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Ambient air pollution triggers wheezing symptoms in infants
  1. Z J Andersen1,2,
  2. S Loft2,
  3. M Ketzel3,
  4. M Stage4,
  5. T Scheike1,
  6. M N Hermansen4,
  7. H Bisgaard4
  1. 1
    Department of Biostatistics, University of Copenhagen, Copenhagen, Denmark
  2. 2
    Department of Environmental and Occupational Health, Copenhagen University, Copenhagen, Denmark
  3. 3
    Department of Atmospheric Environment, National Environmental Research Institute, Aarhus University, Denmark
  4. 4
    Danish Paediatric Asthma Centre, Copenhagen University Hospital, Gentofte, Copenhagen
  1. Mrs Z J Andersen, Department of Biostatistics, Institute of Public Health, Copenhagen University, Øster Farimagsgade 5 Entr. B, P O Box 2099, 1014 Copenhagen K, Denmark; zorana{at}


Background: There is limited evidence for the role of air pollution in the development and triggering of wheezing symptoms in young children. A study was undertaken to examine the effect of exposure to air pollution on wheezing symptoms in children under the age of 3 years with genetic susceptibility to asthma.

Methods: Daily recordings of symptoms were obtained for 205 children participating in the birth cohort study Copenhagen Prospective Study on Asthma in Children and living in Copenhagen for the first 3 years of life. Daily air pollution levels for particulate matter <10 μm in diameter (PM10) and the concentrations of ultrafine particles, nitrogen dioxide (NO2), nitrogen oxide (NOx) and carbon monoxide (CO) were available from a central background monitoring station in Copenhagen. The association between incident wheezing symptoms and air pollution on the concurrent and previous 4 days was estimated by a logistic regression model (generalised estimating equation) controlling for temperature, season, gender, age, exposure to smoking and paternal history of asthma.

Results: Significant positive associations were found between concentrations of PM10, NO2, NOx, CO and wheezing symptoms in infants (aged 0–1 year) with a delay of 3–4 days. Only the traffic-related gases (NO2, NOx) showed significant effects throughout the 3 years of life, albeit attenuating after the age of 1 year.

Conclusions: Air pollution related to traffic is significantly associated with triggering of wheezing symptoms in the first 3 years of life.

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Recurrent wheeze represents a serious health burden in young children and is the cause of a considerable number of hospital admissions and other health care expenditure.1 Evidence for the role of air pollution in causing such symptoms remains equivocal.2 3 Particulate air pollution in terms of particulate matter <10 and 2.5 μm in diameter (PM10 and PM2.5) has been linked to asthma exacerbations in school children,412 suggesting a role as a trigger of symptoms, although two large multicentre studies failed to confirm this.13 14 Recently, ultrafine particles (UFPs; particles <0.1 μm in diameter) generated by traffic emissions were suggested to have particularly strong effects in the airways due to a high level of pulmonary deposition and their ability to induce inflammation and oxidative stress.15 However, the only epidemiological study of the effects of UFPs in children to date found only a weak association with asthma symptoms in school children.8 Traffic-related air pollution, assessed by nitrogen dioxide (NO2) exposure or proximity to traffic, has been more consistently associated with asthma exacerbations in school children, which suggests that air pollution is an important trigger of symptoms.1620

Only a few studies of the effects of air pollution are available in young children and provide mixed evidence.2127 A greater susceptibility of younger children compared with school children may be suspected because of the development of the lung and the higher ventilation rates in early life.2 A case-control study has found significant associations between traffic pollution and asthma before the age of 3 years which attenuated between the ages of 3 and 14 years.20 Understanding the effects of early exposure to air pollution on the development and triggering of wheeze may give important clues to its role in the development of asthma later in life.3 To date, most studies in young children have used geographical variations in air pollutant levels and have focused on traffic.20 21 2326 Only one studied PM1022 and one studied ozone (O3),27 with no evidence to date on the effect of UFPs. One study in preschool children found an association between soot and incident wheeze, asthma and ear/nose/throat infections in a birth cohort of children up to 4 years of age,21 and a second study in preschool children found an association between PM10 and the prevalence and incidence of respiratory symptoms in a survey of children aged 1–5 years.22 While proximity to traffic was recently reported to be associated with wheeze in infants,23 only two birth cohort studies have examined susceptibility in small children by age, reporting associations between doctor-diagnosed asthma24 and respiratory symptoms (cough)25 and traffic to be stronger in the first year of life than in the second year of life. Finally, the potential importance of genetic susceptibility has been suggested by a particularly strong association between triggering of wheezing bronchitis or wheeze and PM2.5 and O3, respectively, among infants with a family history of asthma.26 27

In this study we used the unique data on daily symptom recording in the birth cohort of children of mothers with asthma from the Copenhagen Prospective Study on Asthma in Childhood (COPSAC) to determine the short-term impact of particulate (PM10 and UFPs) and gaseous (nitrogen oxide (NOx), NO2 and carbon monoxide (CO)) air pollution on the triggering of wheezing symptoms during the first 3 years of life in this high-risk population. We examined the susceptibility by age (infants (0–1 year), 1–2 years and 2–3 years) to study the hypothesis of increased susceptibility of infants to air pollution, and tested for effect modification by medication use, gender and paternal asthma history.


Study population

The COPSAC prospective longitudinal birth cohort study includes 411 Danish children born to mothers with asthma and was designed to study the gene-environment interaction in the development of asthma and other atopic diseases.2830 In brief, pregnant women identified through the Danish National Birth Cohort Study and prenatal clinics, living in Copenhagen, fluent in Danish, with a history of physician-diagnosed asthma after the age of 7 years and a history of daily treatment with inhaled β2 agonists or glucocorticoids (minimum of 2 weeks during two seasons or continuously for 1 year) were invited to participate in the study. Infants with a severe congenital abnormally, a gestational age <36 weeks, a need for mechanical ventilation or a lower respiratory tract infection were excluded. The eligible 411 children of 394 mothers (nine pair of twins and eight siblings) were born between 2 August 1998 and 12 December 2001 and enrolled in the cohort at the Clinical Research Unit (CRU) visit at 1 month of age, with follow-up clinical investigations every 6 months until the age of 7 years and at any episode of symptoms.

At the first CRU visit parents were given diary cards ( and instructed to record their child’s wheezing symptoms daily as dichotomised scores (yes/no), as previously described.29 The CRU doctor reviewed the symptom definitions and the diary entries with parents at the 6-monthly clinical sessions and at acute episodes of wheeze. Diary recordings for the first 3 years of life were used in the current analyses. Information on exposure to smoking at home and paternal history of asthma was obtained by interviews. Medication use followed the algorithm described in detail elsewhere.29

Information on addresses, including relocations, was available from the COPSAC database and the Danish National Person Registry, allowing for geocoding and calculation of the exact distance from the residence to the air pollution monitor in the centre of Copenhagen on a daily basis. Thus, 205 children living within a 15 km radius of the central monitor during the first 3 years of life were selected for the study.

Assessment of air pollution exposure

The pollutant and meteorological data were measured by the Danish National Environmental Research Institute at a fixed urban background monitor (20 m height) in the centre of Copenhagen with minimal contribution from local sources, in accordance with WHO guidelines. Main streets are located about 300 m west and 50 m east of the monitor, with 26 000 and 56 000 vehicles passing per working day, respectively. For the study period (12 December 1998 to 19 December 2004), daily (24 h, midnight-to-midnight) mean concentrations were available for PM10 (SM200 monitor; Opsis, Sweden), CO (M300; API, San Diego, USA), NO2 and NOx (M200A; API), O3 (M400; API) and temperature, with missing data on days with equipment malfunctions. Although the total number of concentration measurements included particles 10–700 nm in diameter, we define them as UFPNC in this study because particles <100 nm (by definition, UFP) comprised more than 95% of the total concentration measurements. Monitoring of UFPNC (Differential Mobility Particle Sizer, Roskilde, Denmark)31 and PM2.5 (TEOM Ambient Particulate Monitor; Rupprecht and Patashnick, USA) began in Denmark on 15 May 2001 and 3 October 2003, respectively.

Statistical analysis

The incidence of wheezing symptoms was defined as the first day of a registered symptom. Logistic regression analyses were performed using generalised estimating equations (GEE)32 by the GENMOD procedure with exchangeable correlation structure using SAS Version 9.1. Pollutant concentrations were log-transformed. Analyses were conducted for all 3 years and separately for infants and age groups 1–2 and 2–3 years. We first fitted single-pollutant models, adjusted for age (dummy for each year), gender, exposure to passive smoking, paternal history of asthma, 24 h mean temperature (linearly) and calendar season (dummy). Pollutant concentrations on the same day (lag 0), previous day (lag 1), up to 4 days before a new symptom (lag 4) and the 3-day mean (2–4 days) were considered. In cases where monitor data were missing from single days, 3-day means of available measurements were used. Two-pollutant models were fitted for 3-day mean pollutant concentrations to examine the robustness of one-pollutant associations. Separate analyses were conducted for a subset of data where UFPNC data were available (15 May 2001 to 19 December 2004) to ensure comparability between the effect of UFPNC and other pollutants. Finally, we tested for effect modification by gender, medication use and paternal history of asthma. Effects were reported as odds ratios (ORs) per interquartile range (IQR) increase in exposure.

A number of sensitivity analyses were performed. The results remained unchanged when fitting a GEE model with alternative correlation structures (independent, autoregressive). Various adjustments for meteorology and seasonal/time trend effects were tested. A model with temperature defined by a dummy per 25th percentile showed similar results and indicated a strong negative trend on symptom development by increasing quartiles of temperature, validating linear modelling of temperature. Season was modelled by dummy variables for calendar seasons, with a significant trend seen for an increase of symptoms in colder seasons. After adjusting for season and temperature, additional time trends including calendar time (linearly and by smoothing spline) and dummy for each month were not significant and not included in the model. Finally, we performed analyses for an alternative choice of study population with 110 COPSAC children living within a 5 km radius from the central monitor.


The 205 children (99 boys) in the COPSAC cohort lived within a 15 km radius of the central monitor, with a mean (SD) distance of 6.1 (4.0) km and a minimum distance of 0.3 km. Of the 205 children, 35 had a paternal history of asthma and 94, 64 and 47 were exposed to passive smoking for <10, 10–100 and >100 days/year, respectively. The 205 children seemed representative of the whole COPSAC cohort,29 with a dropout rate of 3 (1.5%), 6 (3%) and 2 (1%) children in the first, second and third year of life, respectively, and 194 (95%) active children at their third birthday. The average observation period per child was 850 days (minimum 23, maximum 1097), with a total of 174 259 person-days and 15.4% diary entries missing. Thirty-three children (16%) experienced no wheezing symptoms in the first 3 years of life. A prevalence of 6.2 per 100 person-days (total of 10 779 symptom-days) and an incidence of 1 per 100 person-days (1591 new symptom-days) was observed.

Tables 1 and 2 show the pollution and meteorological conditions in Copenhagen during the study period. Frequent breakdown or other utilisation of the UFPNC measuring equipment caused missing data gaps (54%). PM2.5 and O3 data were not sufficient for analyses of all three age groups due to the late start of PM2.5 monitoring (3 October 2003) and a monitoring gap for O3 in the middle of the study period (13 July 1999 to 1 July 2003). Strong correlations were observed between PM10 and PM2.5, CO and NO2/NOx, and UFPNC and NO2/NOx (table 2) and weak correlations were observed between PM10 and UFPNC and PM10 and NO2/NOx.

Table 1 Air pollutant levels and meteorological conditions in Copenhagen from 12 December 1998 to 19 December 2004 (2199 days)
Table 2 Spearman correlation coefficients between air pollutants and temperature in Copenhagen from 12 December 1998 to 19 December 2004 (2199 days)

Air pollution showed delayed adverse effects, which were strongest and most significant in infants with a 4-day delay for PM10 and a 3-day delay for NO2, NOx and CO. In this group an increase of one IQR in exposure to PM10, NO2, NOx and CO resulted in an increase in wheezing symptoms of 23% (95% confidence interval (CI) 2% to 48%; lag 4; IQR 14 μg/m3), 42% (95% CI 15% to 77%; lag 3; IQR 6.5 ppb), 30% (95% CI 9% to 53%; lag 3; IQR 8.9 ppb) and 47% (95% CI 10% to 96%; lag 3; 0.12 ppm), respectively (table 3). Associations for NO2 and NOx were also significant with a 4-day lag and 3-day means. Similar lag structures were observed for the pollutants in all 3 years, but the effects were attenuated after 1 year of age. The associations remained positive and significant in all 3 years only for NO2 and NOx, for which an increase of one IQR in 3-day mean exposure was associated with an increase in wheezing symptoms of 19% (95% CI 1% to 30%; 6.5 ppb) and 14% (95% CI 0% to 30%; 8.9 ppb), respectively. The effects of NO2 and NOx were stronger than those of PM10 throughout the study period (table 3), as confirmed in two-pollutant models for infants (table 4).

Table 3 Associations between incident wheezing symptoms and single-day and 3-day mean concentrations (lag 2–4) of air pollutants with maximum data available for each pollutant (12 December 1998 to 19 December 2004)
Table 4 Associations between incident wheezing symptoms for 3-day mean concentrations (lag 2–4) of air pollutants in infants (age 0–1 years) in period with available PM10 measurements (12 December 1998 to 19 December 2004)

UFPNC showed a relatively strong adverse effect with delays of 2–4 days in infants, but without reaching statistical significance, and this changed after the age of 1 year to an apparently protective effect. Analyses for 3-day mean concentrations of other pollutants (PM10, NO2, NOx and CO) were repeated for the subset of data where UFPNC measurements were available (15 May 2001 to 19 December 2004) for infants to enable direct comparisons (table 5). Associations for all pollutants were enhanced, with estimates for UFPNC comparable to those of PM10 and gases in one-pollutant models and higher than the others in two-pollutant models. For COPSAC children living within a 5 km radius of the monitor, the effects of UFPNC were enhanced reaching statistical significance for 3-day means (table 6) and for lags 2–4 (data not shown).

Table 5 Associations between incident wheezing symptoms for 3-day mean concentrations (lag 2–4) of air pollutants in infants (age 0–1 years) in period with available UFPNC measurements (15 May 2001 to 19 December 2004)
Table 6 Associations between incident wheezing symptoms for 3-day mean concentrations (lag 2–4) of air pollutants in 110 children living within a 5 km radius of the central monitor

No effect modification was detected between air pollution and gender, paternal asthma history or medication use.


We found strong adverse effects of air pollution with PM10, NO2, NOx and CO on the triggering of wheezing symptoms in infants which attenuated after the age of 1 year. Independent effects of PM10 and gases were observed, with consistently stronger estimates for gases (NO2 and NOx), proxies of traffic-related pollution. Furthermore, adverse effects of UFPNC in infants, which were enhanced in infants living within a 5 km radius of the monitor, support the relevance of traffic, although an apparently protective effect of UFPNC was observed at the age of 3 years.

The finding that traffic triggers wheezing in infants is in agreement with recent evidence that infant wheezing is associated with proximity to traffic23 and infant wheezing bronchitis is associated with PM2.5.26 Furthermore, two studies found an association between doctor-diagnosed asthma and respiratory symptoms (cough) in children aged <2 years and modelled levels of PM2.5 and NO2 at the children’s residence.24 25 These associations were strongest in the first years of life, as observed in our study. Lung anatomy and physiology, ventilation rates and organ maturity change with age, which may explain the greater vulnerability of infants to air pollution and suggest biological plausibility for these findings.2 Consistent with our findings, a study of asthma symptoms in preschool children21 and several studies of asthma exacerbations in school children have also shown adverse effects of various proxies of traffic.1619

Our study is the first to study the effects of UFPs on triggering of wheezing symptoms in young children. A positive association was found in the first year of life, which was significant in children living within 5 km of the monitor. The only other study of UFPs in children found a significant effect of PM10 and a comparable but not significant effect of UFPs on peak expiratory flow in 39 asthmatic school children over 57 days.8 Three studies of the effects of pollution on asthma exacerbations in adults found that UFPs and PM2.5 had a greater effect than larger particles3335 in a radius of 2 km33 and 5 km34 from the pollution monitor. However, our results should be taken with caution and need to be confirmed because of the unexpected protective effect of UFPs found in children over 1 year of age.

To date, the effects of PM10 have been studied in only one survey of preschool children22 and there have been no studies in infants. In Copenhagen the main source of PM10 is long-range transport from secondary biomass and oil combustion sources with a smaller contribution from traffic,36 which suggests that PM10 is a poor proxy for traffic and enables its effects to be separated from and compared with the effects of traffic proxies (NO2, NOx, and UFPNC). We found that the adverse effects of PM10 were significant only in infants (lag 3) and were generally smaller than those of gases (table 4) and UFPNC (table 5). This suggests an independent effect of particle mass and gases or particle numbers mainly related to traffic.

The increased susceptibility to air pollution in genetically susceptible children was previously suggested by the finding of an increase in the association of incident wheezing bronchitis with PM2.5 in children with a family history of asthma26 and the stronger effects of O3 in triggering wheeze in children of mothers with asthma.27 In our cohort of high-risk infants, associations with traffic-related air pollution were higher than those reported elsewhere in healthy infants,22 24 25 but direct comparisons are difficult because of differences in outcome definitions and the use of geographical variation in exposure assessments in those studies. Furthermore, the association of asthma prevalence with traffic was surprisingly stronger for school children without a family history of asthma.17 This may indicate that early symptoms in infants and later diagnosed asthma—although strongly related—represent children with different susceptibilities.

The incidence of wheezing symptoms was chosen as the health outcome in this study rather than diagnoses of asthma because of the inaccurate diagnostic criteria for asthma in early life.29 30 As the term “wheeze” carries little specific meaning in lay terms and even between specialists,37 we explained the term to the parents in the generic sense as lung symptoms severely affecting the child’s well-being including wheeze or whistling sounds, breathlessness, shortness of breath and persistent troublesome cough. This approach was supported by diary reviews at the 6-monthly visits together with a dedicated book describing the relevant lung symptoms. The diagnosis and day-to-day management of respiratory conditions was conducted solely by the doctors employed for this purpose at the CRU in accordance with predefined algorithms, minimising the risk of symptom misclassification from influence of the prevailing and variable diagnostic criteria and treatment traditions in the medical community. The inaccuracy of symptom recognition and reporting bias was reduced by the fact that all mothers had a personal asthma history and familiarity with the disease. Furthermore, the state-of-the art information on the incidence of wheezing symptoms in the COPSAC cohort provided an opportunity to study the short-term effects of air pollution on triggering symptoms which has previously been reported in only two studies in infants.26 27 Analyses of the cohort at later ages when the diagnoses of health outcomes are more specific will help determine the long-term impact of early exposure to air pollution on the development of specific respiratory diseases, including asthma.

The strengths of our study include the prospective daily recording of symptoms which allowed the study of symptom incidence, and the long study period of 5 years which gave sufficient contrast in exposure over time and power to detect adverse effects. This is an advantage over earlier studies with a short follow-up of a few months,4 610 27 which may limit the power to detect pollutant effects.38 An additional strength is the well-defined birth cohort of children genetically predisposed to asthma, giving a unique opportunity to study the effect of early exposure to air pollution in a high-risk group. Furthermore, continuously updating the residential information allowed inclusion criteria to be based on address (vicinity to the monitor) at the time of symptoms and not at the time of birth, as has previously been the case.21 24 Finally, the availability of both gaseous and particulate pollutants of different sizes allowed comparisons of the effects of traffic-related (gases and UFPs) and non-traffic-related (PM10) air pollution in the development of respiratory disease in young children.

The limitations of the study include assessment of exposure from a single monitor which might result in exposure misclassification.39 A 15 km radius from the central monitor was chosen as it represents the municipality limits of Copenhagen City with similar population and traffic density, and was assumed to be representative of air pollution levels measured at the central monitor. We have previously shown that the UFPNC levels at this site correlated well with levels at a monitoring site at the kerbside (2 m height) of a busy street 3000 m away (Spearman correlation coefficient RS = 0.62) and a rural monitoring site located in a residential area 30 km southwest (RS = 0.80).40 This was despite the fact that UFPNC levels were on average 4.6 times higher at the kerbside and 0.7 times lower at the rural site than at the urban background site. This indicates that the daily oscillations in traffic-related air pollutants on busy streets due to variation in traffic intensity, weather conditions and other factors are also reflected by urban background monitoring, and that a radius of 15 km from the background monitor is reasonable for the assessment of daily variations in population exposure. In children living closer to the central monitor (5 km) there was a slightly stronger association between UFPNC and triggering of wheeze which was statistically significant and exceeded that of other pollutants (table 6). However, if the apparent difference between estimates for a radius of 5 km and 15 km is real, we do not know whether this is due to a greater misclassification of exposure with the larger radius or whether the effects of traffic closer to the monitor—where traffic density and emissions and traffic pollutant levels are higher—really are greater. A further limitation of the study is the counterintuitive and biologically implausible change in the effect of UFPNC from an adverse effect in infants to a protective effect in children over the age of 2 years which was statistically significant in children living within 5 km of the air pollution monitor (table 6). However, the effect of all other pollutants (PM10, NO2, NOx and CO) also attenuated after the age of 2 years to become apparently protective, although this did not reach statistical significance (table 6). This indicates that a decrease in susceptibility to air pollution after the age of 2 years is not confined to UFPs and is therefore probably not the result of measurement error or change in exposure but, rather, is due to the underlying disease process and/or the treatment of wheezing symptoms in this group of children. As the medication protocol in the COPSAC cohort presumes a history of recurring wheezing episodes, children typically do not receive medication until the second year of life or later. The attenuation of the adverse effects of air pollution after the age of 1 year and further after the age of 2 years is therefore most likely explained by the decreased susceptibility of the children to air pollution as a result of medication for their wheezing symptoms. Likewise, the statistical significance with respect to UFPNC is most likely explained by a type I error. Nevertheless, it may question the findings for UFPNC in the first year of life as no other supportive data are available. Further limitations of the study include the large number (54%) of missing data for UFPNC. However, missing data gaps were examined and were found to be random and not associated with pollution levels or weather conditions. Nevertheless, our UFPNC measurements for 602 days are the longest available to date for the study of the effects of UFP on respiratory symptoms.8 3335

In summary, we found independent effects of PM10 and traffic-related pollution measured by exposure to NO2/NOx, CO and UFPNC (5 km radius) in triggering wheezing symptoms in young children genetically predisposed to asthma. Infants were found to be particularly vulnerable. Follow-up of the cohort at later ages will determine the effect of these associations on the development of asthma.


We thank the children and parents who participated in this study.



  • Funding: The statistical analyses were supported by the Danish Research Council grant number 2052-03-16, AIRPOLIFE (Air Pollution in a Life Time Health Perspective) and the Danish Environmental Protection Agency. The COPSAC cohort study is funded by research funds from the Pharmacy Foundation of 1991; the Lundbeck Foundation; the Augustinus Foundation; Ronald McDonald House Charities; the Danish Medical Research Council; the Danish Pediatric Asthma Center; Direktør, cand. pharm. K Gad Andersen og Hustrus Familiefond; Aage Bangs Fond; Danish Lung Association; Kai Lange og Gunhild Kai Langes Fond; Direktør Ib Henriksens Fond; Gerda og Aage Hensch’s Fond; Rosalie Petersens Fond; Hans og Nora Buchards Fond; Dagmar Marshalls Fond; Foundation of Queen Louise Children Hospital; the Danish Hospital Foundation for Medical Research, Region of Copenhagen, the Faroe Island and Greenland; Gangsted Fond; Højmosegård-Legatet; Fonden til Lægevidenskabens Fremme; A P Møller og Hustru Chastine Mc-Kinney Møllers Fond til almene Formaal; The Danish Ministry of the Interior and Health’s Research Centre for Environmental Health. The study received support from the following pharmaceutical companies: AstraZenaca; LEOpharma; Pharmacia-Pfizer and Yamanouchi Pharma.

  • Competing interests: None.

  • Ethics approval: The COPSAC study was conducted in accordance with the Declaration of Helsinki and approved by the Copenhagen Ethics Committee and the Danish Data Protection Agency.