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EFFECT OF SMOKE-FREE LEGISLATION ON PERINATAL AND CHILD HEALTH

Thursday, 27th of March 2014

EFFECT OF SMOKE-FREE LEGISLATION ON PERINATAL AND CHILD HEALTH: A SYSTEMATIC REVIEW AND META-ANALYSIS

The Lancet, Early Online Publication, 28 March 2014

doi:10.1016/S0140-6736(14)60082-9Cite or Link Using DOI

Copyright © 2014 Elsevier Ltd All rights reserved.

Dr Jasper V Been PhD a b c , Ulugbek B Nurmatov PhD b, Bianca Cox MSc d, Tim S Nawrot PhD d e, Prof Constant P van Schayck PhD a b, Prof Aziz Sheikh MD a b f

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http://www.thelancet.com/journals/lancet/article/PIIS0140-6736%2814%2960082-9/fulltext

Summary

Background

Smoke-free legislation has the potential to reduce the substantive disease burden associated with second-hand smoke exposure, particularly in children. We investigated the effect of smoke-free legislation on perinatal and child health.

Methods

We searched 14 online databases from January, 1975 to May, 2013, with no language restrictions, for published studies, and the WHO International Clinical Trials Registry Platform for unpublished studies. Citations and reference lists of articles of interest were screened and an international expert panel was contacted to identify additional studies. We included studies undertaken with designs approved by the Cochrane Effective Practice and Organisation of Care that reported associations between smoking bans in workplaces, public places, or both, and one or more predefined early-life health indicator. The primary outcomes were preterm birth, low birthweight, and hospital attendances for asthma. Effect estimates were pooled with random-effects meta-analysis. This study is registered with PROSPERO, number CRD42013003522.

Findings

We identified 11 eligible studies (published 2008—13), involving more than 2·5 million births and 247 168 asthma exacerbations. All studies used interrupted time-series designs. Five North American studies described local bans and six European studies described national bans. Risk of bias was high for one study, moderate for six studies, and low for four studies. Smoke-free legislation was associated with reductions in preterm birth (four studies, 1 366 862 individuals; −10·4% [95% CI −18·8 to −2·0]; p=0·016) and hospital attendances for asthma (three studies, 225 753 events: −10·1% [95% CI −15·2 to −5·0]; p=0·0001). No significant effect on low birthweight was identified (six studies, >1·9 million individuals: −1·7% [95% CI −5·1 to 1·6]; p=0·31).

Interpretation

Smoke-free legislation is associated with substantial reductions in preterm births and hospital attendance for asthma. Together with the health benefits in adults, this study provides strong support for WHO recommendations to create smoke-free environments.

Funding

Thrasher Fund, Lung Foundation Netherlands, International Paediatric Research Foundation, Maastricht University, Commonwealth Fund.

Introduction

Smoking is estimated to kill 5·7 million people each year, and effective tobacco control is a key instrument for reduction of global mortality and rates of non-communicable diseases.1, 2 The WHO Framework Convention for Tobacco Control provides a valuable means to implement and evaluate tobacco control measures.3, 4 As part of six key recommendations the implementation of smoke-free environments is advocated as an instrument to reduce the estimated 600 000 deaths and 10·9 million disability-adjusted life-years (DALYs) globally due to second-hand smoke exposure each year.1, 5 In support of this recommendation, there is now emerging evidence that smoke-free laws can effectively reduce second-hand smoke exposure and improve population health.6 A 2012 meta-analysis showed a 15% reduction in cardiovascular events and a 24% reduction in admissions to hospital for respiratory diseases after introduction of smoke-free legislation.7 However, although 177 nations have now ratified the Framework Convention for Tobacco Control, at present, only 16% of the worlds population is covered by comprehensive smoke-free laws.5

A broad appreciation of the population health effects of smoke-free legislation might help to further strengthen its mandate. Most studies have focused on the evaluation of adult outcomes; however, children account for more than a quarter of all deaths and more than half of all DALYs due to second-hand smoke exposure.1 The effect of in-utero and early-life exposures on health in childhood and later life is a growing specialty of research interest with major public health implications.8 Children are particularly vulnerable to the adverse effects of second-hand smoke because their lungs and immune system are still undergoing development. The first reports of detrimental health effects of parental smoking on paediatric respiratory health date back to the early 1970s.9 Second-hand smoke has since been linked to a range of adverse outcomes during early-life including stillbirth, preterm birth, low birthweight, congenital anomalies, neonatal and infant mortality, asthma, and respiratory infections.10—12 Furthermore, recent studies implicate childhood second-hand smoke exposure in the development of non-communicable diseases in later life.13, 14 40% of children worldwide are regularly exposed to second-hand smoke,1 which is of great concern because they are generally unable to influence their own level of exposure. Smoke-free legislation has been shown to reduce second-hand smoke exposure in children and increase the proportion of smoke-free homes in several countries.15—19

Emerging evidence now shows particular benefits of smoke-free legislation on early-life health.20—23 However, only a small minority of the worlds population is covered by comprehensive smoke-free laws, and this proportion is increasing slowly.5 This slow increase could at least partly be because of uncertainty about the potential health benefits, particularly concerning child health. A comprehensive assessment is therefore warranted to obtain a well informed appreciation of the effect of smoke-free public and work environments on early-life health and to inform national and international policy decisions on implementation of smoke-free legislation.

Methods

Search strategy and selection criteria

We followed the methods detailed in a peer-reviewed systematic review protocol that is registered with PROSPERO (CRD42013003522).24 We searched online databases of medical literature (Medline, Embase, Google Scholar, ISI Web of Science, Cochrane Central Register of Controlled Trials, Trip, Cumulative Index to Nursing and Allied Health [CINAHL], Allied and Complementary Medicine Database [AMED], CAB International, Global Health, WHO Global Health Library [regional and global indexes], SciELO, IndMED, and KoreaMed) for published studies from January, 1975, to May, 2013, and the overarching WHO International Clinical Trials Registry Platform to identify unpublished work relevant to the research question. Appendix pp 1—3 shows an overview of the search terms used. The search was restricted to studies published from 1975 onwards, when, to the best of our knowledge, the first regional smoking ban was introduced in Minnesota, USA.25 We did not apply any language restriction. Furthermore, we hand-searched reference lists, screened citations of articles of interest with ISI Web of Science and Google Scholar, and approached an international panel of experts in the field to identify additional published and unpublished studies.

Studies were eligible for inclusion if they investigated the association between introduction of a smoking ban in workplaces or public places, or both, and one or more prespecified health outcomes in children. We focused on children aged 12 years or younger to minimise potential confounding by self-smoking, but accepted ages up to 20 years as long as most of the population met the original age criterion. Corresponding authors of eligible studies that included children without reporting their outcomes separately from adults were approached to obtain paediatric subgroup analyses. Primary outcomes were stipulated a priori as preterm birth, low birthweight, and asthma. Secondary outcomes included various perinatal mortality indicators, very preterm birth, very low birthweight, small for gestational age, congenital anomalies, bronchopulmonary dysplasia, upper and lower respiratory infections, otitis media with effusion, wheezing, and chronic cough. Definitions and specifications and consideration of additional outcomes are described elsewhere.24 Surrogate and intermediate outcomes, smoke-related behaviour, smoke-exposure, and economic data were not included. Following the Cochrane Effective Practice and Organisation of Care (EPOC) guidelines, we stipulated that only the following study designs were eligible: randomised controlled trials, controlled clinical trials, controlled before-and-after studies, and interrupted time series.26 Searches were done independently by two reviewers, and final study selection was based on consensus between both reviewers and, if necessary, independent arbitration.

Data extraction and risk of bias assessment

Relevant data were extracted from each paper with customised data extraction forms. We contacted corresponding authors to obtain additional information when necessary. Risk of bias was assessed with EPOC quality criteria.27 Two reviewers independently extracted data and assessed quality, and disagreement was resolved by consensus, or arbitration by a third author. Parental or maternal smoking was defined as the main confounder for which adjustment was recorded.

Statistical analysis

The main findings from the systematic review, including characteristics and key findings of individual studies are presented in tabular form. For the meta-analysis, relative risk differences were selected from the most adjusted model presented in each study and pooled with DerSimonian-Laird random-effects meta-analysis. Several papers evaluated both a step change (direct change in incidence) and a slope change (change in incidence with time) after introduction of a smoking ban (appendix p 4), which were pooled in separate analyses. For this purpose, effect estimates derived from difference-in-difference models were considered step changes (appendix p 5).28Appendix p 6 shows the formulas used to calculate relative risk differences from absolute estimates and 95% CIs from standard errors. Substantial between-study heterogeneity was anticipated and handled through random-effects models. Heterogeneity was assessed with the I2 test. We assessed potential publication bias by visual interpretation of funnel plots for the primary analyses. In view of the small number of studies in each meta-analysis, no attempt was made to formally test for small-study effects.29 For the same reason, we decided not to do the planned sensitivity and subgroup analyses.24 All analyses were done with StatsDirect (version 8) and Stata (version 12.0).

Role of the funding source

The sponsors of the study had no role in study design, data collection, data analysis, data interpretation, or writing of the report. The corresponding author had full access to all the data in the study and had final responsibility for the decision to submit for publication.

Results

We screened 9228 titles from which we identified 20 potentially eligible studies (figure 1).20—23,30—45 One study reporting a mixed paediatric and adult population was excluded because subgroup analyses for children could not be obtained.45 Eight further studies were excluded because they did not meet EPOC design criteria.37—44Appendix p 7 shows the characteristics and findings of the eight excluded studies. We identified 11 eligible studies, reported between 2008 and 2013: eight published papers20—23,32,34—36 and three online reports.30, 31, 33 One study reported the effect of a state-wide smoking ban and the previous overall effect of a combination of local bans introduced at different timepoints.30 Following EPOC guidelines,26 only the evaluation of the state-wide ban was included. Corresponding authors of several reports provided supplementary unpublished data for this report.20—23,30—34

Figure 1 Full-size image (26K) Download to PowerPoint

Study selection

EPOC=Cochrane Effective Practice and Organisation of Care.

Table 1 and appendix p 8 show the detailed study characteristics of the 11 eligible studies. In summary, there were five North American studies describing effects of district-wide or state-wide bans, and six European studies investigating the effect of national bans. Smoke-free legislation was generally comprehensive and introduced at once, except in a Belgian study in which a step-wise approach was used.22 Four studies described the effect on paediatric asthma,20, 23, 32, 36 whereas all other studies reported on perinatal outcomes.21,22,30,31,33—35 All studies were classified as interrupted time series, although specific analytical strategies differed. Three studies used a difference-in-difference design, which can be deemed a form of interrupted time series with a control group (appendix p 5). Outcomes for all studies were based on retrospective interrogation of routinely collected health-care data. Definitions of outcomes were similar between studies with two exceptions: small for gestational age was defined as a birthweight below the 5th centile for gestational age by Kabir and colleagues,34 and as a birthweight below the 10th centile by others;21, 22 and Rayens and colleagues36 evaluated asthma-related emergency department visits, whereas other reports studied admissions to hospital.20, 23, 32

Table 1Table image

Study characteristics—intervention and outcomes

Four studies were deemed to have low risk of bias, six had moderate, and one had high (appendix p 9). Lower risk studies tended to be of European origin, have large study populations, and evaluate national-level bans.

Table 2, Table 3 show the findings from each individual study. Three of five studies reported a significant reduction in preterm births after introduction of smoke-free legislation (figure 2A). A non-significant trend was reported in the Norwegian study, which was likely to be underpowered.31 Meta-analysis showed smoke-free legislation to be associated with a clinically important and statistically significant drop in preterm birth (four studies, 1 366 862 participants; −10·4% [95% CI −18·8 to −2·0], p=0·016; figure 2A) with no subsequent effect on preterm birth rate change over time (appendix p 10). Data from one study could not be obtained and this study was not included in the meta-analysis.33

Table 2Table image

Primary outcomes per study

Table 3Table image

Secondary outcomes per study

Figure 2 Full-size image (86K) Download to PowerPoint

Meta-analysis of step changes in primary outcomes after smoking ban

(A) Preterm birth. (B) Low birthweight. (C) Asthma admissions to hospital. The low-birthweight point estimate for Cox and colleagues22 was based on a backward selection model with a single step and single slope change, similar to the preterm birth model.

One of six studies showed a reduction of low birthweight after smoke-free legislation (figure 2B). Meta-analysis identified an effect that was not statistically significant (six studies, >1·9 million participants; −1·7% [95% CI −5·1 to 1·6], p=0·31; figure 2B, appendix p 10).

Three of four studies reported a significant drop in hospital attendance because of asthma after introduction of smoke-free legislation. Meta-analysis showed both an immediate reduction (three studies, 225 753 events; −10·1% [95% CI −15·2 to −5·0], p=0·0001; figure 2C) and an additional non-significant trend towards an annual rate decrease (three studies, 241 846 events: −7·5% per year [95% CI −16·0 to 0·9], p=0·081; appendix p 11) after introduction of smoking bans.

In line with the overall effect of smoke-free legislation on preterm birth, one study21 also reported a reduction in very preterm birth (table 3). One of two studies showed a significant effect on very low birthweight, with no significant effect observed after meta-analysis (figure 3A). Three studies investigated the effect of smoke-free legislation on being small for gestational age, with two studies showing a significant step reduction. However, neither the step nor the slope reduction was significant after meta-analysis (figures 3B, appendix p 12). Important reductions were observed for very small for gestational age, although this was not a prespecified outcome. These reductions were statistically significant after pooling (two studies, 1 305 965 participants; step change: −5·3% [95% CI −5·4 to −5·2], p<0·0001, figure 3C; slope change: −0·6% per year [95% CI −0·604 to −0·596], p<0·0001; appendix p 12), but mainly driven by one study.34 No effect was noted on birth defects in a small study, albeit with very wide CIs (table 3).31 No studies were identified that investigated any of the other prespecified outcomes.24

Figure 3 Full-size image (65K) Download to PowerPoint

Meta-analysis of step changes in secondary outcomes after smoking ban

(A) Very low birthweight. (B) Small for gestational age. (C) Very small for gestational age. The small-for-gestational-age point estimate for Cox and colleagues22 was based on a backward selection model with a single step and single slope change, similar to the preterm birth model.

Additional outcomes described in individual reports that are not reported here include preterm labour,21 mild preterm birth,21 moderate preterm birth,21 extremely low birthweight,31 birthweight less than 2000 g,31 birthweight less than 3000 g,35 sex,31 Apgar score,31 and birthweight and gestational age quantified on a continuous scale.30

Visual interpretation of funnel plots showed potential publication bias for the studies on preterm birth, but not for those on low birthweight or asthma (appendix p 13).

Discussion

This systematic review and meta-analysis presents strong evidence supporting the effectiveness of smoke-free legislation to improve clinically important perinatal and child health outcomes. In particular, pooled estimates suggest that rates of both preterm birth and paediatric hospital admissions for asthma were reduced by 10% after its introduction. Additional reductions in the risk of being born very small for gestational age were identified. This Article adds important information from an early-life perspective to the growing evidence base supporting the mandate for worldwide implementation of smoke-free legislation as a means to improve population health.

As far as we are aware this is the first systematic review and meta-analysis of the effect of smoke-free legislation on child health. By use of a comprehensive search strategy, we were able to identify a substantial body of evidence, including an important share of unpublished reports.30, 31, 33 Although unpublished reports might suggest that additional unidentified evaluations potentially exist, none were identified by our international panel of experts. The small number of studies hampered visual interpretation of funnel plots and precluded formal assessment of small study effects that might have indicated consequential publication bias.24, 29

Substantial variation was present in sample size and the number of pre-ban and post-ban observations. As a result, precision of the reported point estimates varied greatly between studies and was very low in some, potentially suggesting that they were underpowered.31, 32 We therefore decided to address this issue by doing a meta-analysis to provide a better overall appreciation of the effect of smoke-free legislation on each outcome.24 The inclusion of some very large studies,46 and differences in study design, analysis strategy,47 and smoking ban coverage and enforcement, are likely to contribute to the observed heterogeneity, which was handled through random-effects modelling.48

Intrinsic to community-based policy interventions, which rarely allow evaluation via randomised controlled trial designs,47 the evidence presented here is derived from observational studies, which are inherently at risk of bias. All but one study had low to moderate risk of bias, and overall no clear association between risk of bias and effect size was apparent. To involve only the highest quality evidence available in our report, we followed EPOC guidelines and included only quasiexperimental studies.26 On the suggestion of peer reviewers, we undertook a post-hoc sensitivity analysis that included studies that did not fit EPOC design criteria, but this analysis did not materially affect the results (data not shown). A limitation of this study is that we were unable to do the predefined subgroup and sensitivity analyses because of the low number of studies in each meta-analysis.24

Several within-study aspects that could have affected estimation of the effect of the smoking ban require consideration. Steep incidence changes in hospital attendance for asthma in the pre-ban period could have resulted in overestimation of the effect of the ban in one study,36 and in underestimation in another.32 Complicating interpretation of the difference-in-difference study by Page and colleagues35 is the suggestion that the effect might be attributable to an increasing incidence in the control site rather than a decrease in the intervention site.49 Although these issues underline some of the pitfalls in interpreting interrupted time series studies,47 several studies also showed the robustness of the association with extensive sensitivity and subgroup analyses,20—23,31,34 and by pinpointing the timing of public health benefit to that of introduction of the ban.34 These issues are important in view of the criticism that interrupted time-series analyses might identify the intervention under study as being responsible for an effect that might in fact be due to other unmeasured factors.47

The positive effect of smoke-free legislation on selected indicators of child health adds an important dimension to the substantial body of evidence supporting its benefit to adult health. Reductions in cardiovascular and cerebrovascular events, hospital admissions for respiratory disease, and respiratory and sensory symptoms have been reported in several meta-analyses.7,50—53 Causality is further supported by a dose-response effect for most outcomes, with comprehensive smoking laws having the largest health benefit.7 Many studies have identified that smoking bans effectively reduce maternal smoking during pregnancy and second-hand smoke exposure during childhood.23, 31, 35, 43, 54, 55 The proportion of smoke-free homes increased in several countries after smoke-free laws, disproving initial fears that smoking would be displaced towards the home.18,19,54—57 The health benefit in children is likely to be mediated through these reductions in antenatal and postnatal second-hand smoke exposure. Despite reports that smoke-free laws selectively reduce second-hand smoke exposure in less exposed and less deprived children,15, 18, 54, 55, 58 its effect has been shown to be consistent across socioeconomic backgrounds.20

The association between smoke-free laws and reductions in preterm birth, birthweight for gestational age, and asthma events identified by our meta-analysis accord with recognised associations of these outcomes with second-hand smoke exposure and are biologically plausible.10—12 Findings from meta-analyses have shown that second-hand smoke exposure is associated with an increased risk of low birthweight and intrauterine growth restriction.59, 60 Although observational evidence of the effect of second-hand smoke exposure on length of gestation is inconclusive,59—61 a randomised controlled trial showed a significant reduction in very preterm birth after an intervention that decreased second-hand smoke exposure in the home.62 A strong association between both antenatal and postnatal second-hand smoke exposure and childhood asthma furthermore exists,1, 63, 64 supporting a positive effect of smoke-free legislation on asthma events.20, 23, 36

The public health effect of smoke-free legislation on perinatal and paediatric health is considerable. Worldwide, more than 11% of children are born preterm, amounting to 15 million babies each year.65 Despite efforts to address this issue, preterm birth rates continue to rise in most regions.65 In view of recent estimations that in high-income countries implementation of the five most effective preventive approaches (maternal smoking cessation, progesterone, cervical cerclage, decreasing non-medically indicated caesarean section, and labour induction, and limiting multiple embryo transfer in assisted reproductive technology) could maximally reduce preterm birth by 5%,66 a possible 10% reduction by smoke-free legislation is promising. Although one negative study could not be included in the meta-analysis, potentially resulting in overestimation of the true effect, even a small reduction in preterm birth is relevant at the population level.67 In view of the well recognised long-term adverse consequences of preterm birth, this health benefit holds potential to extend throughout the entire lifecourse.67 The adverse effect of intrauterine growth on adult health suggests that the same might be true for its effect on being very small for gestational age.8 In view of its effect on preterm birth, it seems surprising that our meta-analysis did not identify an effect on low birthweight. Possibly, the risk reduction of preterm birth at the population level attributed to smoking bans is not large enough to translate into an effect on population birthweight. Furthermore, similar to myocardial infarction syndrome, second-hand smoke exposure could act as an onset factor or trigger for preterm birth, whereas birthweight is more importantly affected by its cumulative effect over time. The public health relevance of a reduction in asthma, which is now the most common chronic disease in childhood,68 is unambiguous and might project into adulthood in view of evidence of long-term tracking of reduced lung function and a corresponding link with chronic obstructive pulmonary disease.69

Despite the emerging evidence for early-life health benefits associated with smoke-free legislation, several knowledge gaps remain. Although we searched relevant regional and global health databases, we were unable to identify any studies from low-income or middle-income countries, where the highest burden of early-life adverse outcomes lies.70, 71 Although this burden suggests great potential for smoke-free laws to benefit population health,72 this promise might need to be tempered because of concerns about the challenges of enforcement and as yet contradictory effects on smoking patterns in the home.18, 73 Therefore, the possibility is raised that findings from high-income countries might not easily be globally generalisable, showing the pressing need for studies assessing the effect of tobacco control measures in countries with low and middle income. Additional studies would furthermore help to expand the present evidence base, which is now centred on a small number of studies, with pooled effect estimates consequentially less precise than in adult reports.7,50—53 Most of the health burden of second-hand smoke exposure in childhood is related to respiratory tract infections, and studies are needed to estimate the effect of smoke-free legislation on this outcome.1 In keeping with the recognised link between second-hand smoke and sudden infant death syndrome,12 smoke-free public places have been associated with reductions in its incidence.39 Further large studies are needed to assess their effect on early-life mortality. Finally, comprehensive assessment is needed of the early-life effect of additional tobacco control measures including taxation and advertisement bans, because these effects could be difficult to distinguish from those of smoke-free legislation and they might act synergistically to improve population health.74

Smoke-free legislation is a cost-effective population intervention in view of the magnitude of its public health benefits and the established absence of adverse economic effects that have long been claimed by the tobacco industry.75—77 Formal cost-effectiveness studies are, however, needed to further substantiate this notion.

In conclusion, we provide clear evidence showing reductions in preterm birth, asthma events in childhood, and being born very small for gestational age after introduction of smoke-free legislation. Although the exact mechanisms by which smoke-free legislation exerts its effect are unknown, it is now evident that early-life protection from involuntary second-hand smoke exposure holds great potential to reduce the consequential disease burden and associated economic losses posed to society. This report thus provides support from an early-life perspective for present WHO recommendations to implement smoke-free public environments on a national level.4, 5 These findings should be regarded as an integral part of public health strategy to further reduce the worldwide burden of disease associated with smoking.1, 2, 78

Contributors

JVB coconceived and designed the study; secured funding; was involved in the search process, study selection, quality assessment, data extraction, and analysis; and wrote the manuscript. UBN was involved in the search process, study selection, quality assessment, and data extraction, and commented on the manuscript. BC and TSN were involved in data extraction and analysis, and figure preparation, and commented on drafts of the manuscript. CPvS advised in the study selection and data handling, and commented on drafts of the manuscript. AS coconceived this study, helped to develop the methods, supervised the work, and commented on drafts of the manuscript. JVB had full access to all the data presented and had final responsibility for the decision to submit for publication.

Declaration of interests

We declare that we have no competing interests.

Acknowledgments

This work was supported by the Thrasher Research Fund Early Career Award NR-0166 (JVB); the Lung Foundation Netherlands Long Term Fellowship 3·4.12·128FE (JVB), a Maastricht University Medical Centre Kootstra Talent Fellowship (JVB), and the International Paediatric Research Foundation Young Investigator Exchange Program (JVB). BC is supported by the Flemish Scientific Fund (FWO; G.0873.11.N.10), and TSN is supported by a European Research Council starting grant. AS is supported by The Commonwealth Fund, a private independent foundation based in New York City. The views presented here are those of the author and not necessarily those of The Commonwealth Fund, its directors, officers, or staff. We thank Marshall Dozier for assistance in preparing the search strategy, members of the expert panel for providing advice, Michelle Amaral, Marina Bianchi, Connie Cheverie, Katherine Gaudreau, Julian Johnsen, Zubair Kabir, John Tayu Lee, Katrine Løken, Daniel Mackay, Carol McClure, Chris Millett, Jill Pell, and Anita Ravelli for providing additional data on their studies.

Supplementary Material

Supplementary appendix

PDF (875K)

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a School for Public Health and Primary Care (CAPHRI), Maastricht University, Maastricht, Netherlands

b Allergy and Respiratory Research Group, Centre for Population Health Sciences, The University of Edinburgh, Edinburgh, UK

c Department of Paediatrics, Maastricht University Medical Centre, Maastricht, Netherlands

d Centre for Environmental Sciences, Hasselt University, Hasselt, Belgium

e Department of Public Health, University of Leuven, Leuven, Belgium

f Division of General Internal Medicine and Primary Care, Brigham and Womens Hospital/Harvard Medical School, Boston, MA, USA

Correspondence to: Dr Jasper V Been, Department of Paediatrics, Maastricht University Medical Centre, PO Box 5800, 6202 AZ Maastricht, Netherlands