Quartiles of polycyclic aromatic hydrocarbons in maternal serum and risk of total neural tube defects (NTDs) in a population in Shanxi Province, China, 2010–2013. ΣL-PAHs (left panel) is the sum of acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, and retene; ΣH-PAHs (middle panel) of pyrene, benz[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, and benzo[a]pyrene; ΣPAHs (right panel) of ΣL-PAHs and ΣH-PAHs.