Health risk assessment from inhalation exposure to indoor formaldehyde: a systematic review and meta-analysis

This systematic review and meta-analysis investigated studies on formaldehyde (FA) inhalation exposure in indoor environments and related carcinogenic (CR) and non-carcinogenic (HQ) risk. Studies were obtained from Scopus, PubMed, Web of Science, Medline, and Embase databases without time limitation until November 21, 2023. Studies not meeting the criteria of Population, Exposure, Comparator, and Outcomes (PECO) were excluded. The 45 articles included belonged to the 5 types of sites: dwelling environments, educational centers, kindergartens, vehicle cabins, and other indoor environments. A meta-analysis determined the average effect size (ES) between indoor FA concentrations, CR, and HQ values in each type of indoor environment. FA concentrations ranged from 0.01 to 1620 μg/m3. The highest FA concentrations were stated in water pipe cafés and the lowest in residential environments. In more than 90% of the studies uncertain (1.00×10-61.00×10-4) due to FA inhalation exposure was reported and non-carcinogenic risk was stated acceptable. The meta-analysis revealed the highest CR values due to inhalation of indoor FA in high-income countries. As 90% of the time is spent indoors, it is crucial to adopt effective strategies to reduce FA concentrations, especially in kindergartens and schools, with regular monitoring of indoor air quality. Indoor formaldehyde exposure increased due to interior decorations and chemical products, and its carcinogenicity and mutagenicity was confirmed by IARC. However, not enough actions have been taken by governments and industry to reduce formaldehyde emissions and improve the environmental performance of their products. This study assesses indoor formaldehyde levels worldwide and associated health risks through a systematic presentation of data available in 5 valid databases. The findings can play a crucial role in developing strategies to control and reduce formaldehyde release in indoor environments, especially schools and kindergartens, before the adverse effects of exposure to this pollutant are encountered.