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Respiratory Diseases in Children

4. Can outdoor air pollution contribute to respiratory diseases in children?

  • 4.1 Which pollutants are present in outdoor air?
  • 4.2 What respiratory diseases can outdoor pollution lead to?
  • 4.3 Does exposure to pollen lead to respiratory allergies?

The source document for this Digest states:

There is clear evidence that atmospheric pollution is associated with troublesome respiratory symptoms in children but what is less clear is whether specific pollutants have a causal role in the pathogenesis of respiratory diseases. The principal pollutants of the external (outdoor) environment include nitrogen oxides (NO, NO2), ozone, sulphur dioxide (SO2) and particulates. The well documented rise in the prevalence of asthma in industrialised countries has coincided with a general increase in the density of road traffic in the majority of these countries. Therefore a number of studies have investigated the possibility that traffic pollution is an exposure that is associated with respiratory health in children. The following section gives an overview of the current situation and the trends in ambient air pollution in Europe over the recent years for a number of key pollutants (particulate matter, ozone, sulphur dioxide, nitrogen dioxide) and the emissions from road traffic and industry (7.2). This is followed by a description of the respiratory health effects associated with these ambient air pollutants (7.3). Lastly, the respiratory health effects of exposure to pollen are briefly addressed (7.4).

Source & ©: EU   "Baseline Report on Respiratory Health" in the framework of the European Environment and Health Strategy (COM(2003)338 final), Section 7.1

4.1 Which pollutants are present in outdoor air?

The source document for this Digest states:

Particulate matter (PM) Concentrations of particulate matter with a cut off size of 10 µm (PM10) in ambient air show a downward tendency between 1996 and 2000.

Violations of currently recommended PM10 limit values are widespread in urban areas in Europe, as well in rural areas in some countries [Larsen 2003]. The new EU annual average of 40 µg/m3 that is to be introduced from 2005 has been exceeded at several traffic hot-spots and may de difficult to achieve at such locations. The limit value for average 24-hour concentrations (50 µg/m3 not to be exceeded more than 35 times in a year) was exceeded also at a number of urban and rural background stations. An increase of the PM10 concentration from rural to urban to traffic hot-spot areas is evident.

The situation in Russia is likewise of concern: in 1998, 30% of Russian cities did not comply with the World Health Organisation limits for particulate matter [EEA 2003]. The available data for other PM categories such as PM2.5, PM1.0 and the number of ultra fine particles, are still too scarce to draw firm conclusions.

Ozone (O3) For ground level ozone the tendency is towards an increasing level for annual average concentrations and a stable level for short term peak concentrations.

Violations of the ozone target value are widespread in Europe. Of 1207 stations recorded in AirBase, 275 stations in 12 countries measured levels in excess of the target value, and another 371 had levels above an upper classification level of 100 µg/m3 (as 26th highest daily 8-hour value). Violations occur mainly in South European countries, as well as in Central and Eastern Europe (Switzerland, Austria, South and East Germany, Czech Republic, Slovakia and Poland) [Larsen 2003].

Nitrogen Dioxide (NO2) NO2 shows in Western Europe a downward trend between 1996 and 2000, both for annual and the short-term peak concentrations. The large difference in concentrations from rural to urban to hot-spot is clear. The annual EU limit value is mostly exceeded at traffic sites, and to a lesser extent at some urban background sites. [Larsen 2003].

Sulphur Dioxide (SO2) The urban and local SO2 concentration continues to decrease in most areas in Western Europe, but elevated 24-hour average concentrations of SO2 still represents an air pollution problem in some cities and locations in Europe. In some industrial cities the daily SO2 concentration is still high even at urban background locations. At some hot- spot (industrial type) stations violations of the limit value for 24-hour average occur [Larsen 2002].

Traffic-related air pollution NO2, PM10 and PM2.5 are often used as indicators for traffic-related air pollution. In a study in Amsterdam, outdoor levels of these pollutants were 15-22% higher at homes located in high traffic intensity streets compared to low traffic homes [Fischer et al., 2000]. A substantially larger contrast (about a factor 2) was found for components of the particulate matter like polycyclic aromatic hydrocarbons and soot, and gas-phase components like benzene. Difference of a similar magnitude were also found in the indoor air of these homes.

Recent studies in a number of European cities have shown that the foreseen limit value for benzene of the EC directive (5 µg/m3 by 2010) is likely to be met in most of Europe except in hot-spot locations and in Eastern and Southern European cities [Jantunen et al., 1998, MACBETH and RESOLUTION projects].

Industry-related air pollution The concentration levels of industry-related air pollution depends on the type of industry, the level of process technology and emission control, as well as the emission conditions. Sulphur dioxide, particulate matter and heavy metals are the most significant pollutants emitted from industrial sources. In most of the EU and EFTA countries, efforts to clean up industrial emissions have substantially reduced such problems, but industrial areas with air pollution exposure exceeding WHO-AQGs still exist. High levels of SO2 exposure occur, for example, in Zlatna and Baia Mare (Romania), Asenovgrad (Bulgaria), Sokolov and Teplice regions (Czech Republic) and in Toru’n (Poland) with annual averages around 500 µg/m3. Non-ferrous metal (cadmium and aluminium) industries and coal-fired power plants are those industries most often responsible for very high local industrial pollution in Europe [EEA, 1995]. PM10 levels in adjacent residential areas near certain types of industrial installations can exceed the urban background by a factor of 2. The areas with elevated concentrations can reach one to several square kilometres. Although the size of the population exposed is limited, the total population in Europe affected may be significant in number since there are several hundreds of such installations all around Europe [CAFE WG on PM 2003].

Source & ©: EU   "Baseline Report on Respiratory Health" in the framework of the European Environment and Health Strategy (COM(2003)338 final), Section 7.2

4.2 What respiratory diseases can outdoor pollution lead to?

The source document for this Digest states:

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. [2000] 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. [2002] 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.

Source & ©: EU   "Baseline Report on Respiratory Health" in the framework of the European Environment and Health Strategy (COM(2003)338 final), Section 7.3

4.3 Does exposure to pollen lead to respiratory allergies?

The source document for this Digest states:

Specific aeroallergens released from pollen cause hypersensitivity and lead to the allergic diseases rhinoconjunctivitis and urticaria. In Europe pollen allergens may account for 10-20% of allergic disease, particularly rhinoconjunctivitis [D’Amato 1998] although their role in the causation of asthma is unclear. The main allergies are to grasses and birch pollens in Northern and Central Europe, ragweed in Central and Eastern Europe and olive and cypresses in Southern Europe. The burden of IgE- mediated allergic diseases is related to the length of the pollen season, the total pollen counts and the number and level of the pollen peaks and allergen bioavailability [Huynen and Menne, 2003].

Ambient air pollution may increase the prevalence of pollen allergies in highly polluted areas [Ishizaki 1987]. However, no direct correlation has been observed among pollen release, emission peaks of NOx, SO2 and atmospheric fine dust [Behrendt 1991; Ring 2001]. In a recent study, no additional pro-inflammatory effect of air pollution among children was observed in the pollen season, which suggests that air pollution does not facilitate allergen-induced inflammatory responses [Steerenberg et al., 2003]. There is a growing evidence that global climate change as changing temperature and precipitation might facilitate the geographical spread of particular plant species to new areas which become climatically suitable. Warming is likely to further cause an earlier onset and may extend the duration of flowering and pollen season for some species. The impact of the climate change on the spatial distribution of the incidence and on the severity of allergic disorders is uncertain [Huynen and Menne, 2003].

Source & ©: EU   "Baseline Report on Respiratory Health" in the framework of the European Environment and Health Strategy (COM(2003)338 final), Section 7.4


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