Urban air pollution (UAP) is one of the main environmental health threats for the paediatric age group.1 Although the entire population is affected by poor air quality, children and youth are more vulnerable to its effects due to the physiological immaturity of their respiratory and immune systems, their higher metabolism, higher respiratory rate and greater requirements of air per kilogram of body weight compared to adults.2 A growing body of scientific evidence demonstrates the contribution of UAP to the increased morbidity and mortality of children worldwide.3,4 Exposure to UAP during childhood has been associated with an increase in respiratory complaints and associated hospital admissions, inflammation of the airways, impaired respiratory function, asthma attacks and acute lower airway infections.5–15

The World Health Organization (WHO) regularly publishes guidelines on air quality that constitute the main reference to establish standards worldwide.16 It also offers information on the air quality of more than 3000 cities in 103 countries (http://breathelife2030.org/). Table 1 presents the main types of air pollutants highlighted in studies of respiratory morbidity3 and the maximum levels recommended by the WHO.16 The primary components of UAP involved in morbidity are particulate matter, including fine particles 2.5 μm in diameter and smaller (PM2.5) or 10 μm in diameter and smaller (PM10), nitrogen dioxide (NO2), sulphur dioxide (SO2), and ozone (O3). Tropospheric O3 is found in the lowest layers of the atmosphere and is considered a secondary pollutant, as it is not directly emitted to the atmosphere but rather forms from certain precursors (volatile nonmethane organic compounds, carbon monoxide [CO], nitrogen oxides and, to a lesser degree, methane [CH4]) produced by combustion processes (traffic and industry). Through the action of sunlight, these chemical substances react to form O3. Since sunlight is one of the main factors at play in these reactions, the concentrations of these pollutants peak in spring and summer.

The aim of our study was to analyse the association between the level of UAP and hospital admissions in individuals aged less than 18 years due to acute respiratory illness in the city of Murcia, Spain.

The PED received 12 354 visits of residents of the municipality of Murcia due to acute upper or lower respiratory tract disease in 2015. Cold/upper airway infection, asthma/bronchospasm, pharyngitis or sinusitis/abscesses, otitis, laryngitis, bronchiolitis and pneumonia/bronchitis amounted to 38.7%. 18.9%. 14.9%. 10.0%. 7.9%. 6.9% and 2.6, respectively. A total of 436 patients (corresponding to 3.5% of visits for respiratory disease) were admitted to hospital; Fig. 1 shows the distribution of hospital admissions by respiratory disease. Table 2 presents the profiles of the patients that sought care and data on air pollution.

Figure 1.

Distribution of hospital admissions due to respiratory complaints in 2015 in the city of Murcia.

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The annual mean level of NO2 in the municipality of Murcia, of 46.92 µg/m3 (95% confidence interval, 46.60−47.23), exceeds the limit recommended by the WHO (40 µg/m3), with levels exceeding this recommended limit 52% of days a year.20 The annual mean of PM10 is 34.81 μg/m3 (34.47−35.16), and the levels of PM10 exceed the annual mean recommended by the WHO (50 µg/m3) 10% of the days. Fig. 2 presents the annual distribution of daily mean values for the 4 pollutants.

Figure 2.

Distribution of the daily mean of air pollutants in the city of Murcia throughout the year.

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Table 3 presents the results of the multivariate logistic regression analysis. The risk of hospital admission due to respiratory complaints was greater in male patients, increased with the mean daily level of NO2 and O3 and decreased with age.

Our findings reveal that a higher short-term level of UAP (O3 and NO2), male sex and winter were associated with an increased risk of hospital admission due to respiratory illness. This association was strongest for asthma (with O3 and PM10) and bronchiolitis (with NO2).

The increased hospital admission rate in male patients has already been described in the previous literature,14,15 although it diverges from the increased vulnerability to O3 described in female patients.21 Some of the possible explanations proposed for this are models of exposure (boys tend to play outdoors for longer periods and engage in more aerobic activity) and physiological (differences in hormones, lung size, inflammation of airways).

From a physiological standpoint, it makes sense that air pollutants could increase the severity of asthma and bronchiolitis on account of their known effect on lung function and airway inflammation.22

There is evidence that short-term exposure to particulate matter, O3,18 SO2 and NO2 is associated to an increase in asthma exacerbations.10 The effect of PM10 on asthma could be explained by the oxidative stress induced by the presence of PM10 in the epithelium, with an increase in prostaglandin E that leads to defective macrophage phagocytosis, inflammation, oedema and cytotoxicity. The evidence on hospital admissions due to asthma in relation to O3 exposure requires further research,14,23,24 but a review of 87 studies found that 71 reported a significant increase in the risk of hospital admission due to asthma.15

Bronchiolitis is an infection of the small airways that mainly affects children aged less than 2 years. It causes inflammation of the airways and necrosis of the bronchial epithelium.25 In 80% of cases it is associated with infection by respiratory syncytial virus, and it is one of the main reasons for hospital admission in children aged less than 2 years in both high-income26 and low-income27 countries. Although there are isolated cases throughout the year, there are usually one to two outbreaks of bronchiolitis in the cold months of the year, between November and March. The risk factors known to increase the risk of hospital admission are preterm birth, low birth weight, heart disease or exposure to tobacco smoke. We found an association between the mean daily level of NO2 and the risk of hospital admission due to bronchiolitis. The current evidence suggests an association between short (<24 h) and recent exposure to PM2.5; PM10; NO2 and SO2, among other pollutants, and an increased incidence of lower respiratory tract infections11 and an increased risk of hospital admission due to bronchiolitis.28 The greater effect observed in other studies with chronic exposure to particulate matter (several days or weekly mean) on hospital admission due to bronchiolitis may reflect different pathogenic mechanisms.29 Chronic exposure to PM2.5 and PM10 may have a chronic proinflammatory effect,30 whereas exposure to NO2 and SO2 may be associated with more acute airway damage.31

Ozone levels usually peak during hot seasons, which are far apart from the periods when outbreaks of infection by respiratory syncytial virus tend to occur. This would make it difficult to identify an association with bronchiolitis. In winter, other contaminants, such as NO2, are probably more relevant contributors to respiratory disease. As has been done by other authors,32 we analysed synergistic effects and interactions in a model taking into account multiple pollutants and age groups, and we did not find any differences in the results.

In our study, the annual mean level of NO2 in the city of Murcia exceeded the recommended limit for the health of its residents. Murcia is the city that ranks 8th in Spain in terms of the highest annual mean level of NO2. The increasing urbanization and concentration of the population in megacities,33 in absence of corresponding air quality control measures, has a direct impact on child health. At present, 98% of cities in low- and medium-income countries and 56% of cities in high-income countries have levels of pollution exceeding the limits recommended by the WHO.34

One significant limitation of our study involves the scarcity of sources of pollution data. The municipality of Murcia has 400 000 inhabitants, making it the seventh most populous municipality in Spain, yet it only has one pollutant measurement station, which does not collect information for PM2.5, among other pollutants. The season and the temperature play a role in the variations in levels of UAP, so we controlled our analyses for these factors.35–39 Other population- and individual-based variables, such as smoking, dietary factors and lifestyle, may be a source of bias. Smoking is particularly important, given that approximately 50% of children in Spain live with a smoker at home.40 The low level of documentation and the lack of an established coding scheme to document exposure to second-hand smoke in minors that visited the PED precluded the analysis of its impact on our findings. We believe it essential to train paediatricians in the use of brief opportunistic intervention during PED visits. On the other hand, additional collaborative cohort and longitudinal studies with a standardised methodological approach are needed to better analyse the associations identified in our study.

Although the proportion of patients requiring hospital admission due asthma or bronchiolitis that could be attributed to UAP may seem small, UAP probably has a substantial impact in morbidity and the global burden of disease at the population level. The good news is that exposure to UAP is totally preventable. Successful interventions are already being developed and implemented in numerous cities and regions across the globe. The reduction of UAP has been associated with public health benefits, with a decrease in mortality associated to cardiovascular and respiratory diseases in adults and a decrease in the rate of hospital admission due to respiratory complaints in children (of up to 87% in cities that started out with high levels of pollutants), a decrease in the incidence of preterm birth and a reduction in lung inflammation.33,41–45 Thus, improving air quality surveillance systems and implementing measures for control of UAP (public transport, urban design, banning brush burning, promoting cycling, limiting the use of high-emission motor vehicles, improving monitoring of UAP and its effects in vulnerable groups, among others) contribute to reducing the rate of hospital admission in the paediatric age group.

Such a pathway requires courageous leadership, the creation of new structures that connect health and environment, substantial resources and widespread changes in society.40 The growing social awareness of the association between health and environment is a key driver of change. We take this opportunity to ask Spanish paediatricians to help increase the sample size, providing free access to our data (http://pehsu.org/wp/wp-content/uploads/UAP_IngresosRespi.zip) to other paediatrics research groups in Spain. The combined analysis of clinical and environmental surveillance data is a powerful tool toward achieving adequate monitoring and for creating models that can contribute to improving health and air quality in cities.

The authors have no conflicts of interest to declare.