There is increasing evidence that levels of the most common air pollutants (PM, O3, NO2 and SO2) adversely affect the respiratory health of children. These health effects vary from post neonatal respiratory mortality [Bobak, 1992; Bobak, 1999] and respiratory mortality in infants (< 5 years) [Saldiva, 1994; Conceicao, 2001], decreased exercise capacity, increased respiratory symptoms, lung inflammation, increased airway reactivity and decreases in lung functions [ATS, 1995]. Recently, the respiratory health effects of the traffic-related part of the air pollution mixture has become the focus of interest [Brauer, 2002]. These health effects have been shown both due to short-term exposures (daily variations in air pollution levels) and due to long-term exposures to air pollution. Currently the role of particles is receiving attention as these may be a major component of the adverse effects of air pollution.
A large number of epidemiological studies have focussed on the respiratory health effects due to short-term exposures to air pollution in non-symptomatic children and in children with asthma or chronic respiratory symptoms. Overall, there is an association between the level of air pollution and the prevalence of respiratory symptoms in both healthy and symptomatic children, although some studies have shown conflicting results [Roemer et al., 1998]. The relationship between exposure to air pollution and the exacerbation of childhood asthma has been well studied although until recently, relatively few studies have focused on traffic-related air pollution. Although short-term increases in air pollution levels have been associated with acute reductions in lung function and increased reporting of respiratory symptoms in children, including asthmatic symptoms, it is not clear whether these effects occur exclusively in asthmatic children, or whether, they also adversely affect children without underlying respiratory disease.
Information on the health effects of long-term exposures to air pollution is scarce. In a study in the former East Germany, an association between air pollution levels in the city of residence, presence of chronic respiratory (especially bronchitic) symptoms and lung function growth was found [Frye, 2003]. A study conducted in Switzerland [Braun- Fahrlander, 1997] also found increased occurrence of symptoms with increased air pollution levels in children. Several other studies [Ware et al., 1986; Dockery et al., 1989; Dockery et al., 1996; Raizenne et al., 1996; Baldi et al., 1999] have also found increased bronchitic but not asthmatic symptoms in children and lower lung function at higher air pollution levels. Based on a review of datat from several studies Kuenzli et al.  estimated a 10 µg/m3 increase of the long-term average PM10 concentration was associated with a 31% increase in the prevalence of bronchitic symptoms in children.
Studies that have specifically assessed the role of traffic on adverse respiratory health of children suggest that children attending schools located close to major roads and/or living in their vicinity show adverse respiratory health effects. The health effects reported include reduced lung function, higher prevalence of respiratory symptoms including wheeze and higher asthma rates (Wjst et al. 1993; Duhme et al. 1996; Brunekreef et al. 1997). An attempt to quantify this was made in the study by Künzli et al. (Kunzli et al. 2000), who analysed the public health impact of traffic-related air pollution for Switzerland, France and Austria (with a total population of 74 million inhabitants). The authors estimated that in an average year and in the three countries more than 290 000 episodes of bronchitis and more than 160 000 asthma attacks were attributable to exposure to traffic related air pollution
Experimental and epidemiological studies among adults have suggested that antioxidant supplementation could modulate the acute change in lung functions observed among people exposed to photo-oxidants [Chatham et al. 1987, Trenga et al., 2001, Grievink et al. 1997, Grievink et al., 1999, Samet et al., 2001]. There is only one study that has assessed the effect of antioxidant dietary supplementation on decrements in pulmonary function associated with exposure to air pollution in children. Romieu et al.  evaluated whether acute effects of ozone, nitrogen dioxide, and PM10 could be attenuated by antioxidant vitamin supplementation. 158 Children with asthma living in Mexico City were randomly given a daily supplement of vitamins (50 mg/day of vitamin E and 250 mg/day of vitamin C) or a placebo. In children with moderate and severe asthma, ozone levels were inversely associated significantly with lung function in the placebo group, while no association between ozone and lung functions was observed in the supplement group. The results suggest that supplementation with antioxidants might modulate the impact of ozone exposure on the small airways of children with moderate to severe asthma.
Overall, there is sufficient evidence that exposure to ambient air pollution levels has deleterious respiratory health effects in children. Intervention studies have provided strong circumstantial evidence of the health gains from clean air. One of the best examples is a labour dispute that shut down a large steel mill in Utah Valley [Pope, 1989]. Respiratory hospital admissions in children substantially decreased during the strike and increased to pre strike levels after the dispute was ended. In Hong Kong in 1990 a fuel restriction was introduced that required all power plants and road vehicles to use fuel oil with a sulfur content of not more than 0,5 % by weight. Prevalences of bronchial hyperreactivity in children living in different polluted districts declined on average from 25% to 15% after the fuel restriction [Wong et al 1998]. Another example is the study carried out during the 1996 Summer Olympic Games in Atlanta in which the impact of changes in transportation and community behaviours on air quality and childhood asthma was investigated [Friedman et al 2001]. During these games an alternative transportation strategy was implemented resulting in lower traffic emissions. During the period of the Olympic Games a reduction (41.6%) in the number of childhood asthma acute care events was observed. A study with 110 children investigated whether changes in air quality caused by relocation were associated with changes in lung function growth rates [Avol et al 2001]. As a group, subjects who had moved to areas of lower air pollution levels showed increased growth in lung function and subjects who moved to communities with higher air pollution levels showed decreased growth in lung function [Avol et al 2001]. A stronger trend was found for subjects who had migrated at least 3 years before the follow-up visit than for those who had moved in the pervious 1-2 years [Avol et al 2001]. Another example are several studies on the reduction in respiratory health effects in association with the reduced air pollution levels over several years in the former German Democratic Republic. These studies report a decrease in the prevalence of bronchitis [Heinrich et al 2000, Herbarth et al., 2001], a decrease in prevalence of non- allergic respiratory symptoms [Heinrich et al 2002] and an increase in the mean forced capacity and forced expiratory volume in 1 second in children [Frye et al 2003].
Although the evidence for the contribution of air pollution to exacerbations in children with pre-existing asthma is compelling what is less clear is whether such exposures make any contribution to the cause of asthma. A landmark study of this aspect of lung disease in children was undertaken following the German reunification in 1989. This allowed a study of two genetically similar populations exposed to different levels of atmospheric pollution with higher concentrations of industrial pollutants (SO2 and particulates) in Leipzig, East Germany compared with Munich, West Germany where traffic density was higher. The results of this study demonstrated a higher lifetime prevalence of asthma and a greater prevalence of sensitisation to common aeroallergens in the West German population compared with the East German population, suggesting that prolonged exposure to “classical” air pollutants was not associated with the development of asthma or allegy [von Mutius et al, 1992]. Although the epidemiological evidence is inconclusive, there are data to support a possible role for air pollutants as adjuvants in the process of airway sensitisation to common inhalent allergens The results from some recent studies [Salvi, Frew, Holgate 1999; Diaz-Sanchez et al., 1999 and Janssen et al., 2003] suggest that ambient air pollution, in particular diesel-related compounds, may enhance allergic sensitisation or may increase the allergic response among children who are already sensitised to common allergens.