Discharges of oil from shipping, offshore extraction of oil, and transport of oil in pipelines is the result of either accidents or "normal", deliberate operational discharges. Accidental discharges (oil spills) occur when vessels collide or come in distress at sea (engine breakdown, fire, explosion) and break open, or run aground close to the shore, or when there is a blowout of an offshore oil well, or when a pipeline breaks. Much can be done to avoid accidents, but there will always be unfortunate circumstances and situations that cause accidents to happen. Operational discharges, on the other hand, are mostly deliberate and "routine", and can to a very large extent be effectively controlled and avoided. It is much a question combining available technical solutions with information, education and a change of attitude among ship-owners, mariners, offshore platform and pipeline operators.
Volatile organic compounds (VOCs) are a mixture of propane, butane and several other gases given off through the vaporisation of crude oil and refined products, such as petrol. VOCs are usually divided into non-methane (nmVOCs) and methane. As soon as petroleum is handled in the open air, vapours escape unless something is done to contain them. These hydrocarbons in gaseous form can be emitted into the atmosphere from platforms (extraction of oil), tankers (transportation of oil), terminals (loading and unloading of oil), filling stations (petrol tanking) refineries (processing of oil), pipelines (leakages of oils and gas), and aircraft (refueling and fuel dumping). Hydrocarbon vapours can be recovered and "recycled" as liquid fuel. They are collected and condensed when petroleum products are being loaded and unloaded at terminals, refineries and petrol stations.

Polycyclic aromatic hydrocarbons (PAHs) include around a hundred of the petroleum hydrocarbons and are natural constituents of oil. When fossil fuels are burnt, especially on a small scale when combustion is often incomplete, PAH compounds form and escape to the atmosphere or to water. PAHs occur both in gaseous form and bound to particles (soot). Large amounts of PAHs are emitted from recreational vessels due to inadequate fuel combustion in two-stroke (outboard) engines.


Tankers: When crude oil is unloaded from a tanker, the cargo tanks will contain a mixture of gases, including VOCs. When the tank is filled again with crude oil, the gases are vented to the open air. However, with new technology the VOCs can be condensed and recovered and used as tanker fuel (instead of the much "dirtier" bunker fuel). In other words, when an oil tanker is making a delivery, vapours that escape will be captured and go back into the tanker. On board a tanker, emissions of VOCs can be controlled by allowing a slight overpressure in the tanks.

Pleasure craft: Two-stroke petrol outboard engines are popular and common in pleasure craft (recreational vessels, leisure craft). It is a compact, simple engine which requires little maintenance. In relation to its weight, the two-stroke outboard engine generates much power. It is reliable in operation and comparatively cheap. From an environmental point of view, however, these engines are bad news. They are extremely fuel-consuming due to low thermal efficiency and cause high emissions of uncombusted hydrocarbons (PAHs) and carbon monoxide. The PAHs virtually "disappear" into the water stirred up by the propeller and get dissolved in the water, i.e., transformed from air pollutants to water pollutants. Considerable damage is caused by these hydrocarbon compounds to water-living organisms (to which they are toxic), especially in shallow coastal areas which serve as marine "nurseries".


Activities that cause air pollution (VOCs, methane, carbon dioxide, and nitrogen oxides) from offshore installations include gas venting on shuttle tanks when oil is loaded onto these tanks for transport, as well as gas flaring, gas turbine exhausts, diesel or fuel exhausts, well testing and diffuse emissions from the platform and drilling place. About two-thirds of the air pollutants are VOCs and they mainly derive from the offshore storing and loading of crude oil. About 70 per cent of the VOCs can be recovered (see above).


Crude oil is transported to an oil terminal — a land-based facility which receives and stores crude oil and other products from offshore oil production — by ship or pipeline. In port, at the terminal, it is technically possible to recover hydrocarbon vapours.


Human activities causing atmospheric deposition of hydrocarbons on the sea include military and commercial jets that occasionally jettison excess fuel over the ocean.