Pathophysiological effects of air pollutants
While experimental studies have shown a range of effects related to single pollutants, it should be emphasised that the effects of ambient air pollution cannot be assigned to a single pollutant in the mixture. As in the case of tobacco smoke, many pollutants act together in a series of partly interrelated mechanisms, which result in the observed associations between levels of air pollution and a range of health outcomes. Oxidative stress and both local and systemic inflammation are suggested to be the main harmful mechanisms set in train following the inhalation of these pollutants. A first step may be the generation of reactive oxygen species in the lung cells (e.g. from contact with the carbon core of inhaled particles where toxic substances such as sulfates, nitrates and metals are adsorbed). PM of various sizes and highly oxidative gases (e.g. O3 or NOx) have been shown to induce local pulmonary reactions related to oxidative stress. Both local and systemic inflammatory reactions, mediated through cytokines and chemokines, have been found in experimental studies in cellular systems and in animals, as well as in exposure-chamber studies with human subjects.
Claims that one specific aspect or constituent of ambient air pollution is ‘more important’ than others need to be interpreted with great caution. Such comparisons are inherently difficult to make, as the effect also depends on the health outcomes under study, the timescale (e.g. acute versus long-term effects) being considered, the underlying susceptibilities of the exposed individuals, and possibly the nature and concentrations of the co-pollutants.
Marker of air pollution | Questionnaire-based respiratory health data |
Objective respiratory markers | Biomarkers of effect |
Health system use |
Acute effects on mortality | Long-term effects on mortality/life expectancy |
---|---|---|---|---|---|---|
Particulate matter |
Respiratory symptoms |
Diminished lung function, impaired growth of lung function | Airway inflammation, chronic lung disease |
Emergency admissions for respiratory diseases, asthma |
Increase in daily mortality | Increased mortality from cardiopulmonary diseases |
Ozone | Respiratory symptoms |
Diminished lung function |
Airway inflammation | Emergency admissions for asthma |
Increase in daily mortality | Suggestive for respiratory death |
Nitrogen oxides |
Respiratory symptoms |
Increased bronchial reactivity, impaired growth of lung function | Airway inflammation alteration in lung immune defences |
Emergency admissions for respiratory diseases |
Increase in daily mortality | Increased long-term mortality from cardiopulmonary diseases |