Formaldehyde Exposure and Asthma in Children: A Systematic Review

Gerald McGwin Jr.; Jeffrey Lienert; John I. Kennedy Jr.

Disclosures

Environ Health Perspect. 2010;118(3) 

In This Article

Results

Overall, 10 studies involving 6,387 participants including 635 with diagnosed or self-reported asthma were selected for systematic review, of which seven studies were able to be used in the meta-analysis involving a total of 5,930 participants, 364 of whom had diagnosed asthma (Table 1). Most studies were cross-sectional; half relied on self-reported information on asthma diagnoses, whereas the remainder used actual physician diagnoses. Studies of the former type generally queried participants about whether they had ever been diagnosed with asthma, and thus those responding affirmatively are best characterized as prevalent cases. With respect to those studies using physician diagnosis, based on the study designs as they were described, it was frequently clear that those with asthma would also be best characterized as prevalent cases. In only one study was it entirely clear that the cases were truly incident (i.e., newly diagnosed). Participation rates ranged from 46% to 99%; however, this information was not available for half of the studies.

The results for each individual study as well as the fixed- and random-effects pooled ORs and 95% CIs are shown in Figure 1. The forest plots for both fixed- and random-effects models can be seen in Figures 1 and 2, respectively. The results of the fixed-effects model indicate a 3% increase (95% CI, 1.021.04, p < 0.0001) in asthma risk for each 10-µg/m3 unit increase in formaldehyde, whereas the random-effects model indicates a 17% increase (95% CI, 1.011.22, p = 0.0158). The Q and I 2 statistics were 14.28 (p < 0.0001) and 51%, respectively, indicating the presence of moderate between-study heterogeneity.

Figure 1.

Forest plot of the relative risk estimates and their 95% CIs from the studies included in the meta-analysis of the association between formaldehyde exposure and asthma in children based upon a fixed-effects model.

Figure 2.

Forest plot of the relative risk estimates and their 95% CIs from the studies included in the meta-analysis of the association between formaldehyde exposure and asthma in children based on a random-effects model.

The influence plot indicated that one study (Rumchev et al. 2002) may have had an undue influence on the study results (data not shown). When this study was excluded, the resulting ORs from fixed- and random-effects models were 1.24 (95% CI, 1.091.42) and 1.24 (95% CI, 1.071.44), respectively (Table 2). The Q and I 2 statistics were 6.76 and 11.2%, respectively.

Table 2 presents the pooled results stratified according to specific study characteristics. Based on the fixed-effects model, the OR for self-reported asthma was 1.22 (95% CI, 1.021.46), whereas the OR for diagnosed asthma was 1.03 (95% CI, 1.021.04). For the random-effects model, the ORs for self-reported and diagnosed asthma were 1.26 (95% CI, 0.971.64) and 1.12 (95% CI, 0.881.44), respectively. When stratified according to study design, the ORs for cross-sectional studies were 1.25 (95% CI, 1.081.44) for fixed effects and 1.26 (95% CI, 1.031.55) for random effects. There was only one cohort study and one casecontrol study; the results of these individual studies appear in Table 1. With respect to the exposure setting (home vs. school), assuming fixed effects, the OR was 1.03 (95% CI, 1.021.04) for home exposure, whereas for school exposure the OR was 1.32 (95% CI, 1.051.66). For the random-effects model, the OR for home exposure was 1.10 (95% CI, 0.951.27), and the OR for school exposure was 1.33 (95% CI, 1.021.74). Only one study (Zhao et al. 2008) evaluated outdoor exposure, and its results appear in Table 1. Finally, the fixed- and random-effects results for the four studies (Mi et al. 2006; Smedje and Norbäck 2001; Smedje et al. 1997; Zhao et al. 2008) that provided participation rates (66%, 82%, 90%, and 99%) were stronger than the results for those for which participation rates were unknown.

The funnel plot did not show evidence of publication bias either with or without Rumchev et al. 2002 (data not shown).

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