Table 1.

Summaries of etiology of particulate matter and pathways implicated until reaching the CNS in the brain.

PM by Diameter	1 Main Etiology	Deposited in:	References
PM0.1 or UFPM ≤100 nm	Smoke from wildfire	Cerebral cortex and cerebellum secondary to transport via the olfactory nerves	[11,28]
PM2.5 ≤2.5 μm	Traffic-related activities, industrial sites, factories, and agriculture	2 Lungs and is also subject to olfactory transport and deposition in the olfactory cortex and other brain regions	[16,18,19,30]
PM10 ≤10 μm	Pollen, mold, and dust particles	Filtered out by the nose and upper airways	[16,18,19,29,30]

UFPM: ultrafine particles; CNS: central nervous system. Notes: ¹ Outdoor environmental particles are primarily produced from road transport, agriculture, power plants, industry, and forest fires [11,16]. Indoor environmental particles are derived from combustion activities such as cooking, as well as and heating with coal, wood or dung, candles, incense, kerosene lamps, tobacco smoking, non-combustion sources, and volatile organic compounds such as cleaning and insecticide products, electric devices, and printers [2]. ² PM enters through the nose; once the lungs are reached, PM may also travel through the blood into the brain [16,30].