mainly carbon monoxide. but this largely depends on what smoke you are referring to for example wood smoke contains the following

carbon monoxide, 66methane, volatile organic compounds (C2-C7), aldehydes: formaldehyde, 72acrolein, propionaldehyde, butyraldehyde, acetaldehyde, furfural; substituted furans, benzene, alkyl benzenes: Otoluene, acetic acid, formic acid; nitrogen oxides (NO, NO2), sulfur dioxide, methyl chloride,naphthalene, substituted naphthalenes, oxygenated monoaromatics: guaiacol (and derivatives), phenol (and derivatives), syringol (and derivatives), catechol (and derivatives); particulate organic carbon, oxygenated polycyclic aromatic hydrocarbons, polycyclic aromatic hydrocarbons: fluorene, phenanthrene, anthracene, methylanthracenes, fluoranthene, pyrene,benzo(a)anthracene, chrysene, benzofluoranthenes, benzo(e)pyrene, benzo(a)pyrene, perylene, pyrene, benzo(ghi)perylene, coronene, Odibenzo(a,h)pyrene, retene, dibenz(a,h)anthracene; trace elements: Sodium, Magnesium, Aluminum, Silicon, Sulfur, Chlorine, Potassium, Calcium, Titanium, Vanadium, OChromium, Manganese, Iron, Nickel, Copper, Zinc, Bromine, Lead; particulate elemental carbon, normal alkanes (C24-C30), cyclic di-and triterpenoids, dehydroabietic acid, isopimaric acid, lupenone, friedelin, Ochlorinated dioxins