Greenhouse gases refer to the sum of seven gases that have direct effects on climate change : carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulphur hexafluoride (SF6) and nitrogen trifluoride (NF3). The data are expressed in CO₂ equivalents and refer to gross direct emissions from human activities. CO₂ refers to gross direct emissions from fuel combustion only and data are provided by the International Energy Agency. Other air emissions include emissions of sulphur oxides (SOx) and nitrogen oxides (NOx) given as quantities of SO₂ and NO₂, emissions of carbon monoxide (CO), and emissions of volatile organic compounds (VOC), excluding methane. Air and greenhouse gas emissions are measured in thousand tonnes, tonnes per capita or kilogrammes per capita except for CO₂, which is measured in million tonnes and tonnes per capita.