Related by context. Frequent words. (Click for all words.) 76 air pollution 65 pollution 64 particulate matter 64 ozone 60 soot 58 haze 58 pollutant 57 particulates 55 pollutants 52 nitrogen oxides 52 particulate 52 sulfur dioxide 51 acid rain 51 Pollution 51 Air Quality 50 emission 50 emissions 50 greenhouse gas 49 greenhouse gases 48 carbon monoxide 48 Clean Air 46 humidity 45 Ozone 45 cigarette smoke 44 Emissions 44 mercury 44 traffic jams 44 carbon dioxide emissions 44 greenhouse gas emissions 43 dust 43 fog 43 greenhouse emissions 43 polluted 43 congestion 43 forest fires 42 mercury emissions 42 fumes 42 respiratory illnesses 42 smoke 41 polluting 41 traffic congestion 41 toxic chemicals 41 asthma 41 noxious 41 respiratory illness 41 heatwave 41 benzene 41 polluters 41 carbon dioxide 41 carbon emissions 40 wildfires 40 secondhand smoke 40 smokestacks 40 carbon emission 40 gridlock 40 pollen 40 cleaner burning 40 global warming 40 ash 40 volatile organic compounds 39 formaldehyde 39 hydrogen sulfide 39 polluter 39 urban sprawl 38 freezing temperatures 38 freeways 38 lung disease 38 air 38 sweltering heat 38 gases 38 burning fossil fuels 38 scorching heat 38 idling 38 temperatures 38 heat trapping gases 38 Lung Association 38 CO2 emissions 38 radon 37 humid 37 GHG 37 stench 37 dense fog 37 smoky 37 asthmatic 37 aerosols 37 vapors 37 skies 37 volcanic ash 37 searing heat 37 muggy 37 drifting snow 37 odors 37 sulfur 37 GHG emissions 36 Environmental Protection Agency 36 CO2 36 toxins 36 respiratory diseases 36 water vapor 36 mist 36 atmospheric