be ▁in ▁varying ▁sizes ▁and ▁compos itions ▁and ▁is ▁the ▁focus ▁of ▁many ▁ep ide mi ological ▁studies . ▁ ▁Part ic ulate ▁matter ▁( PM ) ▁is ▁generally ▁class ified ▁into ▁two ▁main ▁size ▁categories : ▁PM 1 0 ▁and ▁PM 2 . 5 . ▁PM 1 0 , ▁also ▁known ▁as ▁co arse ▁partic ulate ▁matter , ▁consists ▁of ▁particles ▁ 1 0 ▁mic rom eters ▁( μ m ) ▁and ▁smaller , ▁while ▁PM 2 . 5 , ▁also ▁called ▁fine ▁partic ulate ▁matter , ▁consists ▁of ▁particles ▁ 2 . 5 ▁ μ m ▁and ▁smaller . ▁Part icles ▁ 2 . 5 ▁ μ m ▁or ▁smaller ▁in ▁size ▁are ▁especially ▁notable ▁as ▁they ▁can ▁be ▁in h aled ▁into ▁the ▁lower ▁resp ir atory ▁system , ▁and ▁with ▁enough ▁expos ure , ▁absor bed ▁into ▁the ▁blood stream . ▁Part ic ulate ▁poll ution ▁can ▁occur ▁directly ▁or ▁indirect ly ▁from ▁a ▁number ▁of ▁sources ▁including , ▁but ▁not ▁limited ▁to : ▁agricult ure , ▁autom ob iles , ▁construction , ▁forest ▁fires , ▁chemical ▁poll ut ants , ▁and ▁power ▁plants . ▁ ▁Ex pos ure ▁to ▁particul ates ▁of ▁any ▁size ▁and ▁composition ▁may ▁occur ▁ac utely ▁over ▁a ▁short ▁duration , ▁or ▁chron ically ▁over ▁a ▁long ▁duration . ▁Part ic ulate ▁expos ure ▁has ▁been ▁associated ▁with ▁ad verse ▁resp ir atory ▁sympt oms ▁r anging ▁from ▁ir rit ation ▁of ▁the ▁air ways , ▁ag gra v ated ▁ast h ma ,