to ozone exposure. However, inflammatory responses to ozone exposure are not reduced.

In children who exercise more or spend more time outdoors, the effects of ozone exposure on lung function, symptoms, and school absences are greater. More...

2.5 Is there a threshold below which nobody’s health is affected by Ozone?

WHO states: "There is little evidence from short-term effect epidemiological studies to suggest a threshold at the population level. It should be noted that many studies have not investigated this issue. Long-term studies on lung function do not indicate a threshold either. However, there may well be different concentration-response curves for individuals in the population, since in controlled human exposure and panel studies there is considerable individual variation in response to O₃ exposure. From human controlled exposure studies, which generally do not include especially sensitive subjects, there is evidence for a threshold for lung damage and inflammation at about 60 to 80 ppb (120-160 mg/m3) for short-term exposure (6.6 hours) with intermittent moderate exercise. Where there are thresholds, they depend on the individual exercise levels." More...

Source & © : WHO Europe
"Health Aspects of Air Pollution" (2003) Section 6.2

See also: General Issues and Recommendations on Air Pollutants:

• question 1.3 on uncertainties in defining thresholds
• question 3.1 recommendations regarding the concept of threshold