Air Quality Guidelines Global Update 2005 - World Health ...

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AIR QUALITY GUIDELINES
in patients with COPD: chronic bronchitis, emphysema or acinar enlargement,
and narrowing of small, distal airways (66). The pathophysiology of these disease
types may confer different susceptibilities to the effects of air pollutant exposure.
However, the information available for analysis in epidemiological studies (generally
death or a diagnosis on discharge) does not permit distinction between the
types of disease.
Smoking is by far the most important etiological risk factor for COPD. Occupational
exposures also contribute. Genetic deficiency of the α-1 protease inhibitor
enzyme is another proven, but rare, cause. Other postulated risk factors
include increased airways responsiveness to nonspecific stimuli, asthma, childhood
respiratory infections and air pollution (66).
The mechanisms by which particle exposure may cause adverse effects in patients
with COPD have not been determined. However, insights may be gained
by examining the factors that contribute to the frequent occurrence of exacerbations
in COPD. One of the most important of these is infection. The airways of
patients with chronic bronchitis are often colonized with microorganisms such as
Haemophilus influenzae and Moraxella catarrhalis. These organisms rarely cause
respiratory infections in healthy adults, but frequently participate in the worsening
of COPD and chronic bronchitis, evidence that host defence mechanisms are
impaired. Mucociliary clearance is slowed, leading to retention of secretions and
bacteria. Exposure to particles and sulfur dioxide could increase susceptibility to
infectious complications of COPD by further impairing mucociliary clearance,
increasing adhesion of bacteria to epithelial cells, altering natural host resistance
factors in epithelial cells or mucus as a consequence of epithelial injury, impairing
the function of cells that fight infection in the lung, or impairing specific or
nonspecific functions of the immune system. Exposure to ozone appears to alter
particle distribution in the lung (67), which may further increase the dose of
particles to susceptible lung units. Such effects could also increase susceptibility
to respiratory viral infections, important contributors to declining lung function
and death in patients with COPD.
In a recent study in Los Angeles (68), 13 elderly patients with COPD and 6
healthy elderly people were exposed for two hours to 200 μg/m 3 concentrated
ambient fine particles. Neither group showed effects on lung function. There
were small reductions in arterial oxygenation, but no effects on heart arrhythmias
or HRV. In this study, those with COPD did not appear to be more susceptible
than the healthy subjects.
Asthma
In contrast to COPD, asthma is often a disease of the young; the incidence is
highest in the first 10 years of life. It is a very common condition, affecting
some 6–9% of the United States population. The hallmark features of asthma
are reversible airway obstruction, hyperresponsiveness and inflammation. A