Nitrogen-containing organic compounds in household solid-fuel combustion PM and residential urinary inflammatory and targeted amino-acid profiles: an exploratory personal exposure study

Nitrogen-containing organic compounds (NOCs) are important constituents of fine particulate matter emitted from household solid-fuel combustion, yet their personal-exposure characteristics and potential biological relevance remain insufficiently understood. We conducted an exploratory cross-sectional study among 15 rural homemakers, including 12 solid-fuel users and three electric-heating controls. Personal breathing-zone PM_2.5 and PM_0.25 samples were collected during two consecutive 24-h periods, and six classes of filter-collected PM-associated NOCs were quantified. Urinary inflammatory and oxidative-stress-related biomarkers and a targeted panel of 62 amino-acid-related metabolites were also analyzed. Personal exposure concentrations of the measured NOC classes were substantially higher in solid-fuel households than in electric-heating households, with particularly pronounced differences in free amino acids (FAAs) and C6-C20 alkyl nitriles. Urea dominated the total measured NOC mass, whereas FAAs constituted the largest non-urea fraction in the solid-fuel group. Correlation and exploratory mixture-response analyses indicated that FAAs and alkyl nitriles were the NOC classes most consistently associated with urinary C-reactive protein and vascular endothelial growth factor. Targeted urinary amino-acid profiling identified lower urinary concentrations of 4-hydroxyphenyllactic acid and methylguanidine in solid-fuel users based on nominal criteria, together with a borderline reduction in serotonin. Exploratory KEGG analysis further organized the measured metabolites into broad amino-acid-related biochemical categories, but was not interpreted as evidence of pathway activation or impairment. Overall, household solid-fuel use was associated with an NOC-rich personal PM exposure profile and selected urinary biomarker and targeted amino-acid-related patterns. Given the small and imbalanced sample and cross-sectional design, these findings should be interpreted as hypothesis-generating associations rather than confirmed biomarkers, causal effects, or mechanistic evidence.