Volatile Organic Compounds (VOC) are light hydrocarbon vapours, mainly methane and heavier hydrocarbons, that evolve from crude oil and petroleum products. Causes and stages:

• At source/vent evolution: crude oil under pressure and temperature contains dissolved light fractions (methane, ethane, propane, butane and heavier volatile fractions). When the tank's vapour space is opened to atmosphere or the vapours are displaced, the flash vapours escape.

• During loading: as the tank fills, the vapour space is displaced and the rich hydrocarbon vapour is forced out through the venting/vapour recovery line unless a vapour recovery or inerting arrangement retains it. The turbulence of incoming crude, splash filling before the discharge pipes are submerged, increases evaporation and aerosol/VOC generation. Boil-off from cargo and wax, e.g. high-RVP cargoes, increases vapour.

• During transportation: dissolved gases (especially methane/ethane) continue to come out of solution as pressure drops or as the cargo is heated, and with a large vapour space, vapour fills the space; any venting to atmosphere, cargo heating, tank heating or tank breathing (thermal expansion and contraction of vapour) releases VOC when vents open or when vapour condenses at high temperature.

• During unloading/ballasting: as the tank empties, the vapour space expands; to avoid collapse, vapour flows out through vents; similarly when ballasting with dirty ballast, displaced VOC escapes. Stripping operations and pump/vent recoveries release vapours.

• Tank cleaning, crude oil washing (COW) and gas freeing also liberate large VOC; the venting of vapours during these stages is a major emission.

• Human health: inhalation of VOC causes dizziness, headaches, nausea and respiratory irritation; exposure to benzene (a VOC) is associated with leukaemia; long-term exposure may damage the central nervous system, liver and kidneys; acute overexposure in confined spaces can cause asphyxiation as vapours displace oxygen and create flammable/toxic atmospheres.
• Marine life: VOC dissolved or floating on the sea surface can harm plankton and the eggs/larvae of marine organisms; hydrocarbons in water are toxic to fish and invertebrates; bio-accumulation may occur in the food chain.
• Air quality: VOCs react with NOx in sunlight to form photochemical smog and ground-level ozone (tropospheric ozone), which is harmful to human respiratory health and vegetation.
• Climate change: methane (a strong VOC and greenhouse gas) has a global warming potential many times (28-80x) that of CO2 over 100/20 years; other volatile HC contribute to radiative forcing, so VOC release from tankers contributes to GHG emissions and to the ships' contribution to climate change.
• Marine ecosystems: oil vapour/oil sheens from VOC sinks can pollute the sea surface, affect birds and otters (oil coating), and degrade the coastal and pelagic environment; VOC emissions also add to the overall atmospheric HC loading in port regions.

• Inert gas system (IGS): by keeping the vapour space inert (below Lower Flammable Limit) the oxygen level is kept low which both prevents explosion and, in conjunction with a vapour space that does not vent to sea, reduces the amount flashed; however the venting of vapour still occurs if tank level changes; IGS does not eliminate VOC but improves safety and reduces air ingress reducing aromatic regeneration.
• Vapour recovery systems (VRS) / vapour emission control systems (VECS) during loading/unloading: recovering the vapour displaced, by condensation, absorption/adsorption and by returning the recovered product; these are highly effective in loading terminals where vapour is collected and returned or processed, reducing emissions at the discharge/loading interface.
• Closed loading/vent lines and shut-off at the fill point, submersed loading, and using the vapour balance line between the ship and terminal (vessel-to-shore vapour return).
• Use of low-vapour-pressure/suppression additives and blending; choosing crude with lower RVP (reduced vapour pressure); minimizing heating and agitating cargo; installing better constructed/sloping vent systems.
• Improving tank vents with flame-trap/routeing the vents, and controlling pressure within the tank by regulating the inert gas pressure and temperature.
• On modern tankers: environmental protection by using terminal VOC recovery units and by the shipboard vacuum/recovery arrangement; and in some ports, shore-side VOC recovery (e.g. in the EU/UK and US) that capture the returning vapour.
• Operational measures: segregating the vapour space, performing tank cleaning and gas freeing when VOC content is low, cargo heating optimisation, better maintenance of vents and seals, and use of vapour return connections.
• Effectiveness: vapour recovery and vapour balance are the most effective, achieving 90-99% capture at terminals; IGS and operational measures are less effective alone but are essential for safety and reduce the quantity emitted; VOC capture is strongest when ship and terminal cooperate and where national/regional rules (e.g. US EPA, EU VOC directives under MARPOL Annex VI Reg 15) mandate vapour emission control at terminals. Overall, a combination of marine terminal vapour recovery and improved tanker operation produces the greatest reduction in VOC.