The indoor air sampling was performed upon the child’s parents’ agreement to
participate, and the sampling time for the case and the matched control households
was within the same month. The air samples for the analysis of the concentrations of
HCHO and NO2 were collected using a diffusion sampler packed with silica gel
containing triethanolamine (passive gas tube for HCHO and NO2, Sibata Scientific
Technology, Japan). The air samples for the analysis of the sixteen VOCs were
collected using a diffusion sampler packed with activated charcoal (passive gas tube
for organic solvents, Sibata Scientific Technology, Japan). The duration of sampling
was approximately 24 h. Sampling was carried out in the child’s bedroom where the
children spend the most time. The samples were taken at a height of 60e70 cm
above thefloor, corresponding to the height at which the children were positioned
when sleeping