PM stands for particulate matter, which is a mixture of solid particles and liquid droplets found in the air. Size of PM ranges from 2.5 micrometers (μm) to more than 40 μm. PM2.5 represents fine particles equal or less than 2.5 μm in diameter, which can be inhaled and get deep into the lungs. The major components, or species, are metals, polycyclic aromatic hydrocarbons (PAHs), carbon, sulfate and nitrate compounds, and crustal materials; however, significant diurnal, seasonal, local and regional variations are observed in both composition and concentrations. PM2.5 can cause alveolar inflammation, asthma and the other health issues. More importantly, it may trigger long term immune system perturbation when exposed in infancy. Previous studies reported the association of ambient air pollution using PMs as indicators with autism. Prenatal exposure and autism has been suspected due to increasing air pollutant (NOx, PM2.5, and PM10) (Suades-González E, et al. 2015). No consistent results are available yet; however, the composition of PMs changes by location and season, inconsistent results do not necessarily suggest the lack of association. Therefore, a prospective study with direct measurement of such toxicants in blood and urine samples is highly required to understand temporal relationship between exposures during pregnancy and autism.
As mentioned in the chapter 7 of Book Metabolic Phenotyping in Personalized and Public Healthcare: "The human metabolome represents expression of an individual’s metabolic phenotype and is a combination of chemicals from core biological processes, the microbiome, dietary exposures, drugs, and exposure to xenobiotics from the environment and commercial products. " (Walker DI, et al. 2016) It was estimated that human has more than 106 metabolites. However, given the individual differences, chemical mesurements vary in both concentration and diversity within human population.
Fig. Human metabolome (Walker DI, et al. 2016)
The LC-HRMS is a untargeted approach which allows us to identify chemicals without prior knowledge. LC-HRMS is also capable to measure large number (15,000 to 20,000) of chemicals arose from the external environment and produced within our body. The GC-MS/MS is a targeted method which can measure the compounds of interest. Additionally, GC-MS/MS is required for low level, semi-volatile enviromental chemicals which compensates the LC-HRMS platform.

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