PAHs can enter ecosystems via unscrubbed engine and boiler emissions to air however exhaust gas cleaning Systems remove ~80% of particulate matter and hence the heavier molecular weight and generally more toxic PAHs from the exhaust stream. Before wash water can be returned to the sea a treatment plant must remove the particulate matter. Low molecular weight PAHs may also be dissolved in the wash water so continuous online monitoring of PAH is used to ensure that the treatment is effective and marine ecosystems are not impacted. Furthermore, as PAHs are also found naturally in petroleum their monitoring ensures that un-burned oil does not enter the sea.

The IMO has set limits for the online monitoring of phenanthrene [10], based on studies to date which have shown no negative influences of EGCS wash water on port environments and that this is the most prevalent of the 16 US EPA PAHs to be found in the wash water systems onboard. [4]

There are a very wide variety of sources for PAHs to enter the environment, both natural and man-made. These include industrial wastewater, road runoff, fossil fuel combustion, oil spills, forest and grass fires, volcanic particles, and natural oil seeps.  There are also seasonal variations in concentration, for example increases can be seen in winter because of the heating of buildings in towns and cities. Low molecular weight PAHs with two or three rings are present normally in dissolved form in water or gaseous in the atmosphere. However the higher the molecular weight the more hydrophobic they behave and the more they are bound to particles. The highest PAH concentrations are therefore found in sediments [4].

Sediments can be disturbed during shallow water manoeuvring of a ship and as a result may enter the wash water system. The IMO therefore requires the background concentration of PAH at the wash water inlet to be taken into account when measuring the PAH concentration at system discharge. It is also required that monitoring at discharge is before any dilution for correction of the wash water pH.

PAH must be continuously measured online and data securely logged to confirm compliance with the IMO EGCS Guidelines.