By the time you've finished reading this sentence, you've likely taken two breaths and inhaled about two litres of air. In a day, the average person will breathe in about 20,000 litres.
Consider the amount of pollution that air contains and it's easy to imagine how ill you can become from just breathing. It's no surprise, then, that air pollution is a major contributor to asthma, the most common chronic disorder in children.
Almost half of all children have at least one episode of wheezing before six years of age, according to a report which was published in The Lancet to coincide with World Asthma Day last week. Forty-eight per cent of children under five years of age with asthma report an attack in the preceding year, a rate higher than any other age group.
In Hong Kong, more than 330,000 people suffer from asthma, according to 2011 figures from the Hong Kong Thoracic Society. About 8 per cent of primary students aged six to seven years, and 10 per cent of secondary students aged 13 to 14 years, have asthma. Every year, about 70 to 90 people in the city die from asthma attacks; among them, 20 to 30 people are aged between 15 and 44.
Evidence from studies during the past several decades makes it clear that air pollution can exacerbate pre-existing asthma. But recent research suggests that air pollution might cause new-onset asthma as well, say the authors of The Lancet report, professors Michael Guarnieri and John Balmes, of the University of California, San Francisco.
Reviewing studies from the past five years since 2009, the authors show that short-term exposure to ozone, nitrogen dioxide, sulphur dioxide, PM2.5 particulate matter and traffic-related air pollution can increase the risk of exacerbations of asthma. They also found that long-term exposure to air pollution, especially from traffic, can lead to new cases of asthma in adults and children.
A study of 10 European cities published last year in the European Respiratory Journal estimated that 14 per cent of chronic childhood asthma is due to exposure to traffic pollution near busy roads.
"Because many urban centres in the developing world are undergoing rapid population growth accompanied by increased outdoor air pollution, the global burden of asthma is likely to increase," write Guarnieri and Balmes.
"In view of the burden of asthma attributed to outdoor air pollution, better understanding of why asthmatic individuals are susceptible to this exposure should enable the design of effective preventive strategies."
Britain's Committee on the Medical Effects of Air Pollutants proposes four main mechanisms of how pollutants affect asthma: oxidative stress and damage, inflamed pathways, airway remodelling, and enhancement of respiratory sensitisation to aeroallergens.
In a genetically predisposed person, this could result in new-onset asthma or exacerbations of existing asthma.
Some evidence shows that particulate matter causes new cases of asthma, though findings have not been consistent. There is substantial evidence that ambient levels of particular matter exacerbate existing asthma, particularly by contributing to oxidative stress and allergic inﬂammation.
Particulate matter is characterised according to size: coarse (a diameter of 2.5 to 10 micrometres) deposits mainly in the head and large conducting airways. Fine particular matter (PM2.5) deposits throughout the respiratory tract, particularly in small airways and alveoli. Ultrafine PM (smaller than 0.1 micrometres) deposits in the alveoli.
Oxidising gases, such as nitrogen dioxide, ozone and sulphur dioxide, are another component of air pollution.
There have been extensive studies on the short-term controlled exposure to ozone and sulphur dioxide at relevant concentrations.
Ozone exposure results in airway inﬂammation, airway hyper-responsiveness, and reductions in lung function in healthy and asthmatic adults, whereas sulphur dioxide causes more prominent broncho-constriction, especially in asthmatics.
Experimental data on nitrogen dioxide has been inconsistent. But studies of asthmatic children and adults in the past five years have found links between nitrogen dioxide and symptoms of asthma, reduced response to bronchodilators, decrements in lung function, and exacerbation of asthma.
Several studies have identiﬁed an increase in asthma incidence or prevalence associated with exposure to nitrogen dioxide.
A third component of air pollution comes from traffic, which is made up of a complex mixture of particulate matter from combustion and non-combustion sources, as well as primary gaseous emissions, including nitrogen oxides.
These primary emissions lead to the generation of secondary pollutants such as ozone, nitrates, and organic aerosol.
There's much data that suggests exposure to traffic-related air pollution, especially in urban areas, has a tremendous effect on disease morbidity in individuals with asthma.
A growing body of evidence, including several recent prospective studies of children with no asthma at enrolment, also shows it's responsible for cases of incident asthma. These findings from the reviewed studies have a few clinical implications, the authors note.
First, local governments should publicise air quality monitoring data widely and on a daily basis, and issue smog alerts on days when ozone or PM2.5 levels are forecast to be high. People with asthma or other pre-existing cardiopulmonary disorders should be urged to stay indoors on such days.
Second, asthmatic patients should ideally live at least 300 metres from major roadways.
Third, patients should maintain inhaled corticosteroid therapy, which decreases adverse responses to pollutant exposures.
Finally, policy initiatives can incentivise the development of alternatively powered vehicles and renewable electricity production, to help reduce overall pollution and mitigate climate change.
Smart urban growth plans are needed. "Government investment in efficient public transportation systems and support of high-density, energy-efficient housing along transit system corridors will reap long-term ... public health benefits," the authors say.