Particulate matter (PM) is a major component of urban air pollution and has a significant effect on human health. Natural PM sources are volcanic eruptions, dust storms, forest and grassland fires, living vegetation and sea spray. Traffic, domestic heating, power plants and various industrial processes generate significant amounts of anthropogenic PM. PM consists of a complex mixture of solid and liquid particles of organic and inorganic substances suspended in the air. The chemical composition of particles is very complex and depends on emission sources, meteorological conditions and their aerodynamic diameter. Several epidemiological studies have demonstrated that exposure to PM of varying size fractions is associated with an increased risk of respiratory and cardiovascular diseases. Adverse health effects have been documented from studies of both acute and chronic exposure. The most severe effects in terms of overall health burden include a significant reduction in life expectancy by a several months for the average population, which is linked to long-term exposure to moderate concentrations of PM. Nevertheless, numerous deaths and serious cardiovascular and respiratory problems have also been attributed to short-term exposure to peak levels of PM. Although many studies attribute greater toxicity to smaller size fractions, which are able to penetrate deeper into the lung, the molecular mechanisms and the size fractions of the PM that are responsible for the observed diseases are not completely understood. Prašni delci so eden najpogostejših onesnaževal zraka in imajo velik vpliv na zdravje ljudi. Kemijska sestava prašnih delcev je zelo zapletena in je odvisna od vira emisij, meteoroloških okoliščin in od aerodinamskega premera delcev. Naravni izvori prašnih delcev so vulkanski izbruhi, puščavski pesek, naravni požari in morska sol. Promet, domača kurišča, termoelektrarne in industrijski obrati pa so glavni vir antropogenih prašnih delcev. Prašni delci so sestavljeni iz zapletene mešanice trdnih in tekočih delcev, sestavljenih iz organskih in anorganskih snovi. Številne epidemiološke študije so pokazale, da je izpostavljenost prašnim delcem različnih velikosti povezana s povečanim tveganjem za razvoj dihalnih in srčno-žilnih obolenj. Škodljiv učinek na zdravje je bil ugotovljen na osnovi raziskav, ki so proučevale akutno, in na osnovi raziskav, ki so proučevale kronično izpostavljenost prašnim delcem. Najbolj škodljiv učinek prašnih delcev je skrajšanje življenjske dobe povprečne populacije za nekaj mesecev, kar je povezano z dolgoročno izpostavljenostjo prašnim delcem zmernih koncentracij. Kljub temu pa so vzroki za številne smrti in resne dihalne ter srčno-žilne bolezni povezane s kratkoročno izpostavljenostjo visokim koncentracijam prašnih delcev. Številne raziskave pripisujejo večje negativne učinke na zdravje ljudi manjšim prašnim delcem, ker lahko globlje prodrejo v pljuča. Kljub številnim raziskavam še ni popolnoma pojasnjeno, kateri prašni delci so ključni za povzročene neželene učinke in prek katerih molekularnih mehanizmov sprožajo neželene učinke.