Introduction
Air pollution is a mixture of solid particles and gases in the air. The six common and harmful air pollutants are particulate matter, ground-level ozone, carbon monoxide, sulfur oxides, nitrogen oxides, and lead; of these, particle pollution and ground-level ozone are the most widespread health threats [1, 2]. Because of their importance, a brief summary about these common pollutants, also known as criteria air pollutants, is provided here: particulate matter or PM consists of a heterogeneous mixture of very small particles and liquid droplets suspended in air. The size of particles in PM is directly linked to their potential to cause health problems. Particles with diameter 10 m are the particles that generally pass through the throat and nose and enter the lungs. Then, they can affect various body organs, especially the heart and lungs, and may cause serious health effects. Based on particle size, particle pollution is grouped into: a) “inhalable coarse particles” which have a diameter of 2.5 m to 10 m, and are found near roadways and industries, and b) “fine particles” < 2.5 m in diameter such as those found in smoke and haze; they can form when gases emitted from power plants, industries and automobiles react in the air. Ozone (O3) is a gas composed of three oxygen atoms. In the presence of sunlight, it is created at ground level by a chemical reaction between oxides of nitrogen and volatile organic compounds. Ozone might have harmful effects when formed in the earth’s lower atmosphere, i.e. at ground level. Hot weather and sunlight cause ground-level ozone to form in harmful concentrations in the air. Carbon monoxide (CO) is an odourless, colourless gas formed by incomplete carbon combustion. It is mainly emitted from motor vehicle exhaust, followed by non-road engines such as construction equipment, industrial processes and wood burning. The increasing number of cars has an important role in the increase in CO emission worldwide. Sulfur dioxide (SO2) is a gas formed when fuel containing sulfur, such as coal and oil, is burned, and when gasoline is extracted from oil or metals are extracted from ore. Nitrogen oxides (NOx) are a group of highly reactive gases containing various levels of nitrogen and oxygen. Lead is usually emitted from motor vehicles and industrial sources [2, 3]. Other stationary sources are waste incinerators, utilities, and lead-acid battery manufacturers. In addition to exposure to lead in air, other major exposure pathways include ingestion of lead in drinking water and lead-contaminated food as well as incidental ingestion of lead-contaminated soil and dust. Lead-based paint remains a major exposure pathway in older homes. Some toys might contain considerable amounts of lead that would be harmful for children’s health [1, 4].
Two types of air quality standards are considered: primary standards set limits to protect public health, including the health of “sensitive” populations such as asthmatics, children, and the elderly. Secondary standards set limits to protect public welfare, including protection against decreased visibility, damage to animals, crops, vegetation, and buildings. Numerous scientific studies have linked particle pollution exposure to a variety of health problems, including: increased respiratory symptoms, such as irritation of the airways, cough, difficult breathing, decreased lung function, trigger of asthma, chronic bronchitis, arrhythmias, heart attacks, premature death in people with cardiovascular or respiratory diseases, cough, dyspnoea, wheezing and chronic lung diseases. Carbon monoxide reduces oxygen delivery to the body’s organs, and cardiovascular patients might experience its most serious effects. In addition, it may cause vision problems, reduced ability to work or learn and difficulty in performing complex tasks. At extremely high levels, CO is poisonous and can cause death. In addition, CO contributes to the formation of smog and its consequent respiratory problems. Sulfur dioxide can cause breathing difficulty for asthmatic patients. Longer-term exposure to high levels of Sulfur dioxide gas and particles may be carcinogenic and may cause respiratory disorders and aggravate cardiovascular diseases; it may also cause eye burning and headache. Sulfur dioxide and nitrogen oxides react with other substances in the air to form acids, which fall to earth as rain, fog, snow, or dry particles. Nitric dioxide (NO2) can cause lung irritation, viral infection, airway resistance and chest tightness [3, 4].
Lead is distributed throughout the body in the blood and is accumulated in the bones. The most common effects of lead exposure are neurological effects in children and cardiovascular effects in adults. Infants and young children are especially sensitive to even low levels of lead, which may contribute to behavioural problems, learning deficits and lowered intelligence quotient (IQ). Infants and children are among the most susceptible age groups for air pollutants, because children may have greater exposure than adults to air pollutants. Infants and children have higher respiratory rates than adults, which would increase their exposure to air pollutants. Mouth breathing is more prevalent in infants and children than in adults; hence they bypass the filtering effect of the nose, and consequently they would inhale higher levels of pollutants than adults. Children generally spend significantly more time outdoors than adults, especially during summer time when smog levels are the highest. In addition, children’s immune systems and developing organs are still immature [5].
Air pollution might have various adverse effects on children’s health; some of the most important effects include perinatal effects, infant mortality, respiratory disorders, allergy, malignancies, cardiovascular disorders, increase in oxidative stress, endothelial dysfunction, mental disorders and vitamin D deficiency [6]. However, so far several studies and most information given to health professionals as well as to communities have focused on short-term respiratory effects of air pollution on children’s health. In this review, we provide a summary of studies conducted on non-respiratory effects of air pollution on children’s health to draw more attention to the wide range of hazards of air pollution from early life, and their possible implications for chronic non-communicable diseases of adulthood.
Search strategy
Electronic databases used for a search of the literature to find relevant studies were as follows:
1) Ovid MEDLINE(R) (1978 to 2008 with weekly update),
2) Ovid MEDLINE(R) in process and other non-indexed citations (2008),
3) AMED (Allied and Complementary Medicine) (1988 to 2008),
4) CINAHL (Cumulative Index to Nursing and Allied Health Literature) (1988 to 2008),
5) EMBASE (1980 to 2008),
6) CAB Abstracts (1978 to 2008),
7) Global Health (1978 to 2008).
The following search terms were used: air pollution, air pollutants, infants, children, adolescents, and youths. In a secondary search, we used terms related to health problems including abnormal development (low birth weight, preterm birth, prematurity, intrauterine growth restriction, congenital defects, intrauterine and infant mortality, malignancy, cancer, development, behavioural problems, neurocognitive decrements, etc).
Data on study design and location, air pollutants, confounding factors, health outcomes measured, and study results were extracted from the selected studies. Relevant articles cited by selected publications were also included. Search results
Studies varied by design, study location, age group of study subjects, study duration and type of health outcomes studied. Some studies were cross-sectional, some had a case-control design, and the most relevant studies used time-series analysis to investigate associations between daily variations in air pollutants and variations in health outcomes.
The most prominent health hazards are summarized in Tables I–V. Discussion
The findings of this review underscore the importance of paying more attention to different aspects of hazards of air pollution on children’s health, and not only to the direct effects on the respiratory system. Of special concern are the late-onset effects of air pollution in early life that may contribute to many chronic diseases later in life. Chronic non-communicable diseases are rapidly escalating in low- and middle-income countries [7], and their risk factors have a considerably high prevalence in different age groups even in children and adolescents [8-11]. The association of dietary and physical activity habits and these risk factors has been documented [12-15]; however, studies about the relationship of environmental factors, notably air pollution, with risk factors of chronic diseases are scarce in the paediatric age group.
Many studies have documented the effects of criteria air pollutants on low birth weight and or prematurity. The association of intrauterine growth retardation and low birth weight with increased risk of chronic non-communicable diseases such as obesity, hypertension and cardiovascular disease later in life is well documented [16]. In addition, prematurity can be associated with higher risk of such disorders [17]. Until now, most studies have linked maternal malnutrition to low birth weight of their children [18], and its implications for the risk of adult chronic diseases in low-income and middle-income countries have been documented [19].
Furthermore, low socio-economic position in early life is known as a predisposing factor for chronic diseases [20] and mortality [21] in adulthood. Usually improper lifestyle habits and low educational levels have been considered as the underlying process of such associations. We suggest that exposure to air pollutants and its effects on low birth weight and premature birth should be considered as well.
According to World Bank data, “urbanization is progressing much faster in developing countries than in developed countries, and most of the world’s most populous cities are in developing countries. Many of these cities are in Asian countries with low per capita incomes but big populations. These cities have high concentrations of poor residents and suffer from social and environmental problems including severe air pollution” [22].
Similarly, the low-income and middle-income countries are facing an emerging epidemic of chronic non-communicable diseases in the near future [7, 23]. Lifestyle modifications as well as strengthening primary care in the health system have been considered as the main strategies to tackle chronic diseases in low- and middle-income countries [24].
We suggest that environmental protection actions, notably for reducing the emission of criteria air pollutants, should be considered for public health measures taken into account for primordial/primary prevention of chronic diseases, especially in developing countries.
The association between air pollution and chronic diseases may be mediated through systemic inflammatory responses [3, 25]. Generation of reactive oxygen species is linked to a variety of environmental factors. The association of air pollution and inflammation/oxidative stress has been demonstrated [26-28], even among healthy children [29] who might have the early stages of atherosclerosis.
The effects of air pollution on oxidative stress and endothelial dysfunction from early life confirm the necessity of implications of these findings in relation to public health and regulatory policies for prevention and control of adult chronic diseases from childhood.
Similar to cardiovascular diseases, the prevalence of malignancies is rapidly escalating worldwide. Although lifestyle behaviours such as smoking [30], as well as unhealthy dietary and physical activity habits leading to obesity and diabetes, are known as a major contributing factor in this regard [31], air pollution should be considered as another potential risk factor for developing countries [32], especially Asian countries where cancer has become an emerging health threat [33]. This issue is particularly important for children, who are susceptible to short-term and long-term effects of air pollutants.
Facilities should be provided for families to become aware of the quality of the air year round and to check daily air-quality levels and air-pollution forecasts by mass media, local weather reports and other available public information sources. This is especially important for smog levels during hot weather. Protective measures should be taken into account for children and pregnant women to reduce their exposure to air pollutants, e.g. children and pregnant women should avoid congested streets and rush hour traffic, and families should try to limit the amount of time their child spends outdoors in vigorous activity if the air quality is unhealthy. Conclusions
Air pollution is a global health issue with serious public health implications, particularly for children. Usually respiratory effects of air pollutants are considered, but the importance of other health hazards should also be highlighted. In addition to short-term effects, exposure to criteria air pollutants from early life might have long-term risks principally for chronic non-communicable diseases such as cardiovascular diseases and cancers. In view of the emerging epidemic of chronic disease in low- and middle-income countries, the vicious cycle of rapid urbanization in such communities resulting in increasing levels of air pollution and its consequent effects on chronic diseases, as well as the limited financial resources of these countries for planning effective air pollution control programmes, public health and regulatory policies for air quality protection should be integrated into the main priorities of the primary health care system and into the educational curriculum of health professionals.
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