The meta-analyses did not include a recent study using RDs to estimate the effect of prenatal PM2.5 exposure on asthma development of nearly 20,000 American children.24 In this study, the authors found significant positive associations between log-transformed prenatal PM2.5 exposure (per 2.7-fold increase) and cumulative asthma incidences from 2 to 6 years of age with RDs ranging from 0.015 to 0.035 after adjustment for confounders. Sensitivity analysis of modeling exposure by quintiles also revealed significant associations between prenatal PM2.5 exposure and asthma incidence and persistence by 5 years of age. However, modeling PM2.5 linearly resulted in positive associations but with no statistical significance (eTable 4; http://links.lww.com/EE/A35, which provides details).
Infant PM2.5 exposure and asthma
There were nine studies evaluating the associations between infant PM2.5 exposure and asthma. These included one for birth year exposure,25 four for exposure during the first of life,20,23,24,35 and four for exposure during first 2 years of life.29–31,34 After excluding four studies either with repeated data25,29,31 or estimating the effect by RDs,24 five remained in the meta-analyses.20,23,30,34,35 Our meta-analyses showed a trend toward a positive association that was not statistically significant (overall OR, 1.14; 95% CI = 0.96, 1.35) with low heterogeneity (I2 = 0%; P = 0.480) (Figure 3). The results were robust to multiple sensitivity analyses (Table 2; eFigures 2 and 3; http://links.lww.com/EE/A35, which provides details).
One study also analyzed the outcomes as current asthma or ever asthma plus current wheeze in their regression models,35 which was not included in the meta-analyses. According to the results of those analyses, infant PM2.5 exposure was found to be significantly associated with an increased risk of current asthma of 35% (95% CI = 7%, 70%) at 6–8 years of age, whereas ever asthma plus current wheeze did not show statistically significant associations (eTable 4; http://links.lww.com/EE/A35, which provides details).
In the study assessing RDs,24 significant associations were observed for PM2.5 exposure during the first year of life and incident or persistent asthma when modeling exposure as a log-transformed continuous variable and by quintiles. Similar with the results of prenatal PM2.5 exposure, modeling the PM2.5 as a continuous variable without log-transformation revealed nonsignificant associations. However, goodness-of-fit analyses suggested that the log-transformed modeling was better than the linear continuous modeling. Other sensitivity analyses all suggested significant associations (eTable 4; http://links.lww.com/EE/A35, which provides details).
Prenatal PM2.5 exposure and wheezing
Meta-analysis was not applicable for the five studies of prenatal PM2.5 exposure and wheezing because most of the studies categorized PM2.5 exposure by median and had different outcome definitions.21,26–28
There was only one study that modeled PM2.5 as a continuous variable using regression analyses.33 The authors evaluated the effect of PM2.5 exposure during different trimesters of pregnancy on ever or current wheeze (wheeze in the past year) in five hundred fifty-two 4-year-old children. No significant association was observed in any trimester PM2.5 exposure and wheezing outcomes.
Another study suggested that higher prenatal PM2.5 exposure (>11.22 µg/m3) was significantly associated with a 102% increase (95% CI = 20%, 240%) in the risk of repeated wheezing in children from birth to 2 years of age compared with the lower exposure group (≤11.22 µg/m3), with consistent results from multiple sensitivity analyses.21
The other three studies were from the same project—the Krakow study26–28 which used PEMS to measure PM2.5 exposure during the second trimester of pregnancy. All studies suggested significant associations between prenatal PM2.5 exposure and wheezing duration in the first 2 years of life; however, although the association for 3–4 years of age was also positive, it was not statistically significant (eTable 4; http://links.lww.com/EE/A35, which provides details).
Infant PM2.5 exposure and wheezing
Meta-analyses included three of the four studies investigating the association between infant PM2.5 and wheezing,30,31,35 whereas the other one containing repeated data was excluded.29 Infant PM2.5 exposure was not associated with wheezing development in either random- or fixed-effects models (overall OR, 1.49; 95% CI = 0.99, 2.26) (Figure 4; eFigure 4; http://links.lww.com/EE/A35, which provides details). Low heterogeneity was found in the three studies as indicated by an I2 = 0% and a P value = 0.770. PM2.5 was also not significantly associated with current wheeze at 6–8 years of age35 [eTable 4; http://links.lww.com/EE/A35, which provides details).
Small studies with negative findings have not been published on the associations between prenatal or infant PM2.5 exposure and asthma. The distribution was symmetrical in the funnel plot of infant exposure and wheezing, despite the small number of studies included in the meta-analysis (eFigures 5–7; http://links.lww.com/EE/A35, which provides details).
Outcomes by specific characteristics
There were nine studies including stratified analyses by gender,12,13,20,22–24,29 heredity,23,24 maternal stress during pregnancy,12,33 race,24 atopic status,23 and other characteristics including birth weight, gestational length, maternal age, parity, neighborhood socioeconomic status (SES),13 and genotype35 (eTable 4; http://links.lww.com/EE/A35, which provides details).
The differences of effects by gender were inconsistent among the seven studies. To illustrate, two studies suggested larger magnitudes of effects in males compared with females,12,22 whereas the other five suggested stronger effects in females.13,20,23,24,29 Of those studies, Hsu and colleagues22 reported significant associations in men exposed to PM2.5 during the 12–26th gestational weeks with asthma development, whereas Pennington and colleagues24 reported significant associations between infant PM2.5 exposure and asthma development in women. Other studies did not show significant results among different genders.
Higher risk was shown for children with a family history of asthma than those without in one study,23 whereas the other one24 only found significantly increased risks of asthma in children of mothers without asthma, but not in children of mothers with asthma.
Stratified analyses by maternal stress during pregnancy revealed a consistently significant and increased risk in children whose mothers were highly stressed during pregnancy compared with those slightly stressed.12,33
Only one study24 tested for potential effect modification by race or ethnicity and found no statistical differences between groups described as “white” or “black.”
Studies that evaluated atopic status23 and other characteristics including birth weight, maternal age, parity, gestational length, and SES13 did not find any significant associations with asthma. However, evidence of effect modification was seen with birth weight. Children with a birth weight <2,500 g were at a higher risk of developing asthma associated with prenatal PM2.5 exposure. Children with the Glutathione S-transferase P1 (GSTP1) rs1138272 (Ala114Val) or rs1695 (IIe105Val) minor alleles were more susceptible to developing asthma associated with infant PM2.5 exposure.35
Our meta-analyses demonstrated positive associations between prenatal PM2.5 exposure and asthma and infant PM2.5 exposure, and both wheezing and asthma; however, there were a limited number of relevant studies, and the results were inconsistent. There was high heterogeneity among the studies for prenatal PM2.5 exposure and asthma. This might be due to the variability in children’s ages, exposure measurement methods, sources of particulate matter, outcome definitions, and adjustment of confounding factors. Studies investigating prenatal PM2.5 exposure and subsequent wheezing were not amenable to meta-analysis but consistently reported significant associations, especially in infants (2 years or younger).
This is the largest review assessing long-term effects of prenatal and infant PM2.5 exposure on subsequent wheezing or asthma. We added three more studies12,32,33 to a previous systematic review and meta-analysis of the effects of prenatal exposure to all types of air pollutants including PM2.5 on the development of wheezing and asthma.9 Our results of meta-analyses of the association between prenatal PM2.5 exposure and asthma were similar to this previous review, observing no significant associations and high heterogeneity. In contrast, the other new study not included in meta-analysis reported significantly increased risk of asthma by 2–6 years of age after prenatal exposure to PM2.5.24 However, the evidence was mixed, with more significant associations seen in children followed to school age12,24,32 than preschool age.20,24 This phenomenon might be explained by the difficulties in the diagnosis of asthma among young children,36 leading to the underestimation of physician-diagnosed asthma in this population. The significant associations between prenatal PM2.5 exposure and wheezing in infants26–28 rather than in older children28,33 could indirectly support this explanation. However, some researchers argue that it is difficult to predict asthma based solely on early life wheezing as less than half of children with episodes of preschool wheezing will have continuing childhood asthma.37
For infant PM2.5 exposure and the subsequent development of wheezing or asthma, our meta-analyses did not demonstrate an association. However, these studies were of higher risk of bias due to potential for selection bias,23,30,31,34,35 recall bias,23,30,31,34,35 not adjusted for important confounding factors,20,35 and a case-control design.20,23 In contrast, a recent large, high-quality cohort study of nearly 20,000 children revealed positive associations between PM2.5 exposure during the first year of life and asthma incidence by 6 years of age, despite not adjusting for important confounders.24 This result was robust to different asthma definitions but sensitive to PM2.5 modeling decisions and covariate controls. Overall, the small number of studies identified in this systematic review limited our confidence in conclusively suggesting the presence or absence of associations. Studies with a larger sample size, a standardized exposure estimate method, more accurate outcome assessment approaches, and greater statistical power are needed to further explore the long-term effects of prenatal and infant PM2.5 exposure on asthma or wheeze development.
Our review also highlights the limited evidence of susceptible populations to prenatal and infant PM2.5 exposure. Children whose mothers were exposed to negative life events during pregnancy were more likely to develop wheezing or asthma after prenatal and infant PM2.5 exposure than those not exposed. The different effects of PM2.5 exposure by gender and heredity were inconsistent between studies. There was insufficient evidence to suggest that race, low birth weight, and specific genotypes could increase the risk of wheezing or asthma development after PM2.5 exposure, whereas the effects of atopic status, gestational length, maternal age, parity, and SES require further investigation.
The main strength of our systematic review was the comprehensive search strategy and reproducible evaluation of current evidence. Our findings provide a timely contribution to the rapidly developing field, which could highlight limitations and guide future studies. However, some limitations should also be acknowledged. First, evidence of prenatal and infant PM2.5 exposure and wheezing or asthma is still limited. In addition, publication bias might be present in studies evaluating early life PM2.5 exposure and asthma. Therefore, any conclusions should be made with caution and confirmed by further investigations. Second, high variability was found between studies in study design, exposure estimating methods, outcome assessment approaches, participants’ ages at assessment, and adjustment of confounders, especially in those evaluating prenatal PM2.5 exposure and asthma. Future syntheses of evidence in this area will benefit from more studies using standardized designs and methods. In addition, diagnosis of asthma in young children is difficult, and outcome misclassification is inevitable in this population. Finally, the major source of PM2.5 in this systematic review was traffic, with scarce evidence regarding the long-term respiratory effects of early life PM2.5 exposure from other sources such as wildfire smoke, which is an increasing global concern due to climate change.38,39 More research on PM2.5 from other sources is needed to guide public health responses.
Prenatal and infant PM2.5 exposure was not clearly associated with subsequent development of wheezing or asthma in our review of the literature. The strongest evidence was for an association between prenatal PM2.5 exposure and wheezing in infants, whereas in utero exposure and asthma had a borderline positive overall effect estimate. However, evidence was insufficient and mixed, indicated by a small number of studies included in the meta-analyses and inconsistent results. Further research is necessary to explore the associations using harmonized exposure methods and appropriate statistical analyses controlling for important covariates. Furthermore, studies of susceptible populations and other sources of PM2.5 are needed to help policy makers improving public health.
Conflicts of interest statement
The authors declares that they have no conflicts of interest with regard to the content of this report.
We thank Ulrike Gehring for providing their unpublished data.
1. Cohen AJ, Brauer M, Burnett R, et al. Estimates and 25-year trends of the global burden of disease attributable to ambient air pollution: an analysis of data from the Global Burden of Diseases Study 2015.Lancet20173891907–1918
2. Castner J, Guo L, Yin Y. Ambient air pollution and emergency department visits for asthma
in Erie County, New York 2007-2012.Int Arch Occup Environ Health201891205–214
3. Bowatte G, Lodge C, Lowe AJ, et al. The influence of childhood traffic-related air pollution exposure on asthma
, allergy and sensitization: a systematic review and a meta-analysis
of birth cohort studies.Allergy201570245–256
4. Yang SI. Particulate matter
and childhood allergic diseases.Korean J Pediatr20196222–29
5. Fan J, Li S, Fan C, Bai Z, Yang K. The impact of PM2.5 on asthma
emergency department visits: a systematic review and meta-analysis
.Environ Sci Pollut Res Int201623843–850
6. Khreis H, Kelly C, Tate J, Parslow R, Lucas K, Nieuwenhuijsen M. Exposure to traffic-related air pollution and risk of development of childhood asthma
: a systematic review and meta-analysis
7. Burri PH. Structural aspects of postnatal lung development: alveolar formation and growth.Biol Neonate200689313–322
8. Dietert RR, Etzel RA, Chen D, et al. Workshop to identify critical windows of exposure for children’s health: immune and respiratory systems work group summary.Environ Health Perspect2000108suppl 3483–490
9. Hehua Z, Qing C, Shanyan G, Qijun W, Yuhong Z. The impact of prenatal exposure to air pollution on childhood wheezing
: a systematic review.Environ Res2017159519–530
10. Herbert C, Siegle JS, Shadie AM, et al. Development of asthmatic inflammation in mice following early-life exposure to ambient environmental particulates and chronic allergen challenge.Dis Model Mech20136479–488
11. Wang PL, You DH, Saravia J, Shen HH, Cormier SA. Maternal exposure to combustion generated PM inhibits pulmonary Th1 maturation and concomitantly enhances postnatal asthma
development in offspring.Part Fibre Toxicol20131029doi: 10.1186/1743-8977-10–29
12. Lee A, Leon Hsu HH, Mathilda Chiu YH, et al. Prenatal fine particulate exposure and early childhood asthma
: effect of maternal stress and fetal sex.J Allergy Clin Immunol20181411880–1886
13. Sbihi H, Tamburic L, Koehoorn M, Brauer M. Perinatal
air pollution exposure and development of asthma
from birth to age 10 years.Eur Respir J2016471062–1071
14. Clarke M, Oxman AD, Paulsen E, Higgins JPT, Green Se. Higgins JPT, Green S. Appendix A: guide to the contents of a Cochrane methodology protocol and review.In: Cochrane Handbook for Systematic Reviews of Interventions Version 5.1.0 (Updated March 2011)2011The Cochrane CollaborationAvailable at: www.handbook.cochrane.org
15. Moher D, Liberati A, Tetzlaff J, Altman DG; PRISMA GroupPreferred Reporting Items for Systematic Reviews and Meta-Analyses: the PRISMA statement.PLoS Med20096e1000097
18. Davies HT, Crombie IK, Tavakoli M. When can odds ratios mislead?BMJ1998316989–991
19. Anderson HR, Favarato G, Atkinson RW. Long-term exposure to air pollution and the incidence of asthma
of cohort studies.Air Qual Atmos Health2013647–56
20. Clark NA, Demers PA, Karr CJ, et al. Effect of early life exposure to air pollution on development of childhood asthma
.Environ Health Perspect2010118284–290
21. Chiu YH, Coull BA, Sternthal MJ, et al. Effects of prenatal community violence and ambient air pollution on childhood wheeze in an urban population.J Allergy Clin Immunol2014133713–722.e4
22. Hsu HH, Chiu YH, Coull BA, et al. Prenatal particulate air pollution and asthma
onset in Urban children. Identifying sensitive windows and sex differences.Am J Respir Crit Care Med20151921052–1059
23. Nishimura KK, Galanter JM, Roth LA, et al. Early-life air pollution and asthma
risk in minority children. The GALA II and SAGE II studies.Am J Respir Crit Care Med2013188309–318
24. Pennington AF, Strickland MJ, Klein M, et al. Exposure to mobile source air pollution in early-life and childhood asthma
incidence: the Kaiser Air Pollution and Pediatric Asthma
25. Carlsten C, Dybuncio A, Becker A, Chan-Yeung M, Brauer M. Traffic-related air pollution and incident asthma
in a high-risk birth cohort.Occup Environ Med201168291–295
26. Jedrychowski W, Flak E, Mroz E, et al. Modulating effects of maternal fish consumption on the occurrence of respiratory symptoms in early infancy attributed to prenatal exposure to fine particles.Ann Nutr Metab2008528–16
27. Jedrychowski W, Perera F, Maugeri U, et al. Effect of prenatal exposure to fine particles and postnatal indoor air quality on the occurrence of respiratory symptoms in the first two years of life.Int J Environ Health20082314–329
28. Jedrychowski WA, Perera FP, Maugeri U, et al. Intrauterine exposure to polycyclic aromatic hydrocarbons, fine particulate matter
and early wheeze. Prospective birth cohort study in 4-year olds.Pediatr Allergy Immunol2010214 pt 2e723–e732
29. Gehring U, Cyrys J, Sedlmeir G, et al. Traffic-related air pollution and respiratory health during the first 2 yrs of life.Eur Respir J200219690–698
30. Morgenstern V, Zutavern A, Cyrys J, et al. Respiratory health and individual estimated exposure to traffic-related air pollutants in a cohort of young children.Occup Environ Med2007648–16
31. Brauer M, Hoek G, Van Vliet P, et al. Air pollution from traffic and the development of respiratory infections and asthmatic and allergic symptoms in children.Am J Respir Crit Care Med20021661092–1098
32. Yap PS. .Risk Factors for Atopic Diseases in a Czech Birth Cohort [Ph.D.]2007Ann Arbor, MIUniversity of California, Davis
33. Rosa MJ, Just AC, Kloog I, et al. Prenatal particulate matter
exposure and wheeze in Mexican children: effect modification by prenatal psychosocial stress.Ann Allergy Asthma
34. Gehring U, Wijga AH, Hoek G, et al. Exposure to air pollution and development of asthma
and rhinoconjunctivitis throughout childhood and adolescence: a population-based birth cohort study.Lancet Respir Med20153933–942
35. MacIntyre EA, Brauer M, Melén E, et al; TAG Study GroupGSTP1 and TNF Gene variants and associations between air pollution and incident childhood asthma
: the traffic, asthma
and genetics (TAG) study.Environ Health Perspect2014122418–424
36. Cave AJ, Atkinson LL. Asthma
in preschool children: a review of the diagnostic challenges.J Am Board Fam Med201427538–548
37. Sears MR. Predicting asthma
outcomes.J Allergy Clin Immunol2015136829–836quiz 837
38. Flannigan M, Cantin AS, de Groot WJ, Wotton M, Newbery A, Gowman LM. Global wildland fire season severity in the 21st century.For Ecol Manage201329454–61
39. Westerling AL, Hidalgo HG, Cayan DR, Swetnam TW. Warming and earlier spring increase western U.S. forest wildfire activity.Science2006313940–943
Perinatal; Particulate matter; Wheezing; Asthma; Meta-analysis
Supplemental Digital Content
Copyright © 2019 The Authors. Published by Wolters Kluwer Health, Inc.