Carbon offset production flows to regions or industries that have a lower price of carbon—unless carbon can be purchased from that area, which offsets effectively permit, equalizing the price. Carbon offsets may represent six primary categories of greenhouse gases: carbon dioxide (), methane (CH), nitrous oxide (NO), perfluorocarbons (PFCs), hydrofluorocarbons (HFCs), and sulfur hexafluoride (SF). Carbon offsets have several common features: In 2009, 8.2 billion metric tons of carbon dioxide equivalent changed hands worldwide, up 68 per cent from 2008, according to the study by carbon-market research firm Point Carbon, of Washington and Oslo. But at EUR94 billion, or about $135 billion,
Industrial gas ambient temperature is low enough; this includes ethylene (R1150), carbon dioxide (R744), ethane (R170), nitrous oxide (R744A), and sulfur hexafluoride; however, these can only be liquefied under pressure if kept below their critical temperatures which are 9 °C for CH ; 31 °C for CO ; 32 °C for CH ; 36 °C for NO ; 45 °C for SF. All of these substances are also provided as a "Gas" (not a vapor) at the 200 bar pressure in a gas cylinder because that pressure is above their critical pressure. Other gases can only be supplied as liquid if also