Related by context. Frequent words. (Click for all words.) 66 Nitrogen oxides 61 potent greenhouse gas 61 nitrous oxides 60 respiratory irritant 58 sulfur oxide 58 Burning fossil fuels 57 nitrogen NOx 57 nitrogen oxide NOx emissions 57 smog forming 57 pollutants emitted 57 nitrogen oxides NOx 56 smog forming pollutants 56 sulfur dioxide SO2 56 nitrogen oxides 56 Sulfur dioxide 55 sulfur dioxide nitrogen oxide 55 hydrogen chloride 54 Hydrogen sulfide 54 benzene toluene 54 greenhouse gases GHGs 53 fluorocarbons 53 nitrogen oxide 53 aromatic hydrocarbons 53 Particulates 53 greenhouse gases GHG 53 particulate matter PM 52 fine particulates 52 pollutants 52 gaseous emissions 52 smog forming nitrogen oxides 52 aggravate asthma 52 SO2 emissions 52 toxic pollutant 52 sulfur dioxide nitrogen oxides 51 sulfur oxides 51 incomplete combustion 51 carbon dioxide methane nitrous 51 particulate matter emissions 51 hydroxyl radicals 51 #,# butadiene 51 absorbs carbon dioxide 51 heat trapping gases emitted 51 sulfur dioxide 51 Particulate matter 50 particulate emissions 50 Diesel exhaust 50 volatile organic compounds 50 deplete oxygen 50 odorless colorless gas 50 pollutant 50 nitrogen oxide emissions 50 biogenic 50 animal dander 50 reduce smog forming 50 smoker inhales 50 pollutant emissions 49 fine particulate 49 diesel particulate matter 49 toxic pollutants 49 sulfur hexafluoride 49 particulates 49 furans 49 mercury cadmium 49 greenhouse gasses 49 carcinogenic compounds 49 particulate matter 49 Greenhouse gases 48 CO -2 48 oxidizers 48 toxic byproducts 48 elemental sulfur 48 dioxide 48 furan 48 nitrous oxide emissions 48 PCBs polychlorinated biphenyls 48 co2 48 trigger allergic reactions 48 isoprene 48 gases emitted 48 halogenated 48 carbon dioxide 48 GHGs 48 Dioxins 48 Prolonged exposure 47 nitrogen oxide NOx 47 airborne particulates 47 SOx 47 soot pollution 47 polution 47 purines 47 gases 47 mercury arsenic 47 aerosol particles 47 gasoline vapors 47 Carbon dioxide emissions 47 inorganic compounds 47 carcinogenic chemicals 47 air pollution 47 ethanol methanol 47 harmful pollutants 47 nitrogen trifluoride