The Effects of Lead and Zinc on Asthma: A Systematic Review and Meta-Analysis

Context: In recent decades, the growing number of factories and vehicles has accelerated the release of environmental contaminants such as heavy metals. These inhalable pollutants often cause respiratory diseases and can endanger human health. Some heavy metals (e.g., zinc), on the other hand, are beneficial micronutrients, playing critical roles in the body. Objectives: This systemic review and meta-analysis study aimed to examine the role of lead (Pb) and zinc (Zn) in asthma. Methods: MeSH keywords, including lead, asthma, and zinc, along with their Persian equivalents were searched in the PubMed, Google, Web of Sciences, Google Scholar, and SID databases. The articles published between 2000 and 2018 were included. For assessing heterogeneity, the Q and I2 statistics were applied. STATA version 14 was used for data analysis. The review was conducted in accordance with the preferred reporting items for systematic reviews and meta-analyses (PRISMA) statement. Results: The frequency of asthma was 12% (95% CI: 11 - 13%, P < 0.001). The frequencies of high blood lead levels (BLL) in controls and patients with asthma were 2% (95% CI: 0 - 2.03, P < 0.001) and 3% (95% CI: 0 - 3.04, P < 0.001), respectively. The ratios of BLL of < 5, 5 - 10, and > 10 µg/dL were found to be 8% (95% CI: 0 - 9.7, P < 0.001), 10% (95% CI: 0.8 - 11, P < 0.001), and 9% (95% CI: 0.08 - 11, P < 0.001), respectively. The forced expiratory volume (FEV) was significantly correlated with zinc level. The results from meta-regression analysis suggested an increasing trend in asthma occurrence from 2000 to 2018. Conclusions: While zinc may have been protective against asthma, lead was a risk factor for respiratory diseases and asthma. Educating children and their parents about lead poisoning may have prevented or reduced lead intoxication and asthma in children.