Respiratory health effects
The respiratory tract is the portal of entry of air pollutants, and thus the lung is the first organ affected. The range of respiratory diseases that can be caused by air pollution exposure is large. Studies on the health impacts of air pollution differentiate between acute and chronic effects. The acute effects of pollution may be apparent within hours or days of exposure, but other health effects of air pollution result from long-term exposure, leading to chronic disease. While the acute and chronic effects of air pollution are partly interrelated, the distinction is important when planning and interpreting epidemiological studies as well as for policy making. Table 1 summarises the most important respiratory health effects of air pollution and how they can be measured.
Short-term respiratory effects of air pollution
Several epidemiological studies have shown that the daily number of deaths, mainly from cardiovascular and respiratory diseases, tracks daily fluctuations in air pollution. A seminal European multi-city time-series analysis, APHEA (Air Pollution and Health: A European Approach), carried out in 29 study centres, found an increase of deaths from illness of 0.6% per 10 μg·m-3 increase in daily PM10 concentration, and data from hundreds of cities around the world have shown similar results. Studies on short- term mortality show that in general the air pollution-related relative risk is higher for respiratory outcomes than for cardiovascular ones, but since more people die from cardiovascular diseases, the absolute number of cardiovascular deaths related to air pollution is as large as, or larger than, the number of respiratory deaths attributable to air pollution.
Daily respiratory exacerbations
The daily variation in disease burden due to urban pollution is also shown by increases in the number of emergency hospital visits and admissions due to respiratory diseases, including asthma. The APHEA study reported increases per 10 μg·m-3 change in daily PM10 concentration of: 1.2% for asthma in children; 1.1% for asthma in adults aged up to 64 years; and 0.9% for all respiratory diseases (including chronic obstructive pulmonary disease (COPD), asthma and other respiratory diseases) in the elderly.
Patients with asthma, especially children who are not receiving anti-inflammatory or bronchodilator therapy, suffer more on or after days with higher pollution levels. Because of the large individual day-to-day variation in, and the many concomitant factors that influence, asthma symptoms, effects in asthmatic patients are not easily demonstrable without strict adherence to the study protocol and individualised exposure assessment. However, panel studies (longitudinal studies in which participants repeatedly provide information over some period of time) on asthmatic patients employing such rigorous methods have noted increased wheezing, cough and attacks of breathlessness, accompanied by poorer lung function and the need for additional medication, associated with daily variations in PM, NO2 and/or O3.
Weather influences the daily variation of pollutant concentrations considerably, with both unduly high (such as heatwaves) and low temperatures having consequences for health. Therefore, all studies on the short-term effects of air pollution need to take account of the effects of weather and other factors that vary over time. Modern epidemiological methods enable the effects of such covarying factors to be disentangled from those attributable to the pollutants.